Python Enum shows weird behavior when using same dictionary for member values

Question

I don't understand why this Enum doesn't have all the members I defined, when I assign a dict as each member's value:

from enum import Enum

class Token(Enum):
    facebook = {
    'access_period': 0,
    'plan_name': ''}

    instagram = {
    'access_period': 0,
    'plan_name': ''}

    twitter = {
    'access_period': 0,
    'plan_name': ''}

if __name__ == "__main__":
    print(list(Token))

The output is:

[<Token.twitter: {'plan_name': '', 'access_period': 0}>]

… but I expected something like:

[<Token.facebook:  {'plan_name': '', 'access_period': 0}>,
 <Token.instagram: {'plan_name': '', 'access_period': 0}>,
 <Token.twitter:   {'plan_name': '', 'access_period': 0}>]

Why aren't all the members shown?

It's because they all have the same value. That's not how enums are supposed to work. — Aran-Fey, Apr 20 '17 at 14:51
I'm not sure why this question has received a down-vote. The writing could be improved -- yes. However the question shows a not necessarily intuitive behavior of the Enum module. — Michael Hoff, Apr 20 '17 at 15:07
@Fartash you might want to double-check your spelling and rewrite the question title. — Michael Hoff, Apr 20 '17 at 15:10
@MichaelHoff: Agreed. The question itself is well worded, shows the code to reproduce the problem, and describes the desired outcome. I wish all questions were this good! — Ethan Furman, Apr 20 '17 at 15:14
This use case is likely a really bad fit for an `Enum`. Have you considered a plain `dict` instead? If you think you must use an `Enum` at all costs, please explain why. — blubberdiblub, Apr 29 '17 at 09:34

Michael Hoff · Answer 1 · 2017-04-20T15:47:19.820

Enum enforces unique values for the members. Member definitions with the same value as other definitions will be treated as aliases.

Demonstration:

Token.__members__
# OrderedDict([('twitter',
#               <Token.twitter: {'plan_name': '', 'access_period': 0}>),
#              ('facebook',
#               <Token.twitter: {'plan_name': '', 'access_period': 0}>),
#              ('instagram',
#               <Token.twitter: {'plan_name': '', 'access_period': 0}>)])

assert Token.instagram == Token.twitter

The defined names do all exist, however they are all mapped to the same member.

Have a look at the source code if you are interested:

# [...]
# If another member with the same value was already defined, the
# new member becomes an alias to the existing one.
for name, canonical_member in enum_class._member_map_.items():
    if canonical_member._value_ == enum_member._value_:
        enum_member = canonical_member
        break
else:
    # Aliases don't appear in member names (only in __members__).
    enum_class._member_names_.append(member_name)
# performance boost for any member that would not shadow
# a DynamicClassAttribute
if member_name not in base_attributes:
    setattr(enum_class, member_name, enum_member)
# now add to _member_map_
enum_class._member_map_[member_name] = enum_member
try:
    # This may fail if value is not hashable. We can't add the value
    # to the map, and by-value lookups for this value will be
    # linear.
    enum_class._value2member_map_[value] = enum_member
except TypeError:
    pass
# [...]

Further, it seems to me that you want to exploit the Enum class to modify the value (the dictionary) during run-time. This is strongly discouraged and also very unintuitive for other people reading/using your code. An enum is expected to be made of constants.

The line "The members do exist" could be worded more clearly, maybe "The different names exist, but they all map to the same member" ? — Ethan Furman, Apr 20 '17 at 15:45
Fair point. I was not too sure about the correct terminology. As `len(Token.__members__) == 3` one could understand that there are three "members". Answer updated. — Michael Hoff, Apr 20 '17 at 15:49

score 4 · Answer 2 · edited May 23 '17 at 11:47

4

As @MichaelHoff noted, the behavior of Enum is to consider names with the same values to be aliases¹.

You can get around this by using the Advanced Enum² library:

from aenum import Enum, NoAlias

class Token(Enum):
    _settings_ = NoAlias
    facebook = {
        'access_period': 0,
        'plan_name': '',
        }

    instagram = {
        'access_period': 0,
        'plan_name': '',
        }

    twitter = {
        'access_period': 0,
        'plan_name': '',
        }

if __name__ == "__main__":
    print list(Token)

Output is now:

[
  <Token.twitter: {'plan_name': '', 'access_period': 0}>,
  <Token.facebook: {'plan_name': '', 'access_period': 0}>,
  <Token.instagram: {'plan_name': '', 'access_period': 0}>,
  ]

To reinforce what Michael said: Enum members are meant to be constants -- you shouldn't use non-constant values unless you really know what you are doing.

A better example of using NoAlias:

class CardNumber(Enum):

    _order_ = 'EIGHT NINE TEN JACK QUEEN KING ACE'  # only needed for Python 2.x
    _settings_ = NoAlias

    EIGHT    = 8
    NINE     = 9
    TEN      = 10
    JACK     = 10
    QUEEN    = 10
    KING     = 10
    ACE      = 11

¹ See this answer for the standard Enum usage.

² Disclosure: I am the author of the Python stdlib Enum, the enum34 backport, and the Advanced Enumeration (aenum) library.

edited May 23 '17 at 11:47

Community

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answered Apr 20 '17 at 15:09

Ethan Furman

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Is there a way to achieve the `NoAlias` behavior for the stdlib? – Michael Hoff Apr 20 '17 at 15:16
@MichaelHoff: Not easily, no. It falls into the category of "you can if you really want to, but nobody is going to enjoy reading that code!" ;) – Ethan Furman Apr 20 '17 at 15:19
How dislikable is `def __init__(self, *args): self._value_ = (self.name,) + args`? ;) – Michael Hoff Apr 21 '17 at 12:36
@MichaelHoff: That will not work with mixin types, such as `int`, and would also make by-value lookups fail (instead of `CardNumber(8)` it would be `CardNumber(('EIGHT', 8))`. – Ethan Furman Apr 21 '17 at 13:38
Surprisingly it does work* with `IntEnum` but yes, the lookup via constructor will fail. (*at least the use cases I tested have been just fine) – Michael Hoff Apr 21 '17 at 13:51
@MichaelHoff, Huh, interesting. Guess I should have tested that statement first! But hey, I was right on the lookup! :) Plus it's still early here... ;) – Ethan Furman Apr 21 '17 at 13:52

Python Enum shows weird behavior when using same dictionary for member values

2 Answers2