I wrote a very simple program to subclass a dictionary. I wanted to try the __missing__
method in python.
After some research i found out that in Python 2 it's available in defaultdict
. ( In python 3 we use collections.UserDict though..)
The __getitem__
is the on responsible for calling the __missing__
method if the key isn't found.
When i implement __getitem__
in the following program i get a key error, but when i implement without it, i get the desired value.
import collections
class DictSubclass(collections.defaultdict):
def __init__(self,dic):
if dic is None:
self.data = None
else:
self.data = dic
def __setitem__(self,key,value):
self.data[key] = value
########################
def __getitem__(self,key):
return self.data[key]
########################
def __missing__(self,key):
self.data[key] = None
dic = {'a':4,'b':10}
d1 = DictSubclass(dic)
d2 = DictSubclass(None)
print d1[2]
I thought i needed to implement __getitem__
since it's responsible for calling __missing__
. I understand that the class definition of defaultdict has a __getitem__
method. But even so, say i wanted to write my own __getitem__
, how would i do it?
The
dict
type will always try to call__missing__
. All thatdefaultdict
does is provide an implementation; if you are providing your own__missing__
method you don't have to subclassdefaultdict
at all.See the
dict
documentation:However, you need to leave the default
__getitem__
method in place, or at least call it. If you overridedict.__getitem__
with your own version and not call the base implementation,__missing__
is never called.You could call
__missing__
from your own implementation:or you could call the original implementation:
In Python 2, you can just subclass
UserDict.UserDict
: