When you can update locals()

There are two built-in functions, globals and locals.  These return dicts of the contents of the global and local scope.

Locals usually refers to the contents of a function, in which case it is a one-time copy.  Updates to the dict do not change the local scope:

>>> def local_fail():
...    a = 1
...    locals()['a'] = 2
...    print 'a is', a
... 
>>> local_fail()
a is 1

However, in the body of a class definition, locals points to the __dict__ of the class, which is mutable.

>>> class Success(object):
...    locals().update({'a': 1})
... 
>>> Success.a
1

identity theft

>>> class JSON(int):
...     from json import *
...
>>> json = JSON()
>>> json.dumps()
'0'