Why it is important to use immutable as default arguments for function

example.py

# -*- coding: utf-8 -*-
"""A short example of why you shouldn't use lists or dicts as default. In python
default parameteres are bounded when the function definition is executed as
part of module loading. So after the function is created the default arguments
always point the same object (being list or dict, list in this case.). One
should only use inmutable objects as None or strings as default arguments.
"""

def foo(bar=[]):
    """Never do this
    """
    print bar
    bar.append('fubar')

foo()
foo()  # Oh noes situation fubar


def fixdfoo(bar=None):
    if bar is None:
        bar = []
    print bar
    bar.append('fubar')

fixdfoo()
fixdfoo()