Question

Possible Duplicate:
“Least Astonishment” in Python: The Mutable Default Argument

Can anyone explain the following strange behaviour?

I have the following class:

class Zoo:
    def __init__(self,alist=[]):
        self.animals = alist

    def __len__(self):
        return len(self.animals)

    def add(self,a):
        self.animals.append(a)

and when I do the following,

In [38]: z=Zoo()
In [39]: z.add(2)
In [40]: z.add(23)
In [41]: len(z)
Out[41]: 2

In [42]: z2=Zoo()

In [43]: len(z2)
Out[43]: 2

Why is z2.animals not an empty list?

Thanks, Matthias

Answer 1

You are mutating the default argument in your constructor (you are just copying a reference to the same list into each of your instances). You can fix this as follows:

class Zoo:
    def __init__(self,alist=None):
        self.animals = alist or []

    def __len__(self):
        return len(self.animals)

    def add(self,a):
        self.animals.append(a)