[Python] Why is this optional method argument shared across different Python objects?...

Possible Duplicate:
“Least Astonishment” in Python: The Mutable Default Argument
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What is the explanation for this?

class Strange(object):
def mutate(self, x=[]):
x.append(1)
return x

obj = Strange()
print obj.mutate()
another_obj = Strange()
print another_obj.mutate()

>> [1]
>> [1, 1]


mutate() is called without the optional parameter. Shouldn't x then always be []? What confuses me also is that the value of x is also shared across different objects...

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