Editing Default argument (section)

==Extent==
Generally a default argument will behave identically to an argument passed by parameter or a local variable declared at the start of the function, and have the same [[scope (computer science)|scope]] and extent (lifetime) as a parameter or other local variable, namely an [[automatic variable]] which is deallocated on function termination.

In other cases a default argument may instead be statically allocated. If the variable is mutable, it will then retain its value across function calls, as with a [[static variable]].

This behavior is found in Python for mutable types, such as lists. As with evaluation, in order to ensure the same extent as a local variable, one can use a sentinel value:
<syntaxhighlight lang="python">
def eager(a=[]):
    return a

x = eager()
x += [1]
assert eager() == [1]

def lazy(a=None):
    if a is None:
        a = []
    return a

x = lazy()
x += [1]
assert lazy() == []
</syntaxhighlight>