Question

What is the difference between type(obj) and obj.__class__? Is there ever a possibility of type(obj) is not obj.__class__?

I want to write a function that works generically on the supplied objects, using a default value of 1 in the same type as another parameter. Which variation, #1 or #2 below, is going to do the right thing?

def f(a, b=None):
  if b is None:
    b = type(a)(1) # #1
    b = a.__class__(1) # #2

Answer 1

type(obj) and type.__class__ do not behave the same for old style classes in Python 2:

>>> class a(object):
...     pass
... 
>>> class b(a):
...     pass
... 
>>> class c:
...     pass
... 
>>> ai = a()
>>> bi = b()
>>> ci = c()
>>> type(ai) is ai.__class__
True
>>> type(bi) is bi.__class__
True
>>> type(ci) is ci.__class__
False
>>> type(ci)
<type 'instance'>
>>> ci.__class__
<class __main__.c at 0x7f437cee8600>

This is explained in the documentation:

For an old-style class, the statement x.__class__ provides the class of x, but type(x) is always <type 'instance'>. This reflects the fact that all old-style instances, independent of their class, are implemented with a single built-in type, called instance.