[Tutor] Re: confusion with dictionaries

[Lee Harr]

>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}


>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.
>


That is pretty much true. The trick is that the default values for
parameters are evaluated only once -- when the function is created.
And since the function itself still exists, the dictionary it uses for that
default value has to be kept alive too.

If you want a new dict with each call, try something like this:

def foo(l=None):
    if l is None:
        l = {}


I believe that is the standard python idiom. It's the one I use
regularly, anyhow.

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