-- Python 2.0 Win32 --

It seems to be that _default argument values_ can be 
changed permanently if they are dictionaries.

Code sample:

def f(a = 1):
  print a
  a = 2

>>>f()
1
>>>f()
1

--> ok

def g(mydic = {}):
  print mydic
  mydic["test"] = 1

>>>g()
{}
>>>g()
{'test': 1}

--> bad (default argument value for 'mydic' changed)

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user_id=31435

Not a bug.  Please see the Python FAQ entry 6.25, "Why are 
default values sometimes shared between objects?"

http://www.python.org/cgi-bin/faqw.py?
req=show&file=faq06.025.htp