On Mon, Jun 10, 2019 at 6:44 PM Steve Dower <steve.do...@python.org> wrote:
>
> I'd expect people coming from other languages to get it wrong this way too:
>
> class Status(Enum):
> on
> off
>
> Which will of course raise NameError and be just as opaque to the naive
> user as the AttributeError later on. I'm not sure we can do as much to
> help this case, but perhaps we can update __getattr__ to check
> __annotations__ on the class before failing and provide a clearer
> message? e.g. "AttributeError: 'on' was specified without assigning a
> value, did you use ':' instead of '='?" Or we could do this on
> construction, but that may rule out some interesting uses in the future
> if you have a need to delay specifying enum values.
As a side note, the aenum package on PyPI [1] supports this bare-name
notation with its AutoNumberEnum class, which uses some metaclass
magic to do it. aenum's code is complex, but an extremely bare-bones
version can be written in about 15 lines of code:
```
class AutoNumberDict(dict):
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
self.count = 0
def __missing__(self, key):
if isinstance(key, str) and key.startswith('__') and key.endswith('__'):
raise KeyError(key)
self[key] = self.count
self.count += 1
class EnumType(type):
def __prepare__(name, bases):
return AutoNumberDict()
class Enum(metaclass=EnumType):
pass
```
Of course, there are some cases that this doesn't handle (such as
duplicate names or bit flags), but for a one-off script or a personal
project, it could be enough.
[1] https://pypi.org/project/aenum/
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