My question:
It seems that __getattr__ is not called for indexing operations, ie I can't use __getattr__ on a class A to provide A[...]. Is there a reason for this? Or a way to get around it so that __getattr__ can provide that functionality without having to explicitly define __getitem__, __setitem__, etc on A?
Minimal Example:
Let's say I define two nearly identical classes, Explicit and Implicit. Each creates a little list self._arr on initiation, and each defines a __getattr__ that just passes all attribute requests to self._arr. The only difference is that Explicit also defines __getitem__ (by just passing it on to self._arr).
# Passes all attribute requests on to a list it contains
class Explicit():
def __init__(self):
self._arr=[1,2,3,4]
def __getattr__(self,attr):
print('called __getattr_')
return getattr(self._arr,attr)
def __getitem__(self,item):
return self._arr[item]
# Same as above but __getitem__ not defined
class Implicit():
def __init__(self):
self._arr=[1,2,3,4]
def __getattr__(self,attr):
print('called __getattr_')
return getattr(self._arr,attr)
This works as expected:
>>> e=Explicit()
>>> print(e.copy())
called __getattr_
[1, 2, 3, 4]
>>> print(hasattr(e,'__getitem__'))
True
>>> print(e[0])
1
But this doesn't:
>>> i=Implicit()
>>> print(i.copy())
called __getattr_
[1, 2, 3, 4]
>>> print(hasattr(i,'__getitem__'))
called __getattr_
True
>>> print(i.__getitem__(0))
called __getattr_
1
>>> print(i[0])
TypeError: 'Implicit' object does not support indexing
