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?

Answer 1

You can say that pass means NOP (No Operation) operation. You will get a clear picture after this example :-

C Program

#include<stdio.h>

void main()
{
    int age = 12;

    if( age < 18 )
    {
         printf("You are not adult, so you can't do that task ");
    }
    else if( age >= 18 && age < 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 < 18:

    print "You are not adult, so you can't do that task"

elif age >= 18 and age < 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 < 18:

    print "You are not adult, so you can't do that task"

elif age >= 18 and age < 60:

    pass

else:

    print "You are too old to do anything , sorry "

Now I think its clear to you.

Answer 2

if you don't know what you are going to put in a certain code block

try:
   int(someuserinput)
except ValueError:
   pass

I like to use it when stubbing out tests too. I often times am aware of what i would liek to test but dont' quite know how to do it. Testing example looks like what sebastian_oe suggested

class TestFunctions(unittest.TestCase):

   def test_some_feature(self):
      pass

   def test_some_other_feature(self):
      pass

Answer 3

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:

>>> while True: ... pass # Busy-wait for keyboard interrupt (Ctrl+C) ...

This is commonly used for creating minimal classes:

>>> 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:

>>> def initlog(*args): ... pass # Remember to implement this! ...

Answer 4

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 < 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

Answer 5

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.

Answer 6

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.