default arguments evaluated once at function definition...

default_arguments.py

>>> def a(x=False, y=[]):
...   if not x:
...     y.append (5)
...   else:
...     print y
...
>>> a()
>>> a()
>>> a()
>>> a()
>>> a()
>>> a(True)
[5, 5, 5, 5, 5]
>>> a()
>>> a(True)
[5, 5, 5, 5, 5, 5]
>>> 


>>> def b(x=False, y=None):
...   if y is None:
...     y=[]
...   if not x:
...     y.append (5)
...   else:
...     print y
... 
>>> b()
>>> b()
>>> b()
>>> b(True)
[]