What does if name main do

Question

Given the following code  what does the if   name         main     do     Threading example import time  thread  def myfunction string  sleeptime  lock   args       while True          lock acquire           time sleep sleeptime          lock release           time sleep sleeptime   if   name         main         lock   thread allocate lock       thread start new thread myfunction    Thread    1   2  lock       thread start new thread myfunction    Thread    2   2  lock

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Every module in Python has a special attribute called name  The value of name  attribute is set to  main   when the module is executed as  the main program  e g  running python foo py   Otherwise  the value of name  is set to the name of the module that it was called from

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Simply  it is the entry point to run the file  like the main function in the C programming language

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Consider   if   name         main         main     It checks if the   name   attribute of the Python script is    main     In other words  if the program itself is executed  the attribute will be   main    so the program will be executed  in this case the main   function    However  if your Python script is used by a module  any code outside of the if statement will be executed  so if    name          main    is used just to check if the program is used as a module or not  and therefore decides whether to run the code

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Before explaining anything about if   name         main    it is important to understand what   name   is and what it does      What is   name        name   is a DunderAlias - can be thought of as a global variable  accessible from modules  and works in a similar way to global   It is a string  global as mentioned above  as indicated by type   name     yielding  lt class  str  gt    and is an inbuilt standard for both Python 3 and Python 2 versions      Where    It can not only be used in scripts but can also be found in both the interpreter and modules packages     Interpreter    gt  gt  gt  print   name      main    gt  gt  gt    Script   test file py   print   name      Resulting in   main    Module or package   somefile py   def somefunction        print   name      test file py   import somefile somefile somefunction     Resulting in somefile  Notice that when used in a package or module    name   takes the name of the file   The path of the actual module or package path is not given  but has its own DunderAlias   file    that allows for this   You should see that  where   name    where it is the main file  or program  will always return   main    and if it is a module package  or anything that is running off some other Python script  will return the name of the file where it has originated from      Practice    Being a variable means that it s value can be overwritten   can  does not mean  should    overwriting the value of   name   will result in a lack of readability   So do not do it  for any reason   If you need a variable define a new variable   It is always assumed that the value of   name   to be   main   or the name of the file   Once again changing this default value will cause more confusion that it will do good  causing problems further down the line   example    gt  gt  gt    name      Horrify    Change default from   main    gt  gt  gt  if   name       Horrify   print   name         gt  gt  gt  else  print  Not Horrify       Horrify  gt  gt  gt    It is considered good practice in general to include the if   name         main    in scripts      Now to answer if   name         main       Now we know the behaviour of   name   things become clearer   An if is a flow control statement that contains the block of code will execute if the value given is true  We have seen that   name   can take either    main   or the file name it has been imported from     This means that if   name   is equal to   main   then the file must be the main file and must actually be running  or it is the interpreter   not a module or package imported into the script   If indeed   name   does take the value of   main   then whatever is in that block of code will execute   This tells us that if the file running is the main file  or you are running from the interpreter directly  then that condition must execute   If it is a package then it should not  and the value will not be   main        Modules      name   can also be used in modules to define the name of a module     Variants      It is also possible to do other  less common but useful things with   name    some I will show here   Executing only if the file is a module or package   if   name         main           Do some useful things    Running one condition if the file is the main one and another if it is not   if   name         main           Execute something else        Do some useful things   You can also use it to provide runnable help functions utilities on packages and modules without the elaborate use of libraries   It also allows modules to be run from the command line as main scripts  which can be also very useful

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It is a special for when a Python file is called from the command line  This is typically used to call a  main    function or execute other appropriate startup code  like commandline arguments handling for instance   It could be written in several ways  Another is   def some function for instance main        dosomething       name         main    and some function for instance main     I am not saying you should use this in production code  but it serves to illustrate that there is nothing  magical  about if   name         main     It is a good convention for invoking a main function in Python files

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You can make the file usable as a script as well as an importable module    fibo py  a module named fibo     Other modules can IMPORT this MODULE to use the function fib def fib n        write Fibonacci series up to n     a  b   0  1     while b  lt  n          print b  end              a  b   b  a b     print      This allows the file to be used as a SCRIPT if   name         main         import sys     fib int sys argv 1      Reference  https   docs python org 3 5 tutorial modules html

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When there are certain statements in our module  M py  we want to be executed when it ll be running as main  not imported   we can place those statements  test-cases  print statements  under this if block   As by default  when module running as main  not imported  the   name   variable is set to    main     and when it ll be imported the   name   variable will get a different value  most probably the name of the module   M    This is helpful in running different variants of a modules together  and separating their specific input  amp  output statements and also if there are any test-cases   In short  use this  if   name       main    block to prevent  certain  code from being run when the module is imported

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Let s look at the answer in a more abstract way   Suppose we have this code in x py        lt Block A gt  if   name         main          lt Block B gt        Blocks A and B are run when we are running x py   But just block A  and not B  is run when we are running another module  y py for example  in which x py is imported and the code is run from there  like when a function in x py is called from y py

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I ve been reading so much throughout the answers on this page  I would say  if you know the thing  for sure you will understand those answers  otherwise  you are still confused  To be short  you need to know several points   import a action actually runs all that can be ran in a py  meaning each line in a py  Because of point 1  you may not want everything to be run in a py when importing it  To solve the problem in point 2  python allows you to put a condition check    name   is an implicit variable in all  py modules     when a py is imported  the value of   name   of a py module is set to its file name  quot a quot  when a py is run directly using  quot python a py quot   the value of   name   is set to a string   main     Based on the mechanism how python sets the variable   name   for each module  do you know how to achieve point 3  The answer is fairly easy  right  Put a if condition  if   name       quot   main   quot      do A   then python a py will run the part    do A and import a will skip the part    do A   You can even put if   name       quot a quot  depending on your functional need  but rarely do  The important thing that python is special at is point 4  The rest is just basic logic

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If this  py file are imported by other  py files  the code under  the if statement  will not be executed   If this  py are run by python this py py under shell  or double clicked in Windows  the code under  the if statement  will be executed   It is usually written for testing

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Let s look at the answer in a more abstract way   Suppose we have this code in x py        lt Block A gt  if   name         main          lt Block B gt        Blocks A and B are run when we are running x py   But just block A  and not B  is run when we are running another module  y py for example  in which x py is imported and the code is run from there  like when a function in x py is called from y py

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The code under if   name         main     will be executed only if the module is invoked as a script  As an example consider the following module my test module py    my test module py  print  This is going to be printed out  no matter what    if   name         main         print  This is going to be printed out  only if user invokes the module as a script     1st possibility  Import my test module py in another module   main py  import my test module  if   name         main         print  Hello from main py    Now if you invoke main py  python main py    gt  gt   This is going to be printed out  no matter what   gt  gt   Hello from main py   Note that only the top-level print   statement in my test module is executed   2nd possibility  Invoke my test module py as a script Now if you run my test module py as a Python script  both print   statements will be exectued  python my test module py   gt  gt  gt   This is going to be printed out  no matter what   gt  gt  gt   This is going to be printed out  only if user invokes the module as a script    For a more comprehensive explanation you can read this blog post

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Before explaining anything about if   name         main    it is important to understand what   name   is and what it does      What is   name        name   is a DunderAlias - can be thought of as a global variable  accessible from modules  and works in a similar way to global   It is a string  global as mentioned above  as indicated by type   name     yielding  lt class  str  gt    and is an inbuilt standard for both Python 3 and Python 2 versions      Where    It can not only be used in scripts but can also be found in both the interpreter and modules packages     Interpreter    gt  gt  gt  print   name      main    gt  gt  gt    Script   test file py   print   name      Resulting in   main    Module or package   somefile py   def somefunction        print   name      test file py   import somefile somefile somefunction     Resulting in somefile  Notice that when used in a package or module    name   takes the name of the file   The path of the actual module or package path is not given  but has its own DunderAlias   file    that allows for this   You should see that  where   name    where it is the main file  or program  will always return   main    and if it is a module package  or anything that is running off some other Python script  will return the name of the file where it has originated from      Practice    Being a variable means that it s value can be overwritten   can  does not mean  should    overwriting the value of   name   will result in a lack of readability   So do not do it  for any reason   If you need a variable define a new variable   It is always assumed that the value of   name   to be   main   or the name of the file   Once again changing this default value will cause more confusion that it will do good  causing problems further down the line   example    gt  gt  gt    name      Horrify    Change default from   main    gt  gt  gt  if   name       Horrify   print   name         gt  gt  gt  else  print  Not Horrify       Horrify  gt  gt  gt    It is considered good practice in general to include the if   name         main    in scripts      Now to answer if   name         main       Now we know the behaviour of   name   things become clearer   An if is a flow control statement that contains the block of code will execute if the value given is true  We have seen that   name   can take either    main   or the file name it has been imported from     This means that if   name   is equal to   main   then the file must be the main file and must actually be running  or it is the interpreter   not a module or package imported into the script   If indeed   name   does take the value of   main   then whatever is in that block of code will execute   This tells us that if the file running is the main file  or you are running from the interpreter directly  then that condition must execute   If it is a package then it should not  and the value will not be   main        Modules      name   can also be used in modules to define the name of a module     Variants      It is also possible to do other  less common but useful things with   name    some I will show here   Executing only if the file is a module or package   if   name         main           Do some useful things    Running one condition if the file is the main one and another if it is not   if   name         main           Execute something else        Do some useful things   You can also use it to provide runnable help functions utilities on packages and modules without the elaborate use of libraries   It also allows modules to be run from the command line as main scripts  which can be also very useful

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What does if   name         main     do      name   is a global variable  in Python  global actually means on the module level  that exists in all namespaces  It is typically the module s name  as a str type    As the only special case  however  in whatever Python process you run  as in mycode py   python mycode py   the otherwise anonymous global namespace is assigned the value of    main    to its   name      Thus  including the final lines  if   name         main         main      at the end of your mycode py script  when it is the primary  entry-point module that is run by a Python process     will cause your script s uniquely defined main function to run    Another benefit of using this construct  you can also import your code as a module in another script and then run the main function if and when your program decides   import mycode       any amount of other code mycode main

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if   name         main    is the part that runs when the script is run from  say  the command line using a command like python myscript py

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When there are certain statements in our module  M py  we want to be executed when it ll be running as main  not imported   we can place those statements  test-cases  print statements  under this if block   As by default  when module running as main  not imported  the   name   variable is set to    main     and when it ll be imported the   name   variable will get a different value  most probably the name of the module   M    This is helpful in running different variants of a modules together  and separating their specific input  amp  output statements and also if there are any test-cases   In short  use this  if   name       main    block to prevent  certain  code from being run when the module is imported

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When you run Python interactively the local   name   variable is assigned a value of   main    Likewise  when you execute a Python module from the command line  rather than importing it into another module  its   name   attribute is assigned a value of   main    rather than the actual name of the module  In this way  modules can look at their own   name   value to determine for themselves how they are being used  whether as support for another program or as the main application executed from the command line  Thus  the following idiom is quite common in Python modules   if   name         main           Do something appropriate here  like calling a       main   function defined elsewhere in this module      main   else        Do nothing  This module has been imported by another       module that wants to make use of the functions        classes and other useful bits it has defined

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This answer is for Java programmers learning Python  Every Java file typically contains one public class  You can use that class in two ways     Call the class from other files  You just have to import it in the calling program  Run the class stand alone  for testing purposes     For the latter case  the class should contain a public static void main   method  In Python this purpose is served by the globally defined label    main

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if   name         main    is the part that runs when the script is run from  say  the command line using a command like python myscript py

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if name     main    We see if   name         main     quite often   It checks if a module is being imported or not   In other words  the code within the if block will be executed only when the code runs directly  Here directly means not imported   Let s see what it does using a simple code that prints the name of the module     test py def test       print  test module name  s      name      if   name         main        print  call test        test     If we run the code directly via python test py  the module name is   main     call test   test module name   main

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The code under if   name         main     will be executed only if the module is invoked as a script  As an example consider the following module my test module py    my test module py  print  This is going to be printed out  no matter what    if   name         main         print  This is going to be printed out  only if user invokes the module as a script     1st possibility  Import my test module py in another module   main py  import my test module  if   name         main         print  Hello from main py    Now if you invoke main py  python main py    gt  gt   This is going to be printed out  no matter what   gt  gt   Hello from main py   Note that only the top-level print   statement in my test module is executed   2nd possibility  Invoke my test module py as a script Now if you run my test module py as a Python script  both print   statements will be exectued  python my test module py   gt  gt  gt   This is going to be printed out  no matter what   gt  gt  gt   This is going to be printed out  only if user invokes the module as a script    For a more comprehensive explanation you can read this blog post

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I ve been reading so much throughout the answers on this page  I would say  if you know the thing  for sure you will understand those answers  otherwise  you are still confused  To be short  you need to know several points   import a action actually runs all that can be ran in a py  meaning each line in a py  Because of point 1  you may not want everything to be run in a py when importing it  To solve the problem in point 2  python allows you to put a condition check    name   is an implicit variable in all  py modules     when a py is imported  the value of   name   of a py module is set to its file name  quot a quot  when a py is run directly using  quot python a py quot   the value of   name   is set to a string   main     Based on the mechanism how python sets the variable   name   for each module  do you know how to achieve point 3  The answer is fairly easy  right  Put a if condition  if   name       quot   main   quot      do A   then python a py will run the part    do A and import a will skip the part    do A   You can even put if   name       quot a quot  depending on your functional need  but rarely do  The important thing that python is special at is point 4  The rest is just basic logic

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All the answers have pretty much explained the functionality  But I will provide one example of its usage which might help clearing out the concept further   Assume that you have two Python files  a py and b py  Now  a py imports b py  We run the a py file  where the  import b py  code is executed first  Before the rest of the a py code runs  the code in the file b py must run completely   In the b py code there is some code that is exclusive to that file b py and we don t want any other file  other than b py file   that has imported the b py file  to run it   So that is what this line of code checks  If it is the main file  i e   b py  running the code  which in this case it is not  a py is the main file running   then only the code gets executed

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You can checkup for the special variable   name   with this simple example   create file1 py  if   name         main         print  file1 is being run directly   else      print  file1 is being imported     create file2 py  import file1 as f1  print    name   from file1      format f1   name     print    name   from file2      format   name      if   name         main         print  file2 is being run directly   else      print  file2 is being imported     Execute file2 py  output   file1 is being imported   name   from file1  file1   name   from file2    main   file2 is being run directly

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I think it s best to break the answer in depth and in simple words     name    Every module in Python has a special attribute called   name    It is a built-in variable that returns the name of the module     main    Like other programming languages  Python too has an execution entry point  i e   main     main    is the name of the scope in which top-level code executes  Basically you have two ways of using a Python module  Run it directly as a script  or import it  When a module is run as a script  its   name   is set to   main     Thus  the value of the   name   attribute is set to   main   when the module is run as the main program  Otherwise the value of   name    is set to contain the name of the module

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The reason for  if   name         main         main     is primarily to avoid the import lock problems that would arise from having code directly imported  You want main   to run if your file was directly invoked  that s the   name         main    case   but if your code was imported then the importer has to enter your code from the true main module to avoid import lock problems   A side-effect is that you automatically sign on to a methodology that supports multiple entry points  You can run your program using main   as the entry point  but you don t have to  While setup py expects main    other tools use alternate entry points  For example  to run your file as a gunicorn process  you define an app   function instead of a main    Just as with setup py  gunicorn imports your code so you don t want it do do anything while it s being imported  because of the import lock issue

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Whenever the Python interpreter reads a source file  it does two things   it sets a few special variables like   name    and then  it executes all of the code found in the file    Let s see how this works and how it relates to your question about the   name   checks we always see in Python scripts  Code Sample Let s use a slightly different code sample to explore how imports and scripts work   Suppose the following is in a file called foo py    Suppose this is foo py   print  quot before import quot   import math  print  quot before functionA quot   def functionA        print  quot Function A quot    print  quot before functionB quot   def functionB        print  quot Function B    quot  format math sqrt 100     print  quot before   name   guard quot   if   name         main         functionA       functionB   print  quot after   name   guard quot    Special Variables When the Python interpreter reads a source file  it first defines a few special variables  In this case  we care about the   name   variable  When Your Module Is the Main Program If you are running your module  the source file  as the main program  e g  python foo py  the interpreter will assign the hard-coded string  quot   main   quot  to the   name   variable  i e    It s as if the interpreter inserts this at the top   of your module when run as the main program    name      quot   main   quot    When Your Module Is Imported By Another On the other hand  suppose some other module is the main program and it imports your module  This means there s a statement like this in the main program  or in some other module the main program imports    Suppose this is in some other main program  import foo  The interpreter will search for your foo py file  along with searching for a few other variants   and prior to executing that module  it will assign the name  quot foo quot  from the import statement to the   name   variable  i e    It s as if the interpreter inserts this at the top   of your module when it s imported from another module    name      quot foo quot   Executing the Module s Code After the special variables are set up  the interpreter executes all the code in the module  one statement at a time  You may want to open another window on the side with the code sample so you can follow along with this explanation  Always  It prints the string  quot before import quot   without quotes    It loads the math module and assigns it to a variable called math  This is equivalent to replacing import math with the following  note that   import   is a low-level function in Python that takes a string and triggers the actual import       Find and load a module given its string name   quot math quot     then assign it to a local variable called math  math     import    quot math quot     It prints the string  quot before functionA quot    It executes the def block  creating a function object  then assigning that function object to a variable called functionA   It prints the string  quot before functionB quot    It executes the second def block  creating another function object  then assigning it to a variable called functionB   It prints the string  quot before   name   guard quot     Only When Your Module Is the Main Program  If your module is the main program  then it will see that   name   was indeed set to  quot   main   quot  and it calls the two functions  printing the strings  quot Function A quot  and  quot Function B 10 0 quot    Only When Your Module Is Imported by Another   instead  If your module is not the main program but was imported by another one  then   name   will be  quot foo quot   not  quot   main   quot   and it ll skip the body of the if statement   Always  It will print the string  quot after   name   guard quot  in both situations   Summary In summary  here s what d be printed in the two cases    What gets printed if foo is the main program before import before functionA before functionB before   name   guard Function A Function B 10 0 after   name   guard    What gets printed if foo is imported as a regular module before import before functionA before functionB before   name   guard after   name   guard  Why Does It Work This Way  You might naturally wonder why anybody would want this   Well  sometimes you want to write a  py file that can be both used by other programs and or modules as a module  and can also be run as the main program itself   Examples   Your module is a library  but you want to have a script mode where it runs some unit tests or a demo   Your module is only used as a main program  but it has some unit tests  and the testing framework works by importing  py files like your script and running special test functions  You don t want it to try running the script just because it s importing the module   Your module is mostly used as a main program  but it also provides a programmer-friendly API for advanced users    Beyond those examples  it s elegant that running a script in Python is just setting up a few magic variables and importing the script   quot Running quot  the script is a side effect of importing the script s module  Food for Thought  Question  Can I have multiple   name   checking blocks   Answer  it s strange to do so  but the language won t stop you   Suppose the following is in foo2 py   What happens if you say python foo2 py on the command-line  Why      Suppose this is foo2 py  import os  sys  sys path insert 0  os path dirname   file       needed for some interpreters  def functionA        print  quot a1 quot       from foo2 import functionB     print  quot a2 quot       functionB       print  quot a3 quot    def functionB        print  quot b quot    print  quot t1 quot   if   name       quot   main   quot       print  quot m1 quot       functionA       print  quot m2 quot   print  quot t2 quot            Now  figure out what will happen if you remove the   name   check in foo3 py     Suppose this is foo3 py  import os  sys  sys path insert 0  os path dirname   file       needed for some interpreters  def functionA        print  quot a1 quot       from foo3 import functionB     print  quot a2 quot       functionB       print  quot a3 quot    def functionB        print  quot b quot    print  quot t1 quot   print  quot m1 quot   functionA   print  quot m2 quot   print  quot t2 quot     What will this do when used as a script   When imported as a module     Suppose this is in foo4 py   name      quot   main   quot   def bar        print  quot bar quot        print  quot before   name   guard quot   if   name       quot   main   quot       bar   print  quot after   name   guard quot

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if name     main    We see if   name         main     quite often   It checks if a module is being imported or not   In other words  the code within the if block will be executed only when the code runs directly  Here directly means not imported   Let s see what it does using a simple code that prints the name of the module     test py def test       print  test module name  s      name      if   name         main        print  call test        test     If we run the code directly via python test py  the module name is   main     call test   test module name   main

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The simplest explanation for the   name   variable  imho  is the following   Create the following files     a py import b   and    b py print  Hello World from  s       name    if   name         main         print  Hello World again from  s       name     Running them will get you this output     python a py Hello World from b    As you can see  when a module is imported  Python sets globals      name     in this module to the module s name  Also  upon import all the code in the module is being run  As the if statement evaluates to False this part is not executed     python b py Hello World from   main    Hello World again from   main      As you can see  when a file is executed  Python sets globals      name     in this file to    main     This time  the if statement evaluates to True and is being run

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When your script is run by passing it as a command to the Python interpreter   python myscript py   all of the code that is at indentation level 0 gets executed   Functions and classes that are defined are  well  defined  but none of their code gets run   Unlike other languages  there s no main   function that gets run automatically - the main   function is implicitly all the code at the top level   In this case  the top-level code is an if block     name   is a built-in variable which evaluates to the name of the current module   However  if a module is being run directly  as in myscript py above   then   name   instead is set to the string    main      Thus  you can test whether your script is being run directly or being imported by something else by testing  if   name         main               If your script is being imported into another module  its various function and class definitions will be imported and its top-level code will be executed  but the code in the then-body of the if clause above won t get run as the condition is not met  As a basic example  consider the following two scripts     file one py def func        print  func   in one py    print  top-level in one py    if   name         main         print  one py is being run directly   else      print  one py is being imported into another module         file two py import one  print  top-level in two py   one func    if   name         main         print  two py is being run directly   else      print  two py is being imported into another module     Now  if you invoke the interpreter as  python one py   The output will be  top-level in one py one py is being run directly   If you run two py instead   python two py   You get  top-level in one py one py is being imported into another module top-level in two py func   in one py two py is being run directly   Thus  when module one gets loaded  its   name   equals  one  instead of    main

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Simply  it is the entry point to run the file  like the main function in the C programming language

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There are lots of different takes here on the mechanics of the code in question  the  How   but for me none of it made sense until I understood the  Why   This should be especially helpful for new programmers   Take file  ab py    def a        print  A function in ab file    a     And a second file  xy py    import ab def main        print  main function  this is where the action is   def x        print   peripheral task  might be useful in other projects   x   if   name         main         main        What is this code actually doing    When you execute xy py  you import ab  The import statement runs the module immediately on import  so ab s operations get executed before the remainder of xy s  Once finished with ab  it continues with xy   The interpreter keeps track of which scripts are running with   name    When you run a script - no matter what you ve named it - the interpreter calls it    main     making it the master or  home  script that gets returned to after running an external script   Any other script that s called from this    main    script is assigned its filename as its   name    e g     name       ab py    Hence  the line if   name         main     is the interpreter s test to determine if it s interpreting parsing the  home  script that was initially executed  or if it s temporarily peeking into another  external  script  This gives the programmer flexibility to have the script behave differently if it s executed directly vs  called externally   Let s step through the above code to understand what s happening  focusing first on the unindented lines and the order they appear in the scripts  Remember that function - or def - blocks don t do anything by themselves until they re called  What the interpreter might say if mumbled to itself    Open xy py as the  home  file  call it    main    in the   name   variable  Import and open file with the   name       ab py   Oh  a function  I ll remember that  Ok  function a    I just learned that  Printing  A function in ab file   End of file  back to    main     Oh  a function  I ll remember that  Another one  Function x    ok  printing  peripheral task  might be useful in other projects   What s this  An if statement  Well  the condition has been met  the variable   name   has been set to    main      so I ll enter the main   function and print  main function  this is where the action is     The bottom two lines mean   If this is the    main    or  home  script  execute the function called main     That s why you ll see a def main    block up top  which contains the main flow of the script s functionality      Why implement this    Remember what I said earlier about import statements  When you import a module it doesn t just  recognize  it and wait for further instructions - it actually runs all the executable operations contained within the script  So  putting the meat of your script into the main   function effectively quarantines it  putting it in isolation so that it won t immediately run when imported by another script   Again  there will be exceptions  but common practice is that main   doesn t usually get called externally  So you may be wondering one more thing  if we re not calling main    why are we calling the script at all  It s because many people structure their scripts with standalone functions that are built to be run independent of the rest of the code in the file  They re then later called somewhere else in the body of the script  Which brings me to this      But the code works without it   Yes  that s right  These separate functions can be called from an in-line script that s not contained inside a main   function  If you re accustomed  as I am  in my early learning stages of programming  to building in-line scripts that do exactly what you need  and you ll try to figure it out again if you ever need that operation again     well  you re not used to this kind of internal structure to your code  because it s more complicated to build and it s not as intuitive to read   But that s a script that probably can t have its functions called externally  because if it did it would immediately start calculating and assigning variables  And chances are if you re trying to re-use a function  your new script is related closely enough to the old one that there will be conflicting variables   In splitting out independent functions  you gain the ability to re-use your previous work by calling them into another script  For example   example py  might import  xy py  and call x    making use of the  x  function from  xy py    Maybe it s capitalizing the third word of a given text string  creating a NumPy array from a list of numbers and squaring them  or detrending a 3D surface  The possibilities are limitless     As an aside  this question contains an answer by  kindall that finally helped me to understand - the why  not the how  Unfortunately it s been marked as a duplicate of this one  which I think is a mistake

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Every module in Python has a special attribute called name  The value of name  attribute is set to  main   when the module is executed as  the main program  e g  running python foo py   Otherwise  the value of name  is set to the name of the module that it was called from

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if   name      quot   main   quot  means that if you are running the python file normally like python foo py it will assign the special variable   name   to  quot   main   quot  but if you are importing the file like  quot import foo quot  it will assign   name   to  quot foo quot  and will not run the function

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The simplest explanation for the   name   variable  imho  is the following   Create the following files     a py import b   and    b py print  Hello World from  s       name    if   name         main         print  Hello World again from  s       name     Running them will get you this output     python a py Hello World from b    As you can see  when a module is imported  Python sets globals      name     in this module to the module s name  Also  upon import all the code in the module is being run  As the if statement evaluates to False this part is not executed     python b py Hello World from   main    Hello World again from   main      As you can see  when a file is executed  Python sets globals      name     in this file to    main     This time  the if statement evaluates to True and is being run

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The reason for  if   name         main         main     is primarily to avoid the import lock problems that would arise from having code directly imported  You want main   to run if your file was directly invoked  that s the   name         main    case   but if your code was imported then the importer has to enter your code from the true main module to avoid import lock problems   A side-effect is that you automatically sign on to a methodology that supports multiple entry points  You can run your program using main   as the entry point  but you don t have to  While setup py expects main    other tools use alternate entry points  For example  to run your file as a gunicorn process  you define an app   function instead of a main    Just as with setup py  gunicorn imports your code so you don t want it do do anything while it s being imported  because of the import lock issue

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if   name         main    is the part that runs when the script is run from  say  the command line using a command like python myscript py

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What does the if   name         main     do    To outline the basics    The global variable    name    in the module that is the entry point to your program  is    main     Otherwise  it s the name you import the module by  So  code under the if block will only run if the module is the entry point to your program  It allows the code in the module to be importable by other modules  without executing the code block beneath on import      Why do we need this   Developing and Testing Your Code  Say you re writing a Python script designed to be used as a module   def do important           This function does something very important      You could test the module by adding this call of the function to the bottom   do important     and running it  on a command prompt  with something like      python important py   The Problem  However  if you want to import the module to another script   import important   On import  the do important function would be called  so you d probably comment out your function call  do important    at the bottom      do important     I must remember to uncomment to execute this    And then you ll have to remember whether or not you ve commented out your test function call  And this extra complexity would mean you re likely to forget  making your development process more troublesome   A Better Way  The   name   variable points to the namespace wherever the Python interpreter happens to be at the moment    Inside an imported module  it s the name of that module    But inside the primary module  or an interactive Python session  i e  the interpreter s Read  Eval  Print Loop  or REPL  you are running everything from its    main      So if you check before executing   if   name         main         do important     With the above  your code will only execute when you re running it as the primary module  or intentionally call it from another script     An Even Better Way  There s a Pythonic way to improve on this  though    What if we want to run this business process from outside the module   If we put the code we want to exercise as we develop and test in a function like this and then do our check for    main    immediately after   def main           business logic for when running this module as the primary one         setup       foo   do important       bar   do even more important foo      for baz in bar          do super important baz      teardown      Here s our payoff idiom  if   name         main         main     We now have a final function for the end of our module that will run if we run the module as the primary module    It will allow the module and its functions and classes to be imported into other scripts without running the main function  and will also allow the module  and its functions and classes  to be called when running from a different    main    module  i e   import important important main     This idiom can also be found in the Python documentation in an explanation of the   main   module  That text states      This module represents the  otherwise anonymous  scope in which the   interpreter   s main program executes     commands read either from   standard input  from a script file  or from an interactive prompt  It   is this environment in which the idiomatic    conditional script    stanza   causes a script to run   if   name         main         main

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Every module in python has a attribute called   name    The value of   name    attribute is    main    when the module is run directly  like python my module py  Otherwise  like when you say import my module  the value of   name    is the name of the module   Small example to explain in short    Script test py  apple   42  def hello world        print  I am inside hello world    if   name         main         print  Value of   name   is       name        print  Going to call hello world       hello world     We can execute this directly as  python test py     Output  Value of   name   is    main   Going to call hello world I am inside hello world   Now suppose we call above script from other script   script external calling py  import test print test apple  test hello world    print test   name      When you execute this  python external calling py   Output  42 I am inside hello world test   So  above is self explanatory that when you call test from other script  if loop   name   in test py will not execute

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PYTHON MAIN FUNCTION is a starting point of any program  When the program is run  the python interpreter runs the code sequentially  Main function is executed only when it is run as a Python program       def main         print   i am in the function   print   i am out of function     when you run script show     i am out of function   and not the code  i am in the function  It is because we did not declare the call function  if  name      main   if you use from it    def main         print   i am in the function     if   name         main         main    print   i am out of function     The output is equal to  i am in the function i am out of function   In Python  if  name      main  allows you to run the Python files either as reusable modules or standalone programs   When Python interpreter reads a source file  it will execute all the code found in it  When Python runs the  source file  as the main program  it sets the special variable  name  to have a value   main     When you execute the main function  it will then read the  if  statement and checks whether name does equal to main   In Python  if  name      main  allows you to run the Python files either as reusable modules or standalone programs

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If the python interpreter is running  a particular module then   name   global  variable will have value    main       def a          print  a     def b          print  b      if   name         main                     print   you can see me              a     else                  print   You can t see me             b     When you run this script prints you can see me   a  If you import this file say A to file B  and execute the file B then if   name         main    in file A becomes false  so it prints  You can t see me  b

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In  simple words  The code you see under  if   name       quot   main   quot   will only get called upon when your python file is executed as  quot python example1 py quot   However  if you wish to import your python file  example1 py  as a module to work with another python file say  example2 py   the code under if   name       quot   main   quot   will not run or take any effect

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Whenever the Python interpreter reads a source file  it does two things   it sets a few special variables like   name    and then  it executes all of the code found in the file    Let s see how this works and how it relates to your question about the   name   checks we always see in Python scripts  Code Sample Let s use a slightly different code sample to explore how imports and scripts work   Suppose the following is in a file called foo py    Suppose this is foo py   print  quot before import quot   import math  print  quot before functionA quot   def functionA        print  quot Function A quot    print  quot before functionB quot   def functionB        print  quot Function B    quot  format math sqrt 100     print  quot before   name   guard quot   if   name         main         functionA       functionB   print  quot after   name   guard quot    Special Variables When the Python interpreter reads a source file  it first defines a few special variables  In this case  we care about the   name   variable  When Your Module Is the Main Program If you are running your module  the source file  as the main program  e g  python foo py  the interpreter will assign the hard-coded string  quot   main   quot  to the   name   variable  i e    It s as if the interpreter inserts this at the top   of your module when run as the main program    name      quot   main   quot    When Your Module Is Imported By Another On the other hand  suppose some other module is the main program and it imports your module  This means there s a statement like this in the main program  or in some other module the main program imports    Suppose this is in some other main program  import foo  The interpreter will search for your foo py file  along with searching for a few other variants   and prior to executing that module  it will assign the name  quot foo quot  from the import statement to the   name   variable  i e    It s as if the interpreter inserts this at the top   of your module when it s imported from another module    name      quot foo quot   Executing the Module s Code After the special variables are set up  the interpreter executes all the code in the module  one statement at a time  You may want to open another window on the side with the code sample so you can follow along with this explanation  Always  It prints the string  quot before import quot   without quotes    It loads the math module and assigns it to a variable called math  This is equivalent to replacing import math with the following  note that   import   is a low-level function in Python that takes a string and triggers the actual import       Find and load a module given its string name   quot math quot     then assign it to a local variable called math  math     import    quot math quot     It prints the string  quot before functionA quot    It executes the def block  creating a function object  then assigning that function object to a variable called functionA   It prints the string  quot before functionB quot    It executes the second def block  creating another function object  then assigning it to a variable called functionB   It prints the string  quot before   name   guard quot     Only When Your Module Is the Main Program  If your module is the main program  then it will see that   name   was indeed set to  quot   main   quot  and it calls the two functions  printing the strings  quot Function A quot  and  quot Function B 10 0 quot    Only When Your Module Is Imported by Another   instead  If your module is not the main program but was imported by another one  then   name   will be  quot foo quot   not  quot   main   quot   and it ll skip the body of the if statement   Always  It will print the string  quot after   name   guard quot  in both situations   Summary In summary  here s what d be printed in the two cases    What gets printed if foo is the main program before import before functionA before functionB before   name   guard Function A Function B 10 0 after   name   guard    What gets printed if foo is imported as a regular module before import before functionA before functionB before   name   guard after   name   guard  Why Does It Work This Way  You might naturally wonder why anybody would want this   Well  sometimes you want to write a  py file that can be both used by other programs and or modules as a module  and can also be run as the main program itself   Examples   Your module is a library  but you want to have a script mode where it runs some unit tests or a demo   Your module is only used as a main program  but it has some unit tests  and the testing framework works by importing  py files like your script and running special test functions  You don t want it to try running the script just because it s importing the module   Your module is mostly used as a main program  but it also provides a programmer-friendly API for advanced users    Beyond those examples  it s elegant that running a script in Python is just setting up a few magic variables and importing the script   quot Running quot  the script is a side effect of importing the script s module  Food for Thought  Question  Can I have multiple   name   checking blocks   Answer  it s strange to do so  but the language won t stop you   Suppose the following is in foo2 py   What happens if you say python foo2 py on the command-line  Why      Suppose this is foo2 py  import os  sys  sys path insert 0  os path dirname   file       needed for some interpreters  def functionA        print  quot a1 quot       from foo2 import functionB     print  quot a2 quot       functionB       print  quot a3 quot    def functionB        print  quot b quot    print  quot t1 quot   if   name       quot   main   quot       print  quot m1 quot       functionA       print  quot m2 quot   print  quot t2 quot            Now  figure out what will happen if you remove the   name   check in foo3 py     Suppose this is foo3 py  import os  sys  sys path insert 0  os path dirname   file       needed for some interpreters  def functionA        print  quot a1 quot       from foo3 import functionB     print  quot a2 quot       functionB       print  quot a3 quot    def functionB        print  quot b quot    print  quot t1 quot   print  quot m1 quot   functionA   print  quot m2 quot   print  quot t2 quot     What will this do when used as a script   When imported as a module     Suppose this is in foo4 py   name      quot   main   quot   def bar        print  quot bar quot        print  quot before   name   guard quot   if   name       quot   main   quot       bar   print  quot after   name   guard quot

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It is a special for when a Python file is called from the command line  This is typically used to call a  main    function or execute other appropriate startup code  like commandline arguments handling for instance   It could be written in several ways  Another is   def some function for instance main        dosomething       name         main    and some function for instance main     I am not saying you should use this in production code  but it serves to illustrate that there is nothing  magical  about if   name         main     It is a good convention for invoking a main function in Python files

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Put simply    name   is a variable defined for each script that defines whether the script is being run as the main module or it is being run as an imported module   So if we have two scripts    script1 py print  Script 1 s name      format   name      and   script2 py import script1 print  Script 2 s name      format   name      The output from executing script1 is  Script 1 s name    main     And the output from executing script2 is   Script1 s name is script1 Script 2 s name    main     As you can see    name   tells us which code is the  main  module  This is great  because you can just write code and not have to worry about structural issues like in C C    where  if a file does not implement a  main  function then it cannot be compiled as an executable and if it does  it cannot then be used as a library   Say you write a Python script that does something great and you implement a boatload of functions that are useful for other purposes  If I want to use them I can just import your script and use them without executing your program  given that your code only executes within the  if   name         main     context   Whereas in C C   you would have to portion out those pieces into a separate module that then includes the file  Picture the situation below     The arrows are import links  For three modules each trying to include the previous modules code there are six files  nine  counting the implementation files  and five links  This makes it difficult to include other code into a C project unless it is compiled specifically as a library  Now picture it for Python     You write a module  and if someone wants to use your code they just import it and the   name   variable can help to separate the executable portion of the program from the library part

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The simplest explanation for the   name   variable  imho  is the following   Create the following files     a py import b   and    b py print  Hello World from  s       name    if   name         main         print  Hello World again from  s       name     Running them will get you this output     python a py Hello World from b    As you can see  when a module is imported  Python sets globals      name     in this module to the module s name  Also  upon import all the code in the module is being run  As the if statement evaluates to False this part is not executed     python b py Hello World from   main    Hello World again from   main      As you can see  when a file is executed  Python sets globals      name     in this file to    main     This time  the if statement evaluates to True and is being run

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There are a number of variables that the system  Python interpreter  provides for source files  modules    You can get their values anytime you want  so  let us focus on the   name   variable attribute   When Python loads a source code file  it executes all of the code found in it   Note that it doesn t call all of the methods and functions defined in the file  but it does define them    Before the interpreter executes the source code file though  it defines a few special variables for that file    name   is one of those special variables that Python automatically defines for each source code file   If Python is loading this source code file as the main program  i e  the file you run   then it sets the special   name   variable for this file to have a value    main      If this is being imported from another module    name   will be set to that module s name   So  in your example in part   if   name         main        lock   thread allocate lock      thread start new thread myfunction    Thread    1   2  lock      thread start new thread myfunction    Thread    2   2  lock     means that the code block   lock   thread allocate lock   thread start new thread myfunction    Thread    1   2  lock   thread start new thread myfunction    Thread    2   2  lock     will be executed only when you run the module directly  the code block will not execute if another module is calling importing it because the value of   name   will not equal to  main  in that particular instance   Hope this helps out

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PYTHON MAIN FUNCTION is a starting point of any program  When the program is run  the python interpreter runs the code sequentially  Main function is executed only when it is run as a Python program       def main         print   i am in the function   print   i am out of function     when you run script show     i am out of function   and not the code  i am in the function  It is because we did not declare the call function  if  name      main   if you use from it    def main         print   i am in the function     if   name         main         main    print   i am out of function     The output is equal to  i am in the function i am out of function   In Python  if  name      main  allows you to run the Python files either as reusable modules or standalone programs   When Python interpreter reads a source file  it will execute all the code found in it  When Python runs the  source file  as the main program  it sets the special variable  name  to have a value   main     When you execute the main function  it will then read the  if  statement and checks whether name does equal to main   In Python  if  name      main  allows you to run the Python files either as reusable modules or standalone programs

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if   name         main     is basically the top-level script environment  and it specifies the interpreter that   I have the highest priority to be executed first        main    is the name of the scope in which top-level code executes  A module   s   name   is set equal to    main    when read from standard input  a script  or from an interactive prompt   if   name         main           Execute only if run as a script     main

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When your script is run by passing it as a command to the Python interpreter   python myscript py   all of the code that is at indentation level 0 gets executed   Functions and classes that are defined are  well  defined  but none of their code gets run   Unlike other languages  there s no main   function that gets run automatically - the main   function is implicitly all the code at the top level   In this case  the top-level code is an if block     name   is a built-in variable which evaluates to the name of the current module   However  if a module is being run directly  as in myscript py above   then   name   instead is set to the string    main      Thus  you can test whether your script is being run directly or being imported by something else by testing  if   name         main               If your script is being imported into another module  its various function and class definitions will be imported and its top-level code will be executed  but the code in the then-body of the if clause above won t get run as the condition is not met  As a basic example  consider the following two scripts     file one py def func        print  func   in one py    print  top-level in one py    if   name         main         print  one py is being run directly   else      print  one py is being imported into another module         file two py import one  print  top-level in two py   one func    if   name         main         print  two py is being run directly   else      print  two py is being imported into another module     Now  if you invoke the interpreter as  python one py   The output will be  top-level in one py one py is being run directly   If you run two py instead   python two py   You get  top-level in one py one py is being imported into another module top-level in two py func   in one py two py is being run directly   Thus  when module one gets loaded  its   name   equals  one  instead of    main

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If the python interpreter is running  a particular module then   name   global  variable will have value    main       def a          print  a     def b          print  b      if   name         main                     print   you can see me              a     else                  print   You can t see me             b     When you run this script prints you can see me   a  If you import this file say A to file B  and execute the file B then if   name         main    in file A becomes false  so it prints  You can t see me  b

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Create a file  a py   print   name      It will print out   main       name   is always equal to   main   whenever that file is run directly showing that this is the main file   Create another file  b py  in the same directory   import a    Prints a   Run it  It will print a  i e   the name of the file which is imported   So  to show two different behavior of the same file  this is a commonly used trick     Code to be run when imported into another python file  if   name         main           Code to be run only when run directly

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What does if   name         main     do      name   is a global variable  in Python  global actually means on the module level  that exists in all namespaces  It is typically the module s name  as a str type    As the only special case  however  in whatever Python process you run  as in mycode py   python mycode py   the otherwise anonymous global namespace is assigned the value of    main    to its   name      Thus  including the final lines  if   name         main         main      at the end of your mycode py script  when it is the primary  entry-point module that is run by a Python process     will cause your script s uniquely defined main function to run    Another benefit of using this construct  you can also import your code as a module in another script and then run the main function if and when your program decides   import mycode       any amount of other code mycode main

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There are a number of variables that the system  Python interpreter  provides for source files  modules    You can get their values anytime you want  so  let us focus on the   name   variable attribute   When Python loads a source code file  it executes all of the code found in it   Note that it doesn t call all of the methods and functions defined in the file  but it does define them    Before the interpreter executes the source code file though  it defines a few special variables for that file    name   is one of those special variables that Python automatically defines for each source code file   If Python is loading this source code file as the main program  i e  the file you run   then it sets the special   name   variable for this file to have a value    main      If this is being imported from another module    name   will be set to that module s name   So  in your example in part   if   name         main        lock   thread allocate lock      thread start new thread myfunction    Thread    1   2  lock      thread start new thread myfunction    Thread    2   2  lock     means that the code block   lock   thread allocate lock   thread start new thread myfunction    Thread    1   2  lock   thread start new thread myfunction    Thread    2   2  lock     will be executed only when you run the module directly  the code block will not execute if another module is calling importing it because the value of   name   will not equal to  main  in that particular instance   Hope this helps out

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When your script is run by passing it as a command to the Python interpreter   python myscript py   all of the code that is at indentation level 0 gets executed   Functions and classes that are defined are  well  defined  but none of their code gets run   Unlike other languages  there s no main   function that gets run automatically - the main   function is implicitly all the code at the top level   In this case  the top-level code is an if block     name   is a built-in variable which evaluates to the name of the current module   However  if a module is being run directly  as in myscript py above   then   name   instead is set to the string    main      Thus  you can test whether your script is being run directly or being imported by something else by testing  if   name         main               If your script is being imported into another module  its various function and class definitions will be imported and its top-level code will be executed  but the code in the then-body of the if clause above won t get run as the condition is not met  As a basic example  consider the following two scripts     file one py def func        print  func   in one py    print  top-level in one py    if   name         main         print  one py is being run directly   else      print  one py is being imported into another module         file two py import one  print  top-level in two py   one func    if   name         main         print  two py is being run directly   else      print  two py is being imported into another module     Now  if you invoke the interpreter as  python one py   The output will be  top-level in one py one py is being run directly   If you run two py instead   python two py   You get  top-level in one py one py is being imported into another module top-level in two py func   in one py two py is being run directly   Thus  when module one gets loaded  its   name   equals  one  instead of    main

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Consider   print   name     The output for the above is   main     if   name         main       print  direct method    The above statement is true and prints  direct method   Suppose if they imported this class in another class it doesn t print  direct method  because  while importing  it will set   name   equal to  first model name

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Every module in python has a attribute called   name    The value of   name    attribute is    main    when the module is run directly  like python my module py  Otherwise  like when you say import my module  the value of   name    is the name of the module   Small example to explain in short    Script test py  apple   42  def hello world        print  I am inside hello world    if   name         main         print  Value of   name   is       name        print  Going to call hello world       hello world     We can execute this directly as  python test py     Output  Value of   name   is    main   Going to call hello world I am inside hello world   Now suppose we call above script from other script   script external calling py  import test print test apple  test hello world    print test   name      When you execute this  python external calling py   Output  42 I am inside hello world test   So  above is self explanatory that when you call test from other script  if loop   name   in test py will not execute

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There are lots of different takes here on the mechanics of the code in question  the  How   but for me none of it made sense until I understood the  Why   This should be especially helpful for new programmers   Take file  ab py    def a        print  A function in ab file    a     And a second file  xy py    import ab def main        print  main function  this is where the action is   def x        print   peripheral task  might be useful in other projects   x   if   name         main         main        What is this code actually doing    When you execute xy py  you import ab  The import statement runs the module immediately on import  so ab s operations get executed before the remainder of xy s  Once finished with ab  it continues with xy   The interpreter keeps track of which scripts are running with   name    When you run a script - no matter what you ve named it - the interpreter calls it    main     making it the master or  home  script that gets returned to after running an external script   Any other script that s called from this    main    script is assigned its filename as its   name    e g     name       ab py    Hence  the line if   name         main     is the interpreter s test to determine if it s interpreting parsing the  home  script that was initially executed  or if it s temporarily peeking into another  external  script  This gives the programmer flexibility to have the script behave differently if it s executed directly vs  called externally   Let s step through the above code to understand what s happening  focusing first on the unindented lines and the order they appear in the scripts  Remember that function - or def - blocks don t do anything by themselves until they re called  What the interpreter might say if mumbled to itself    Open xy py as the  home  file  call it    main    in the   name   variable  Import and open file with the   name       ab py   Oh  a function  I ll remember that  Ok  function a    I just learned that  Printing  A function in ab file   End of file  back to    main     Oh  a function  I ll remember that  Another one  Function x    ok  printing  peripheral task  might be useful in other projects   What s this  An if statement  Well  the condition has been met  the variable   name   has been set to    main      so I ll enter the main   function and print  main function  this is where the action is     The bottom two lines mean   If this is the    main    or  home  script  execute the function called main     That s why you ll see a def main    block up top  which contains the main flow of the script s functionality      Why implement this    Remember what I said earlier about import statements  When you import a module it doesn t just  recognize  it and wait for further instructions - it actually runs all the executable operations contained within the script  So  putting the meat of your script into the main   function effectively quarantines it  putting it in isolation so that it won t immediately run when imported by another script   Again  there will be exceptions  but common practice is that main   doesn t usually get called externally  So you may be wondering one more thing  if we re not calling main    why are we calling the script at all  It s because many people structure their scripts with standalone functions that are built to be run independent of the rest of the code in the file  They re then later called somewhere else in the body of the script  Which brings me to this      But the code works without it   Yes  that s right  These separate functions can be called from an in-line script that s not contained inside a main   function  If you re accustomed  as I am  in my early learning stages of programming  to building in-line scripts that do exactly what you need  and you ll try to figure it out again if you ever need that operation again     well  you re not used to this kind of internal structure to your code  because it s more complicated to build and it s not as intuitive to read   But that s a script that probably can t have its functions called externally  because if it did it would immediately start calculating and assigning variables  And chances are if you re trying to re-use a function  your new script is related closely enough to the old one that there will be conflicting variables   In splitting out independent functions  you gain the ability to re-use your previous work by calling them into another script  For example   example py  might import  xy py  and call x    making use of the  x  function from  xy py    Maybe it s capitalizing the third word of a given text string  creating a NumPy array from a list of numbers and squaring them  or detrending a 3D surface  The possibilities are limitless     As an aside  this question contains an answer by  kindall that finally helped me to understand - the why  not the how  Unfortunately it s been marked as a duplicate of this one  which I think is a mistake

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if   name         main    is the part that runs when the script is run from  say  the command line using a command like python myscript py

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Whenever the Python interpreter reads a source file  it does two things   it sets a few special variables like   name    and then  it executes all of the code found in the file    Let s see how this works and how it relates to your question about the   name   checks we always see in Python scripts  Code Sample Let s use a slightly different code sample to explore how imports and scripts work   Suppose the following is in a file called foo py    Suppose this is foo py   print  quot before import quot   import math  print  quot before functionA quot   def functionA        print  quot Function A quot    print  quot before functionB quot   def functionB        print  quot Function B    quot  format math sqrt 100     print  quot before   name   guard quot   if   name         main         functionA       functionB   print  quot after   name   guard quot    Special Variables When the Python interpreter reads a source file  it first defines a few special variables  In this case  we care about the   name   variable  When Your Module Is the Main Program If you are running your module  the source file  as the main program  e g  python foo py  the interpreter will assign the hard-coded string  quot   main   quot  to the   name   variable  i e    It s as if the interpreter inserts this at the top   of your module when run as the main program    name      quot   main   quot    When Your Module Is Imported By Another On the other hand  suppose some other module is the main program and it imports your module  This means there s a statement like this in the main program  or in some other module the main program imports    Suppose this is in some other main program  import foo  The interpreter will search for your foo py file  along with searching for a few other variants   and prior to executing that module  it will assign the name  quot foo quot  from the import statement to the   name   variable  i e    It s as if the interpreter inserts this at the top   of your module when it s imported from another module    name      quot foo quot   Executing the Module s Code After the special variables are set up  the interpreter executes all the code in the module  one statement at a time  You may want to open another window on the side with the code sample so you can follow along with this explanation  Always  It prints the string  quot before import quot   without quotes    It loads the math module and assigns it to a variable called math  This is equivalent to replacing import math with the following  note that   import   is a low-level function in Python that takes a string and triggers the actual import       Find and load a module given its string name   quot math quot     then assign it to a local variable called math  math     import    quot math quot     It prints the string  quot before functionA quot    It executes the def block  creating a function object  then assigning that function object to a variable called functionA   It prints the string  quot before functionB quot    It executes the second def block  creating another function object  then assigning it to a variable called functionB   It prints the string  quot before   name   guard quot     Only When Your Module Is the Main Program  If your module is the main program  then it will see that   name   was indeed set to  quot   main   quot  and it calls the two functions  printing the strings  quot Function A quot  and  quot Function B 10 0 quot    Only When Your Module Is Imported by Another   instead  If your module is not the main program but was imported by another one  then   name   will be  quot foo quot   not  quot   main   quot   and it ll skip the body of the if statement   Always  It will print the string  quot after   name   guard quot  in both situations   Summary In summary  here s what d be printed in the two cases    What gets printed if foo is the main program before import before functionA before functionB before   name   guard Function A Function B 10 0 after   name   guard    What gets printed if foo is imported as a regular module before import before functionA before functionB before   name   guard after   name   guard  Why Does It Work This Way  You might naturally wonder why anybody would want this   Well  sometimes you want to write a  py file that can be both used by other programs and or modules as a module  and can also be run as the main program itself   Examples   Your module is a library  but you want to have a script mode where it runs some unit tests or a demo   Your module is only used as a main program  but it has some unit tests  and the testing framework works by importing  py files like your script and running special test functions  You don t want it to try running the script just because it s importing the module   Your module is mostly used as a main program  but it also provides a programmer-friendly API for advanced users    Beyond those examples  it s elegant that running a script in Python is just setting up a few magic variables and importing the script   quot Running quot  the script is a side effect of importing the script s module  Food for Thought  Question  Can I have multiple   name   checking blocks   Answer  it s strange to do so  but the language won t stop you   Suppose the following is in foo2 py   What happens if you say python foo2 py on the command-line  Why      Suppose this is foo2 py  import os  sys  sys path insert 0  os path dirname   file       needed for some interpreters  def functionA        print  quot a1 quot       from foo2 import functionB     print  quot a2 quot       functionB       print  quot a3 quot    def functionB        print  quot b quot    print  quot t1 quot   if   name       quot   main   quot       print  quot m1 quot       functionA       print  quot m2 quot   print  quot t2 quot            Now  figure out what will happen if you remove the   name   check in foo3 py     Suppose this is foo3 py  import os  sys  sys path insert 0  os path dirname   file       needed for some interpreters  def functionA        print  quot a1 quot       from foo3 import functionB     print  quot a2 quot       functionB       print  quot a3 quot    def functionB        print  quot b quot    print  quot t1 quot   print  quot m1 quot   functionA   print  quot m2 quot   print  quot t2 quot     What will this do when used as a script   When imported as a module     Suppose this is in foo4 py   name      quot   main   quot   def bar        print  quot bar quot        print  quot before   name   guard quot   if   name       quot   main   quot       bar   print  quot after   name   guard quot

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You can make the file usable as a script as well as an importable module    fibo py  a module named fibo     Other modules can IMPORT this MODULE to use the function fib def fib n        write Fibonacci series up to n     a  b   0  1     while b  lt  n          print b  end              a  b   b  a b     print      This allows the file to be used as a SCRIPT if   name         main         import sys     fib int sys argv 1      Reference  https   docs python org 3 5 tutorial modules html

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When you run Python interactively the local   name   variable is assigned a value of   main    Likewise  when you execute a Python module from the command line  rather than importing it into another module  its   name   attribute is assigned a value of   main    rather than the actual name of the module  In this way  modules can look at their own   name   value to determine for themselves how they are being used  whether as support for another program or as the main application executed from the command line  Thus  the following idiom is quite common in Python modules   if   name         main           Do something appropriate here  like calling a       main   function defined elsewhere in this module      main   else        Do nothing  This module has been imported by another       module that wants to make use of the functions        classes and other useful bits it has defined

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If this  py file are imported by other  py files  the code under  the if statement  will not be executed   If this  py are run by python this py py under shell  or double clicked in Windows  the code under  the if statement  will be executed   It is usually written for testing

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This answer is for Java programmers learning Python  Every Java file typically contains one public class  You can use that class in two ways     Call the class from other files  You just have to import it in the calling program  Run the class stand alone  for testing purposes     For the latter case  the class should contain a public static void main   method  In Python this purpose is served by the globally defined label    main

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Create a file  a py   print   name      It will print out   main       name   is always equal to   main   whenever that file is run directly showing that this is the main file   Create another file  b py  in the same directory   import a    Prints a   Run it  It will print a  i e   the name of the file which is imported   So  to show two different behavior of the same file  this is a commonly used trick     Code to be run when imported into another python file  if   name         main           Code to be run only when run directly

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if   name         main     is basically the top-level script environment  and it specifies the interpreter that   I have the highest priority to be executed first        main    is the name of the scope in which top-level code executes  A module   s   name   is set equal to    main    when read from standard input  a script  or from an interactive prompt   if   name         main           Execute only if run as a script     main

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You can checkup for the special variable   name   with this simple example   create file1 py  if   name         main         print  file1 is being run directly   else      print  file1 is being imported     create file2 py  import file1 as f1  print    name   from file1      format f1   name     print    name   from file2      format   name      if   name         main         print  file2 is being run directly   else      print  file2 is being imported     Execute file2 py  output   file1 is being imported   name   from file1  file1   name   from file2    main   file2 is being run directly

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I think it s best to break the answer in depth and in simple words     name    Every module in Python has a special attribute called   name    It is a built-in variable that returns the name of the module     main    Like other programming languages  Python too has an execution entry point  i e   main     main    is the name of the scope in which top-level code executes  Basically you have two ways of using a Python module  Run it directly as a script  or import it  When a module is run as a script  its   name   is set to   main     Thus  the value of the   name   attribute is set to   main   when the module is run as the main program  Otherwise the value of   name    is set to contain the name of the module

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The simplest explanation for the   name   variable  imho  is the following   Create the following files     a py import b   and    b py print  Hello World from  s       name    if   name         main         print  Hello World again from  s       name     Running them will get you this output     python a py Hello World from b    As you can see  when a module is imported  Python sets globals      name     in this module to the module s name  Also  upon import all the code in the module is being run  As the if statement evaluates to False this part is not executed     python b py Hello World from   main    Hello World again from   main      As you can see  when a file is executed  Python sets globals      name     in this file to    main     This time  the if statement evaluates to True and is being run

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When your script is run by passing it as a command to the Python interpreter   python myscript py   all of the code that is at indentation level 0 gets executed   Functions and classes that are defined are  well  defined  but none of their code gets run   Unlike other languages  there s no main   function that gets run automatically - the main   function is implicitly all the code at the top level   In this case  the top-level code is an if block     name   is a built-in variable which evaluates to the name of the current module   However  if a module is being run directly  as in myscript py above   then   name   instead is set to the string    main      Thus  you can test whether your script is being run directly or being imported by something else by testing  if   name         main               If your script is being imported into another module  its various function and class definitions will be imported and its top-level code will be executed  but the code in the then-body of the if clause above won t get run as the condition is not met  As a basic example  consider the following two scripts     file one py def func        print  func   in one py    print  top-level in one py    if   name         main         print  one py is being run directly   else      print  one py is being imported into another module         file two py import one  print  top-level in two py   one func    if   name         main         print  two py is being run directly   else      print  two py is being imported into another module     Now  if you invoke the interpreter as  python one py   The output will be  top-level in one py one py is being run directly   If you run two py instead   python two py   You get  top-level in one py one py is being imported into another module top-level in two py func   in one py two py is being run directly   Thus  when module one gets loaded  its   name   equals  one  instead of    main

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In  simple words  The code you see under  if   name       quot   main   quot   will only get called upon when your python file is executed as  quot python example1 py quot   However  if you wish to import your python file  example1 py  as a module to work with another python file say  example2 py   the code under if   name       quot   main   quot   will not run or take any effect

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What does the if   name         main     do    To outline the basics    The global variable    name    in the module that is the entry point to your program  is    main     Otherwise  it s the name you import the module by  So  code under the if block will only run if the module is the entry point to your program  It allows the code in the module to be importable by other modules  without executing the code block beneath on import      Why do we need this   Developing and Testing Your Code  Say you re writing a Python script designed to be used as a module   def do important           This function does something very important      You could test the module by adding this call of the function to the bottom   do important     and running it  on a command prompt  with something like      python important py   The Problem  However  if you want to import the module to another script   import important   On import  the do important function would be called  so you d probably comment out your function call  do important    at the bottom      do important     I must remember to uncomment to execute this    And then you ll have to remember whether or not you ve commented out your test function call  And this extra complexity would mean you re likely to forget  making your development process more troublesome   A Better Way  The   name   variable points to the namespace wherever the Python interpreter happens to be at the moment    Inside an imported module  it s the name of that module    But inside the primary module  or an interactive Python session  i e  the interpreter s Read  Eval  Print Loop  or REPL  you are running everything from its    main      So if you check before executing   if   name         main         do important     With the above  your code will only execute when you re running it as the primary module  or intentionally call it from another script     An Even Better Way  There s a Pythonic way to improve on this  though    What if we want to run this business process from outside the module   If we put the code we want to exercise as we develop and test in a function like this and then do our check for    main    immediately after   def main           business logic for when running this module as the primary one         setup       foo   do important       bar   do even more important foo      for baz in bar          do super important baz      teardown      Here s our payoff idiom  if   name         main         main     We now have a final function for the end of our module that will run if we run the module as the primary module    It will allow the module and its functions and classes to be imported into other scripts without running the main function  and will also allow the module  and its functions and classes  to be called when running from a different    main    module  i e   import important important main     This idiom can also be found in the Python documentation in an explanation of the   main   module  That text states      This module represents the  otherwise anonymous  scope in which the   interpreter   s main program executes     commands read either from   standard input  from a script file  or from an interactive prompt  It   is this environment in which the idiomatic    conditional script    stanza   causes a script to run   if   name         main         main

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Whenever the Python interpreter reads a source file  it does two things   it sets a few special variables like   name    and then  it executes all of the code found in the file    Let s see how this works and how it relates to your question about the   name   checks we always see in Python scripts  Code Sample Let s use a slightly different code sample to explore how imports and scripts work   Suppose the following is in a file called foo py    Suppose this is foo py   print  quot before import quot   import math  print  quot before functionA quot   def functionA        print  quot Function A quot    print  quot before functionB quot   def functionB        print  quot Function B    quot  format math sqrt 100     print  quot before   name   guard quot   if   name         main         functionA       functionB   print  quot after   name   guard quot    Special Variables When the Python interpreter reads a source file  it first defines a few special variables  In this case  we care about the   name   variable  When Your Module Is the Main Program If you are running your module  the source file  as the main program  e g  python foo py  the interpreter will assign the hard-coded string  quot   main   quot  to the   name   variable  i e    It s as if the interpreter inserts this at the top   of your module when run as the main program    name      quot   main   quot    When Your Module Is Imported By Another On the other hand  suppose some other module is the main program and it imports your module  This means there s a statement like this in the main program  or in some other module the main program imports    Suppose this is in some other main program  import foo  The interpreter will search for your foo py file  along with searching for a few other variants   and prior to executing that module  it will assign the name  quot foo quot  from the import statement to the   name   variable  i e    It s as if the interpreter inserts this at the top   of your module when it s imported from another module    name      quot foo quot   Executing the Module s Code After the special variables are set up  the interpreter executes all the code in the module  one statement at a time  You may want to open another window on the side with the code sample so you can follow along with this explanation  Always  It prints the string  quot before import quot   without quotes    It loads the math module and assigns it to a variable called math  This is equivalent to replacing import math with the following  note that   import   is a low-level function in Python that takes a string and triggers the actual import       Find and load a module given its string name   quot math quot     then assign it to a local variable called math  math     import    quot math quot     It prints the string  quot before functionA quot    It executes the def block  creating a function object  then assigning that function object to a variable called functionA   It prints the string  quot before functionB quot    It executes the second def block  creating another function object  then assigning it to a variable called functionB   It prints the string  quot before   name   guard quot     Only When Your Module Is the Main Program  If your module is the main program  then it will see that   name   was indeed set to  quot   main   quot  and it calls the two functions  printing the strings  quot Function A quot  and  quot Function B 10 0 quot    Only When Your Module Is Imported by Another   instead  If your module is not the main program but was imported by another one  then   name   will be  quot foo quot   not  quot   main   quot   and it ll skip the body of the if statement   Always  It will print the string  quot after   name   guard quot  in both situations   Summary In summary  here s what d be printed in the two cases    What gets printed if foo is the main program before import before functionA before functionB before   name   guard Function A Function B 10 0 after   name   guard    What gets printed if foo is imported as a regular module before import before functionA before functionB before   name   guard after   name   guard  Why Does It Work This Way  You might naturally wonder why anybody would want this   Well  sometimes you want to write a  py file that can be both used by other programs and or modules as a module  and can also be run as the main program itself   Examples   Your module is a library  but you want to have a script mode where it runs some unit tests or a demo   Your module is only used as a main program  but it has some unit tests  and the testing framework works by importing  py files like your script and running special test functions  You don t want it to try running the script just because it s importing the module   Your module is mostly used as a main program  but it also provides a programmer-friendly API for advanced users    Beyond those examples  it s elegant that running a script in Python is just setting up a few magic variables and importing the script   quot Running quot  the script is a side effect of importing the script s module  Food for Thought  Question  Can I have multiple   name   checking blocks   Answer  it s strange to do so  but the language won t stop you   Suppose the following is in foo2 py   What happens if you say python foo2 py on the command-line  Why      Suppose this is foo2 py  import os  sys  sys path insert 0  os path dirname   file       needed for some interpreters  def functionA        print  quot a1 quot       from foo2 import functionB     print  quot a2 quot       functionB       print  quot a3 quot    def functionB        print  quot b quot    print  quot t1 quot   if   name       quot   main   quot       print  quot m1 quot       functionA       print  quot m2 quot   print  quot t2 quot            Now  figure out what will happen if you remove the   name   check in foo3 py     Suppose this is foo3 py  import os  sys  sys path insert 0  os path dirname   file       needed for some interpreters  def functionA        print  quot a1 quot       from foo3 import functionB     print  quot a2 quot       functionB       print  quot a3 quot    def functionB        print  quot b quot    print  quot t1 quot   print  quot m1 quot   functionA   print  quot m2 quot   print  quot t2 quot     What will this do when used as a script   When imported as a module     Suppose this is in foo4 py   name      quot   main   quot   def bar        print  quot bar quot        print  quot before   name   guard quot   if   name       quot   main   quot       bar   print  quot after   name   guard quot

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Consider   if   name         main         main     It checks if the   name   attribute of the Python script is    main     In other words  if the program itself is executed  the attribute will be   main    so the program will be executed  in this case the main   function    However  if your Python script is used by a module  any code outside of the if statement will be executed  so if    name          main    is used just to check if the program is used as a module or not  and therefore decides whether to run the code

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All the answers have pretty much explained the functionality  But I will provide one example of its usage which might help clearing out the concept further   Assume that you have two Python files  a py and b py  Now  a py imports b py  We run the a py file  where the  import b py  code is executed first  Before the rest of the a py code runs  the code in the file b py must run completely   In the b py code there is some code that is exclusive to that file b py and we don t want any other file  other than b py file   that has imported the b py file  to run it   So that is what this line of code checks  If it is the main file  i e   b py  running the code  which in this case it is not  a py is the main file running   then only the code gets executed

User · Answer

Put simply    name   is a variable defined for each script that defines whether the script is being run as the main module or it is being run as an imported module   So if we have two scripts    script1 py print  Script 1 s name      format   name      and   script2 py import script1 print  Script 2 s name      format   name      The output from executing script1 is  Script 1 s name    main     And the output from executing script2 is   Script1 s name is script1 Script 2 s name    main     As you can see    name   tells us which code is the  main  module  This is great  because you can just write code and not have to worry about structural issues like in C C    where  if a file does not implement a  main  function then it cannot be compiled as an executable and if it does  it cannot then be used as a library   Say you write a Python script that does something great and you implement a boatload of functions that are useful for other purposes  If I want to use them I can just import your script and use them without executing your program  given that your code only executes within the  if   name         main     context   Whereas in C C   you would have to portion out those pieces into a separate module that then includes the file  Picture the situation below     The arrows are import links  For three modules each trying to include the previous modules code there are six files  nine  counting the implementation files  and five links  This makes it difficult to include other code into a C project unless it is compiled specifically as a library  Now picture it for Python     You write a module  and if someone wants to use your code they just import it and the   name   variable can help to separate the executable portion of the program from the library part

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if   name      quot   main   quot  means that if you are running the python file normally like python foo py it will assign the special variable   name   to  quot   main   quot  but if you are importing the file like  quot import foo quot  it will assign   name   to  quot foo quot  and will not run the function

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Consider   print   name     The output for the above is   main     if   name         main       print  direct method    The above statement is true and prints  direct method   Suppose if they imported this class in another class it doesn t print  direct method  because  while importing  it will set   name   equal to  first model name

[python] What does if name == "main": do?

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[python] What does if __name__ == "__main__": do?

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[python] What does if name == "main": do?