[Tutor] confusion with dictionaries

[Alexandre]

>>> def foo(l = {}):
...     l[random.choice(string.lowercase)] = random.random()
...     return l
...
>>> foo()
{'m': 0.33204172040918167}
>>> foo()
{'m': 0.33204172040918167, 'l': 0.49843519723518959}
>>> 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?

Hi Thomi, below a little extract from Python Tutorial :
************************************************************
Important warning: The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes. For example, the following function accumulates the arguments passed to it on subsequent calls: 


def f(a, L=[]):
    L.append(a)
    return L

print f(1)
print f(2)
print f(3)
This will print 


[1]
[1, 2]
[1, 2, 3]
If you don't want the default to be shared between subsequent calls, you can write the function like this instead: 


def f(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L
************************************************************Hope that helps.Alexandre
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mail.python.org/pipermail/tutor/attachments/20031220/f9e4cdda/attachment.html