Difference between type(obj) and obj.__class__

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

Old-style classes are the problem, sigh:

>>> class old: pass
... 
>>> x=old()
>>> type(x)
<type 'instance'>
>>> x.__class__
<class __main__.old at 0x6a150>
>>>

Not a problem in Python 3 since all classes are new-style now;-).

In Python 2, a class is new-style only if it inherits from another new-style class (including object and the various built-in types such as dict, list, set, ...) or implicitly or explicitly sets __metaclass__ to type.

Answer 2

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

>>> 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