pass statements

#python #beginners

In some programming languages there are is a statement that does nothing. In assembly there's nop (no operation). How about Python?

In Python you can use the pass statement. The pass statement does nothing.

Why pass?

Python has the syntactical requirement that code blocks (for, if, except, def, class etc.) are not empty.

Empty code blocks are useful in different contexts, such as when you don't have time to implement right now.

def complexFunction():
    pass

Other purposes are deriving an exception class that does not add new behaviour

class CustomException(Exception):
    pass

if you want to ignore a type of Exception

try:
    self.version = "Expat %d.%d.%d" % expat.version_info
except:
    pass # unknown

Below are some more pass examples

If nothing is supposed to happen in a code block, a pass is needed for such a block to not produce an IndentationError

You can use this for all kinds of purposes. Inside a while loop:

>>> while True:
...     pass

This will wait for a keyboard interrupt (Ctrl+C)

In a class that you need to implement, but will implement later:

>>> class Example:
...     pass
...

But beware, if a class is not implemented you can still create objects:

>>> obj = Example()
>>>

In a function that you implement later:

>>> def hello():
...     pass   # Implement this later

Related links: