Could anyone explain __class__?

[Alex]

> I'm having trouble understanding the function of the built-in class
> method __class__.  The documentation is terse, to say the least.
> 
> From code examples (UserList.py, forex.), it appears that
> self.__class__() returns a new instance of the class of the instance
> in which it is called.

The idea with UserList is that it's to be inherited from.  See
http://www.python.org/doc/current/tut/node11.html#SECTION0011500000000000000000
for a brief explanation.  UserList instantiates return values using the
class referred to by the self.__class__ attribute because in most cases
it's expected that adding two instances of a class will return another
instance of that class, and self.__class__ is just the class of the
instance calling the current method.  Here's a simple example:

from UserList import UserList

class MyList(UserList):

    '''A subclass of UserList with a frivolous attribute.'''

    def __init__(self, initlist=None):
        UserList.__init__(self, initlist)
        self.my_frivolous_attribute = None

l = MyList([1])
m = MyList([2])

new_mylist = l+m
assert (new_mylist.__class__ == MyList) and \
       hasattr(new_mylist, 'my_frivolous_attribute')

HTH
Alex.