bpo-33976: support nested classes in Enum

[edwardcwang]

Methods defined in Enums behave 'normally' but classes defined in Enums get mistaken for regular values and can't be used as classes out of the box. In the example code below, the nested class Outer.Inner gets mistakenly identified as an Enum value. This PR changes it so that nested classes are treated in the same way as methods, fixing this bug.

Sample code:

from enum import Enum

class Outer(Enum):
    a = 1
    b = 2
    class Inner(Enum):
        foo = 10
        bar = 11

Demonstration of bug:

>>> type(Outer)
<class 'enum.EnumMeta'>
>>> type(Outer.Inner)
<enum 'Outer'>
>>> print(Outer.Inner.foo)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Outer' object has no attribute 'foo'
>>> 

https://bugs.python.org/issue33976

[the-knights-who-say-ni]

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[edwardcwang]

CLA is signed and pending.

[eric-wieser]

I don't think this is backwards-compatible- users might create an enum to refer to types deliberately:

class MyTypes(Enum):
	i = int
	f = float
	s = str

>>> MyTypes.i
<MyTypes.i: <class 'int'>>
>>> MyTypes.i.value
<class 'int'>

This is perhaps a little strange, but I don't think your use case is any less strange, and status quo wins.

[eric-wieser]

Perhaps the check should be for classes actually declared within the enum, using __qual_name__

[edwardcwang]

@eric-wieser Thanks for the catch. Added a unit test for your example and updated the PR to fix it.

[eric-wieser]

This catches

class MyEnumInner(object):
    pass

class MyEnum(Enum):
    oops = MyEnumInner

Should be .startswith(enum_class_qualname + '.')

[edwardcwang]

Thanks again - just fixed.

[edwardcwang]

@ethanfurman Were you able to find a moment to look further into this PR? Thanks again in advance.

[ethanfurman]

I have been thinking about it. At this point I'm still not sure how I want to resolve the issue. I'm thinking about having embedded/referenced classes be part of an Enum value, but also be directly callable.

For example:

MyTypes.i()

would result in 0.

For those times when one does not want a class as a member, it's easy enough to write an @skip decorater -- in fact, one is included in aenum.

[ethanfurman]

Edward, thanks for your work. I decided to go a different route where the work is done in _EnumDict instead (which you can find in my aenum project).

I'm closing this PR, but if you are up for a challenge I need a new PR that:

• adds an @member decorator to force an object to become a member
• adds an @nonmember decorator to protect an object from becoming a member
• adds the _is_class() utility for deciding if an object is a class
• updates EnumMeta to forward the class name to _EnumDict, and _EnumDict to store and forward the class name to _is_class()
• issues a DeprecationWarning when a bare class is present in an Enum, and still convert it to a member (warning should mention coming change in behavior, and the @member decorator to maintain the current behavior)

[dignissimus]

Edward, thanks for your work. I decided to go a different route where the work is done in _EnumDict instead (which you can find in my aenum project).

I'm closing this PR, but if you are up for a challenge I need a new PR that:

• adds an @member decorator to force an object to become a member

• adds an @nonmember decorator to protect an object from becoming a member

• adds the _is_class() utility for deciding if an object is a class

• updates EnumMeta to forward the class name to _EnumDict, and _EnumDict to store and forward the class name to _is_class()

• issues a DeprecationWarning when a bare class is present in an Enum, and still convert it to a member (warning should mention coming change in behavior, and the @member decorator to maintain the current behavior)

@ethanfurman
Are these changes still required?

[ethanfurman]

@edwardcwang Hey, I'm adding you to the ACKS file as "Edward C Wang" -- let me know if that should be something different.

[edwardcwang]

"Edward Wang" would actually be fine too!