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“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
290
6.7 CONSTRUCTORS WITH DEFAULT ARGUMENTSIt is possible to have a constructor with default arguments.. It means that if the constructor is defined with n parameters, we can invoke it with less than n arguments specified in the call. Anyname ( para1,para2,para3,para4 =val4, … paran = valn);
In the python method and constructor, overloading is not possible.
In short, Python's instantiation process starts with a call to the class constructor, which triggers the instance creator, . __new__() , to create a new empty object. The process continues with the instance initializer, . __init__() , which takes the constructor's arguments to initialize the newly created object.
The __init__ method is the Python equivalent of the C++ constructor in an object-oriented approach. The __init__ function is called every time an object is created from a class. The __init__ method lets the class initialize the object's attributes and serves no other purpose. It is only used within classes.
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)
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