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?
2 Answers
Answers 1
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.
Answers 2
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.
0 comments:
Post a Comment