[Python-Dev] AutoNumber Enum

[Ethan Furman]

There is a several-month-old request to add aenum's [1] AutoNumberEnum
to the stdlib [2].

The requester and two of the three developers of Enum are in favor (the
third hasn't chimed in yet).

This new addition would enable the following:

from Enum import AutoNumberEnum

class Color(AutoNumberEnum):
# auto-number magic is on
Red
Green
Blue
Cyan
# magic turns off when non-enum is defined

def is_primary(self):
# typos in methods, etc, will raise
return self in (self.Red, self.Grene, self.Blue)

# typos after the initial definition stanza will raise
BlueGreen = Blue + Grene

There is, of course, the risk of typos during the initial member
definition stanza, but since this magic only happens when the user
explicitly asks for it (AutoNumberEnum), I think it is acceptable.

The `start` parameter is still available, and assigning a number is
supported (subsequent numbers will (re)start from the assigned number).

Thoughts? Opinions? Flames?

--
~Ethan~



[1] https://pypi.python.org/pypi/aenum
[2] http://bugs.python.org/issue26988
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[Brett Cannon]

Is it going to subclass Enum or IntEnum? Personally I would be quite happy to never have to specify a value for enums ever again, but only if they subclass Enum (since IntEnum is for compatibility with C stuff where a specific value is needed I don't think users need to mess that up by having the automatic numbering not work how they would expect).

[Guido van Rossum]

And how would you implement that without support from the compiler?
Does it use a hook that catches the NameError?

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--
--Guido van Rossum (python.org/~guido)

[Ethan Furman]

On 06/29/2016 11:15 AM, Brett Cannon wrote:

> On Wed, 29 Jun 2016 at 10:41 Ethan Furman wrote:

>> There is a several-month-old request to add aenum's [1] AutoNumberEnum
>> to the stdlib [2].
>>
>> The requester and two of the three developers of Enum are in favor (the
>> third hasn't chimed in yet).
>>
>> This new addition would enable the following:
>>
>> from Enum import AutoNumberEnum
>>
>> class Color(AutoNumberEnum):
>> # auto-number magic is on
>> Red
>> Green
>> Blue
>> Cyan
>> # magic turns off when non-enum is defined
>

> Is it going to subclass Enum or IntEnum?

Enum.

> Personally I would be quite
> happy to never have to specify a value for enums ever again, but only if
> they subclass Enum (since IntEnum is for compatibility with C stuff
> where a specific value is needed I don't think users need to mess that
> up by having the automatic numbering not work how they would expect).

If a user really wants that they can, of course, specify both
AutoNuberEnum and IntEnum in the class header.

--
~Ethan~

[Ethan Furman]

On 06/29/2016 12:11 PM, Guido van Rossum wrote:

> And how would you implement that without support from the compiler?
> Does it use a hook that catches the NameError?

It's built into the _EnumDict class dictionary used during class creation.

Current (edited) code from the aenum package that implements this:

class _EnumDict(dict):
"""Track enum member order and ensure member names are not reused.

EnumMeta will use the names found in self._member_names as the
enumeration member names.
"""
def __init__(self, locked=True, start=1, multivalue=False):
super(_EnumDict, self).__init__()
# list of enum members
self._member_names = []
# starting value for AutoNumber
self._value = start - 1
# when the magic turns off
self._locked = locked
...

def __getitem__(self, key):
if (
self._locked
or key in self
or _is_sunder(key)
or _is_dunder(key)
):
return super(_EnumDict, self).__getitem__(key)
try:
# try to generate the next value
value = self._value + 1
self.__setitem__(key, value)
return value
except:
# couldn't work the magic, report error
raise KeyError('%s not found' % key)

def __setitem__(self, key, value):
"""Changes anything not sundured, dundered, nor a descriptor.
Single underscore (sunder) names are reserved.
"""
if _is_sunder(key):
raise ValueError('_names_ are reserved for future Enum use')
elif _is_dunder(key):
if key == '__order__':
key = '_order_'
if _is_descriptor(value):
self._locked = True
elif key in self._member_names:
# descriptor overwriting an enum?
raise TypeError('Attempted to reuse name: %r' % key)
elif not _is_descriptor(value):
if key in self:
# enum overwriting a descriptor?
raise TypeError('%s already defined as: %r' % ...
self._member_names.append(key)
if not self._locked:
if isinstance(value, int):
self._value = value
else:
count = self._value + 1
self._value = count
value = count, value
else:
# not a new member, turn off the autoassign magic
self._locked = True
super(_EnumDict, self).__setitem__(key, value)

Disclaimer: some errors may have crept in as I deleted unrelated
content. For the full code check out the _EnumDict class in the aenum
package.

--
~Ethan~

[Ivan Levkivskyi]

It looks like the values in AutoNumberEnum are consecutive integers 1,2,3,...

Have you considered an option (keyword argument) to change this to powers of two 1,2,4,8,...?

--

Ivan

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[Ethan Furman]

On 06/29/2016 01:01 PM, Ivan Levkivskyi wrote:

> It looks like the values in AutoNumberEnum are consecutive integers
> 1,2,3,...
> Have you considered an option (keyword argument) to change this to
> powers of two 1,2,4,8,...?

There is another issue relating to bitwise enums that deals with that.
It is not part of this proposal.

--
~Ethan~
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[Larry Hastings]

On 06/29/2016 01:01 PM, Ivan Levkivskyi wrote:

It looks like the values in AutoNumberEnum are consecutive integers 1,2,3,...

Have you considered an option (keyword argument) to change this to powers of two 1,2,4,8,...?

Why would you want that?  I remind you that this descends from Enum, so its members won't be directly interchangeable with ints.  Presumably you want a bitfield enum, and those should descend from IntEnum.

TBH I'd prefer the AutoNumberEnum *not* have this feature; it's already a little too magical for my tastes.


/arry

[Ivan Levkivskyi]

> Presumably you want a bitfield enum, and those should descend from IntEnum.

Yes, and probably having an AutoNumberIntEnum would indeed be too much magic in one place.

Anyway, it is easy to implement bitfield IntEnum without magic.

To be clear, I like the Ethan's original proposal.

--

Ivan

 

TBH I'd prefer the AutoNumberEnum *not* have this feature; it's already a little too magical for my tastes.


/arry

[Roberto Martínez]

Why the 'start' parameter default is 1? 0 (zero) is more consistent with other parts of the language: indexes, enumerate, range... 

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[Ethan Furman]

On 06/29/2016 03:40 PM, Roberto Martínez wrote:

> Why the 'start' parameter default is 1? 0 (zero) is more consistent with
> other parts of the language: indexes, enumerate, range...

An excerpt from [1]:

> The reason for defaulting to 1 as the starting number and not 0 is that 0 is False in a boolean sense, but enum members all evaluate to True.

--
~Ethan~

[1] https://docs.python.org/3/library/enum.html#functional-api

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[Wes Turner]

It may be worth mentioning that pandas Categoricals are mutable and zero-based:
https://pandas-docs.github.io/pandas-docs-travis/categorical.html

Serialization to SQL and CSV is (also?) lossy, though:
- https://pandas-docs.github.io/pandas-docs-travis/categorical.html#getting-data-in-out
- https://pandas-docs.github.io/pandas-docs-travis/io.html#io-stata-categorical

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