Define __prepare__() in with_metaclass() (#178)

Define `__prepare__()` in `with_metaclass()`'s temporary
metaclass, and make sure that it passes the correct bases to the
real metaclass's `__prepare__()`.

The temporary metaclass previously didn't extend the
`__prepare__()` method, which meant that if the real metaclass
had a `__prepare__()`, it wouldn't get called correctly. This
could lead to bugs in Python 3 code.

The temporary metaclass's `__prepare__()` gets called with
```bases=(temporary_class,)```.  Since there was no proxy in the
middle, that was getting passed directly to the real metaclass's
`__prepare__()`. But then, if the real class's `__prepare__()`
method depended on the bases, the logic would be incorrect.

This was a problem in projects that use `enum` / `enum34` and
try to use `with_metaclass(EnumMeta)`. `enum34.EnumMeta` doesn't
define `__prepare__()`, since it is a Python 2 backport. Python
3's `enum.EnumMeta` does define `__prepare__()`, but originally
didn't depend at all on the bases. But starting in Python 3.6,
`enum.EnumMeta.__prepare__()` will raise `TypeError` if the
bases aren't valid for an enum subclass. Thus, a codebase that
was successfully using `enum` / `enum34` and
`with_metaclass(EnumMeta)` could break on Python 3.6.

1 files changed, 1 insertions, 0 deletions

diff --git a/CONTRIBUTORS b/CONTRIBUTORS
index 530d0e6..5979045 100644
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -27,5 +27,6 @@ Peter Ruibal
 Miroslav Shubernetskiy
 Anthony Sottile
 Lucas Wiman
+Jordan Moldow
 
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