[issue26458] Is the default value assignment of a function parameter evaluated multiple times if it is Parameter=None

[Albert Freeman]

New submission from Albert Freeman:

def f(a, L=[]):
    L.append(a)
    return L

Seems to behave differently to
def f(a, L=None):
    L = []
    L.append(a)
    return L
Which behaves the same as (as far as I noticed) to the below code in the documentation (In the tutorial under 4. More Control Flow Tools)
def f(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L

I am using CPython 3.5.1, what is the point of "if L is None:" in the lowermost above example? And why is None treated differently to []?

----------
assignee: docs at python
components: Documentation
messages: 261000
nosy: docs at python, tocretpa
priority: normal
severity: normal
status: open
title: Is the default value assignment of a function parameter evaluated multiple times if it is Parameter=None
versions: Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue26458>
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