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pass

Simple things first: the pass statement is a no-operation placeholder that is used when the syntax requires a statement, but you have nothing useful to say. It is often used to code an empty body for a compound statement. For instance, if you want to code an infinite loop that does nothing each time through, do it with a pass:

while True: pass                   # Type Ctrl-C to stop me!

Because the body is just an empty statement, Python gets stuck in this loop. pass is roughly to statements as None is to objects—an explicit nothing. Notice that here the while loop’s body is on the same line as the header, after the colon; as with if statements, this only works if the body isn’t a compound statement.

This example does nothing forever. It probably isn’t the most useful Python program ever written (unless you want to warm up your laptop computer on a cold winter’s day!); frankly, though, I couldn’t think of a better pass example at this point in the book.

We’ll see other places where pass makes more sense later—for instance, to ignore exceptions caught by try statements, and to define empty class objects with attributes that behave like “structs” and “records” in other languages. A pass is also sometime coded to mean “to be filled in later,” to stub out the bodies of functions temporarily:

def func1():
    pass                           # Add real code here later

def func2():
    pass

We can’t leave the body empty without getting a syntax error, so we say pass instead.

Note

Version skew note: Python 3.0 (but not 2.6) allows ellipses coded as ... (literally, three consecutive dots) to appear any place an expression can. Because ellipses do nothing by themselves, this can serve as an alternative to the pass statement, especially for code to be filled in later—a sort of Python “TBD”:

def func1():
    ...                   # Alternative to pass

def func2():
    ...

func1()                   # Does nothing if called

Ellipses can also appear on the same line as a statement header and may be used to initialize variable names if no specific type is required:

def func1(): ...          # Works on same line too
def func2(): ...

>>> X = ...               # Alternative to None
>>> X
Ellipsis

This notation is new in Python 3.0 (and goes well beyond the original intent of ... in slicing extensions), so time will tell if it becomes widespread enough to challenge pass and None in these roles.