[Tutor] confusion with dictionaries

[Thomi Richards]

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Hi guys,

I was playing around with dictionaries, and came across something which I 
found rather... unexpected. here's a demonstration:

>>> def foo(l = {}):
...     l[random.choice(string.lowercase)] = random.random()
...     return l
...
>>> foo()
{'m': 0.33204172040918167}
>>> foo()
{'m': 0.33204172040918167, 'l': 0.49843519723518959}
>>> foo()
{'u': 0.92434722065201858, 'm': 0.33204172040918167, 'l': 0.49843519723518959}
>>> foo()
{'u': 0.92434722065201858, 'm': 0.33204172040918167, 'l': 0.43003419699678858}
>>> foo()
{'u': 0.92434722065201858, 'z': 0.48421207726860993, 'm': 0.33204172040918167, 
'l': 0.43003419699678858}


I thought that if the dictionary 'l' was not stored in a variable outside the 
function it would be erased whenever the function foo() was terminated.

This is very wierd... can anyone explain this? Furthermore, how can I ensure 
that a dictionary is clean when passing it as an optional argument to a 
function like this?

Thanks,

- -- 
Thomi Richards,
http://once.sourceforge.net/


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