`len()`, old vs new

example.py

>>> class A:
...     pass
... 
>>> class B(object):
...     pass
... 
>>> class C:
...     def __len__(self):
...         return 0
...     
... 
>>> class D(object):
...     def __len__(self):
...         return 0
...     
... 
>>> len(A())
Traceback (most recent call last):
  File "<input>", line 1, in <module>
AttributeError: A instance has no attribute '__len__'
>>> len(B())
Traceback (most recent call last):
  File "<input>", line 1, in <module>
TypeError: object of type 'B' has no len()
>>> len(C())
0
>>> len(D())
0
>>> try:
...     len(A())
... except (TypeError, AttributeError):
...     print 'A has no length'
...     
... 
A has no length
>>> try:
...     len(B())
... except (TypeError, AttributeError):
...     print 'B has no length'
...     
... 
B has no length