How to use the pass statement

Question

I am in the process of learning Python and I have reached the section about the pass statement  The guide I m using defines it as being a Null statement that is commonly used as a placeholder    I still don t fully understand what that means though  Can someone show me a simple basic situation where the pass statement would be used and why it is needed

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pass in Python basically does nothing  but unlike a comment it is not ignored by interpreter  So you can take advantage of it in a lot of places by making it a place holder  1  Can be used in class class TestClass       pass  2  Can be use in loop and conditional statements  if  something    true      used in conditional statement     pass     while  some condition is true      user is not sure about the body of the loop     pass  3  Can be used in function   def testFunction args     programmer wants to implement the body of the function later     pass  pass is mostly used when programmer does not want to give implementation at the moment but still wants to create a certain class function conditional statement which can be used later on  Since the Python interpreter does not allow for blank or unimplemented class function conditional statement it gives an error   IndentationError  expected an indented block  pass can be used in such scenarios

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The best and most accurate way to think of pass is as a way to explicitly tell the interpreter to do nothing  In the same way the following code   def foo x y       return x y   means  if I call the function foo x  y   sum the two numbers the labels x and y represent and hand back the result    def bar        pass   means  If I call the function bar    do absolutely nothing    The other answers are quite correct  but it s also useful for a few things that don t involve place-holding   For example  in a bit of code I worked on just recently  it was necessary to divide two variables  and it was possible for the divisor to be zero   c   a   b   will  obviously  produce a ZeroDivisionError if b is zero  In this particular situation  leaving c as zero was the desired behavior in the case that b was zero  so I used the following code   try      c   a   b except ZeroDivisionError      pass   Another  less standard usage is as a handy place to put a breakpoint for your debugger  For example  I wanted a bit of code to break into the debugger on the 20th iteration of a for    in statement  So   for t in range 25       do a thing t      if t    20          pass   with the breakpoint on pass

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Here s an example where I was extracting particular data from a list where I had multiple data types  that s what I d call it in R-- sorry if it s the wrong nomenclature  and I wanted to extract only integers numeric and NOT character data   The data looked like    gt  gt  gt  a     1    env    2    gag    1 234    nef    gt  gt  gt  data       gt  gt  gt  type a   lt class  list  gt   gt  gt  gt  type a 1    lt class  str  gt   gt  gt  gt  type a 0    lt class  str  gt    I wanted to remove all alphabetical characters  so I had the machine do it by subsetting the data  and  passing  over the alphabetical data   a     1    env    2    gag    1 234    nef   data      for i in range 0  len a        if a i  isalpha            pass     else          data append a i   print data    1    2    1 234

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The pass statement in Python is used when a statement is required syntactically but you do not want any command or code to execute   The pass statement is a null operation  nothing happens when it executes  The pass is also useful in places where your code will eventually go  but has not been written yet  e g   in stubs for example     Example      usr bin python  for letter in  Python       if letter     h         pass       print  This is pass block     print  Current Letter     letter  print  Good bye     This will produce following result   Current Letter   P Current Letter   y Current Letter   t This is pass block Current Letter   h Current Letter   o Current Letter   n Good bye    The preceding code does not execute any statement or code if the value of letter is  h   The pass statement is helpful when you have created a code block but it is no longer required   You can then remove the statements inside the block but let the block remain with a pass statement so that it doesn t interfere with other parts of the code

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as the book said  I only ever use it as a temporary placeholder  ie     code that does something to to a variable  var if var    2000      pass else      var    1   and then later fill in the scenario where var    2000

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Python has the syntactical requirement that code blocks  after if  except  def  class etc   cannot be empty  Empty code blocks are however useful in a variety of different contexts  such as in examples below  which are the most frequent use cases I have seen   Therefore  if nothing is supposed to happen in a code block  a pass is needed for such a block to not produce an IndentationError  Alternatively  any statement  including just a term to be evaluated  like the Ellipsis literal     or a string  most often a docstring  can be used  but the pass makes clear that indeed nothing is supposed to happen  and does not need to be actually evaluated and  at least temporarily  stored in memory    Ignoring  all or  a certain type of Exception  example from xml    try      self version    Expat  d  d  d    expat version info except AttributeError      pass   unknown   Note  Ignoring all types of raises  as in the following example from pandas  is generally considered bad practice  because it also catches exceptions that should probably be passed on to the caller  e g  KeyboardInterrupt or SystemExit  or even HardwareIsOnFireError     How do you  know you aren t running on a custom box with specific errors defined  which some calling application would want to know about      try      os unlink filename larry  except      pass   Instead using at least except Error  or in this case preferably except OSError  is considered much better practice  A quick analysis of all python modules I have installed gave me that more than 10  of all except      pass statements catch all exceptions  so it s still a frequent pattern in python programming  Deriving an exception class that does not add new behaviour  e g  in scipy    class CompileError Exception       pass   Similarly  classes intended as abstract base class often have an explicit empty   init   or other methods that subclasses are supposed to derive   e g  pebl   class  BaseSubmittingController  BaseController       def submit self  tasks   pass     def retrieve self  deferred results   pass  Testing that code runs properly for a few test values  without caring about the results  from mpmath    for x  error in MDNewton mp  f   1 -2   verbose 0                           norm lambda x  norm x  inf        pass  In class or function definitions  often a docstring is already in place as the obligatory statement to be executed as the only thing in the block  In such cases  the block may contain pass in addition to the docstring in order to say    This is indeed intended to do nothing      for example in pebl   class ParsingError Exception           Error encountered while parsing an ill-formed datafile         pass  In some cases  pass is used as a placeholder to say    This method class if-block     has not been implemented yet  but this will be the place to do it     although I personally prefer the Ellipsis literal     in order to strictly differentiate between this and the intentional    no-op    in the previous example   Note that the Ellipsis literal is a valid expression only in Python 3  For example  if I write a model in broad strokes  I might write  def update agent agent              where others might have  def update agent agent       pass   before  def time step agents       for agent in agents          update agent agent    as a reminder to fill in the update agent function at a later point  but run some tests already to see if the rest of the code behaves as intended   A third option for this case is raise NotImplementedError  This is useful in particular for two cases  Either    This abstract method should be implemented by every subclass  there is no generic way to define it in this base class     or    This function  with this name  is not yet implemented in this release  but this is what its signature will look like

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Suppose you are designing a new class with some methods that you don t want to implement  yet   class MyClass object       def meth a self           pass      def meth b self           print  I m meth b    If you were to leave out the pass  the code wouldn t run   You would then get an   IndentationError  expected an indented block   To summarize  the pass statement does nothing particular  but it can act as a placeholder  as demonstrated here

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You can say that pass means NOP  No Operation  operation  You will get a clear picture after this example  -  C Program   include lt stdio h gt   void main         int age   12       if  age  lt  18                  printf  You are not adult  so you can t do that task               else if  age  gt   18  amp  amp  age  lt  60                   I will add more code later inside it            else                printf  You are too old to do anything   sorry               Now how you will write that in Python  -  age   12  if age  lt  18       print  You are not adult  so you can t do that task   elif age  gt   18 and age  lt  60   else       print  You are too old to do anything   sorry     But your code will give error because it required an indented block after elif   Here is the role of pass keyword   age   12  if age  lt  18       print  You are not adult  so you can t do that task   elif age  gt   18 and age  lt  60       pass  else       print  You are too old to do anything   sorry     Now I think its clear to you

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Besides its use as a placeholder for unimplemented functions  pass can be useful in filling out an if-else statement   Explicit is better than implicit     def some silly transform n         Even numbers should be divided by 2     if n   2    0          n    2         flag   True       Negative odd numbers should return their absolute value     elif n  lt  0          n   -n         flag   True       Otherwise  number should remain unchanged     else          pass   Of course  in this case  one would probably use return instead of assignment  but in cases where mutation is desired  this works best   The use of pass here is especially useful to warn future maintainers  including yourself   not to put redundant steps outside of the conditional statements  In the example above  flag is set in the two specifically mentioned cases  but not in the else-case  Without using pass  a future programmer might move flag   True to outside the condition   thus setting flag in all cases     Another case is with the boilerplate function often seen at the bottom of a file   if   name         main         pass   In some files  it might be nice to leave that there with pass to allow for easier editing later  and to make explicit that nothing is expected to happen when the file is run on its own     Finally  as mentioned in other answers  it can be useful to do nothing when an exception is caught   try      n i    0 except IndexError      pass

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The pass statement does nothing  It can be used when a statement is required syntactically but the program requires no action

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pass is used to avoid indentation error in python    If we take languages like c c   java they have braces like   if i  0       else     some code    But in python it used indentation instead of braces so to avoid such errors we use pass  Remembered as you were playing a quiz and    if dont know the answer        pass   Example program     for letter in  geeksforgeeks           pass   print  Last Letter     letter

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Honestly  I think the official Python docs describe it quite well and provide some examples      The pass statement does nothing  It can be used when a statement is required syntactically but the program requires no action  For example        gt  gt  gt  while True            pass    Busy-wait for keyboard interrupt  Ctrl C             This is commonly used for creating minimal classes        gt  gt  gt  class MyEmptyClass            pass            Another place pass can be used is as a place-holder for a function or conditional body when you are working on new code  allowing you to keep thinking at a more abstract level  The pass is silently ignored        gt  gt  gt  def initlog  args             pass     Remember to implement this

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A common use case where it can be used  as is  is to override a class just to create a type  which is otherwise the same as the superclass   e g   class Error Exception       pass   So you can raise and catch Error exceptions  What matters here is the type of exception  rather than the content

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Pass refers to ignore    as simple as it is     if the given condition is true and the next statement is pass it ignores that value or iteration and proceed to the next line       Example   For i in range  1 100       If i 2  0                    Pass      Else                    Print i    Output   Prints all the odd numbers from 1-100  This is because modulus of even number is equal to zero  hence it ignores the number and proceeds to next number  since odd numbers modulus is not equal to zero the Else part of the loop is executed and its printed

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If you want to import a module  if it exists  and ignore importing it  if that module does not exists  you can use the below code  try      import a module except ImportError      pass  rest of your code  If you avoid writing the pass statement and continue writing the rest of your code  a IndentationError would be raised  since the lines after opening the except block are not indented

[python] How to use the pass statement?

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