What is the purpose of `__metaclass__ = type`?

Question

Python (2 only?) looks at the value of variable __metaclass__ to determine how to create a type object from a class definition. It is possible to define __metaclass__ at the module or package level, in which case it applies to all subsequent class definitions in that module.

However, I encountered the following in the flufl.enum package's __init__.py:

__metaclass__ = type

Since the default metaclass if __metaclass__ is not defined is type, wouldn't this have no effect? (This assignment would revert to the default if __metaclass__ were assigned to at a higher scope, but I see no such assignment.) What is its purpose?

Answer 1

In Python 2, a declaration __metaclass__ = type makes declarations that would otherwise create old-style classes create new-style classes instead. Only old-style classes use a module level __metaclass__ declaration. New-style classes inherit their metaclass from their base class (e.g. object), unless __metaclass__ is provided as a class variable.

The declaration is not actually used in the code you linked to above (there are no class declarations in the __init__.py file), but it could be. I suspect it was included as part of some boilerplate that makes Python 2 code work more like Python 3 (where all classes are always new-style).

Answer 2

Yes, it has no effect. It's probably just a misunderstanding from flufl.enum's author, or a leftover from previous code.

A "superpackage" __metaclass__ declaration would have no effect because there is no such a thing as Python superpackages.