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

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))

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.

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:

1. import a action actually runs all that can be ran in a.py, meaning each line in a.py

2. Because of point 1, you may not want everything to be run in a.py when importing it

3. To solve the problem in point 2, python allows you to put a condition check

4. __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 "a"
• when a.py is run directly using "python a.py", the value of __name__ is set to a string __main__

5. 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__ == "__main__": // do A

• then python a.py will run the part // do A
• and import a will skip the part // do A

6. You can even put if __name__ == "a" 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.

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__".

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.

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__".

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()

Let's look at the answer in a more abstract way:

Suppose we have this code in x.py:

...
<Block A>
if __name__ == '__main__':
    <Block B>
...

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).

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.)

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

This answer is for Java programmers learning Python. Every Java file typically contains one public class. You can use that class in two ways:

1. Call the class from other files. You just have to import it in the calling program.

2. 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__'.

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.

In simple words:

The code you see under if __name__ == "__main__": will only get called upon when your python file is executed as "python example1.py".

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__ == "__main__": will not run or take any effect.

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 

>> 'This is going to be printed out, no matter what'
>> '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

>>> 'This is going to be printed out, no matter what'
>>> '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.

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

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()

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

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 & 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.

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.

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.

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.

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.

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).

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

if __name__ = "__main__" means that if you are running the python file normally like python foo.py it will assign the special variable __name__ to "__main__" but if you are importing the file like "import foo" it will assign __name__ to "foo" and will not run the function.

Let's look at the answer in a more abstract way:

Suppose we have this code in x.py:

...
<Block A>
if __name__ == '__main__':
    <Block B>
...

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).

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__

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()

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.

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.

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 & 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.

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.

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__".

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.

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 

>> 'This is going to be printed out, no matter what'
>> '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

>>> 'This is going to be printed out, no matter what'
>>> '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.

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.

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()

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.

In simple words:

The code you see under if __name__ == "__main__": will only get called upon when your python file is executed as "python example1.py".

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__ == "__main__": will not run or take any effect.

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.

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.

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.

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 < 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

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.

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".

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.

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.

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.

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.

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).

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

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 <class 'str'>), 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:

>>> print(__name__)
__main__
>>>

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:

>>> __name__ = 'Horrify' # Change default from __main__
>>> if __name__ == 'Horrify': print(__name__)
...
>>> else: print('Not Horrify')
...
Horrify
>>>

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.

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__

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.

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 <class 'str'>), 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:

>>> print(__name__)
__main__
>>>

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:

>>> __name__ = 'Horrify' # Change default from __main__
>>> if __name__ == 'Horrify': print(__name__)
...
>>> else: print('Not Horrify')
...
Horrify
>>>

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.

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.

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

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

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

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.

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.

if __name__ = "__main__" means that if you are running the python file normally like python foo.py it will assign the special variable __name__ to "__main__" but if you are importing the file like "import foo" it will assign __name__ to "foo" and will not run the function.

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.

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.

This answer is for Java programmers learning Python. Every Java file typically contains one public class. You can use that class in two ways:

1. Call the class from other files. You just have to import it in the calling program.

2. 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__'.

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 < 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

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".

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__".

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.

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:

1. import a action actually runs all that can be ran in a.py, meaning each line in a.py

2. Because of point 1, you may not want everything to be run in a.py when importing it

3. To solve the problem in point 2, python allows you to put a condition check

4. __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 "a"
• when a.py is run directly using "python a.py", the value of __name__ is set to a string __main__

5. 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__ == "__main__": // do A

• then python a.py will run the part // do A
• and import a will skip the part // do A

6. You can even put if __name__ == "a" 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.

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.

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()

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()

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.)

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.