[New-bugs-announce] [issue17959] Alternate approach to aliasing for PEP 435

[Nick Coghlan]

New submission from Nick Coghlan:

Creating this as a separate issue so as not to delay incorporation of the accepted PEP.

One legitimate criticism of the accepted PEP 435 is that the combination of requiring explicit assignment of values *and* allowing aliasing by default is that aliases may be created inadvertently. I believe we can actually do better than the initial implementation by making the following work:

>>> class Shape(Enum):
...   square = 2
...   diamond = 1
...   circle = 3
...   alias_for_square = square
...
>>> Shape.square
<Shape.square: 2>
>>> Shape.alias_for_square
<Shape.square: 2>
>>> Shape(2)
<Shape.square: 2>

While *disallowing* the following: 

>>> class Shape(Enum):
...   square = 2
...   diamond = 1
...   circle = 3
...   alias_for_square = 2
...

How, do you ask? By wrapping non-descriptors on assignment in a placeholder type, and keeping track of which values we have already seen. If a new attribute is mapped to a placeholder, then that's fine, we accept it as an explicit declaration of an alias. However, if it's mapped directly to a repeat value, then that would be disallowed (as it was in earlier versions of the PEP).

----------
messages: 188981
nosy: ncoghlan
priority: normal
severity: normal
stage: needs patch
status: open
title: Alternate approach to aliasing for PEP 435
type: enhancement
versions: Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17959>
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