Python class constructor with default arguments

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

---

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)

---

The default argument list is the same object for all instances, hence assigning it to a member just assigns a reference to the same object.

here's an example:

>>> class foo():
...   def __init__(self, x = []):
...       print id(x)
... 
>>> x = foo()
140284337344168
>>> y = foo()
140284337344168
>>> z = foo()
140284337344168

you can see that x is the same object in all instances.

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