Python Pocket Reference, Second Edition

Name

The def Statement

Synopsis

def name([arg, arg=value,... *arg, **arg]): 
    suite

Makes new functions. Creates a function object and assigns it to variable name. Each call to a function object generates a new, local scope, where assigned names are local to the function call by default (unless declared global). See also Section 1.9 later in the book. Arguments are passed by assignment; in a def header, they may be defined by any of the formats in Table 1-14.

Table 1-14. Argument definition formats

Argument format	Interpretation
arg	Simple name, matched by name or position
arg=value	Default value if arg not passed
*arg	Collects extra positional args
**arg	Collects extra keyword args passed by name

Mutable default argument values are evaluated once at def statement time, not on each call, so may retain state between calls (but classes are better state-retention tools):

>>> def grow(a, b=[]):
...     b.append(a)
...     print b
...
>>> grow(1); grow(2)
[1]
[1, 2]

lambda expressions

Functions may also be created with the lambda expression form: lambda arg, arg,...: expression. In lambda, arg is as in def, expression is the implied return value, and the generated function is simply returned as the lambda result to be called later, instead of being assigned to a variable. Because lambda is an expression, not a statement, it may be used in places that a def cannot be (e.g., within an argument list of a call).