Python function with mutable default arguments

In python default function arguments are created during executing instruction def and not at each function call. If argument value is immutable object (for example string, integer, tuple) it is ok, but if value is mutable, then there can be a trap:

def foo(l=[]):
    l.append('x')
    return l

It seems, that every call to foo() will return list [‘x’]. But:

>>> foo()
['x']
>>> foo()
['x', 'x']
>>> foo()
['x', 'x', 'x']

So, if it is needed to create empty list at every call, you should do:

def bar(l=None):
    if l is None:
        l = []
    l.append('x')
    return l

However, sometimes this behaviour can be usefull. Here is a way to know function call count:

from itertools import count

def bar(call_count=count()):
    return next(call_count)

>>> bar()
0
>>> bar()
1
>>> bar()
2
>>> bar()
3