What turns following to __builtin__?

Question

I have following snippet and output

with metaclass:

def some(*args):
    return type(args)

__metaclass__ = some

class Foo:
   a = 'khkjh'


print Foo.__module__

Output: __builtin__

without metaclass:

class Foo:
   a = 'khkjh'

print Foo.__module__

Output: __main__

So,

What is __builtin__ ?

why or how metaclass is affecting it?

Answer 1

__builtin__ is the module that provides all built-in functions, exceptions, etc.

You're getting this returned from __module__ because the metaclass you're providing is essentially turning Foo into the tuple type:

>>> def some (*args):
...  return type(args)  # This returns <type 'tuple'>
...
>>> class Hmm(object):
...  __metaclass__ = some
... 
>>> class Foo(object):
...  pass
... 
>>> print Hmm
<type 'tuple'>
>>> print Foo
<class '__main__.Foo'>
>>> print tuple
<type 'tuple'>
>>> print tuple.__module__
__builtin__

As you can see Hmm is now the type tuple . The tuple type is provided by the __builtin__ module, hence the output you're seeing.