[英]Python Enum shows weird behavior when using same dictionary for member values
当我将一个dict分配为每个成员的值时,我不明白为什么此Enum没有我定义的所有成员:
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))
输出为:
[<Token.twitter: {'plan_name': '', 'access_period': 0}>]
…但是我期望的是:
[<Token.facebook: {'plan_name': '', 'access_period': 0}>,
<Token.instagram: {'plan_name': '', 'access_period': 0}>,
<Token.twitter: {'plan_name': '', 'access_period': 0}>]
为什么不显示所有成员?
枚举强制成员的唯一值。 与其他定义具有相同值的成员定义将被视为别名。
示范:
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
定义的名称确实存在,但是它们都映射到同一成员。
如果您感兴趣,请看一下源代码 :
# [...]
# 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
# [...]
此外,在我看来,您似乎想利用Enum类在运行时修改值(字典)。 不鼓励这样做,对于其他阅读/使用您的代码的人来说也很不直观。 枚举应由常量组成。
正如@MichaelHoff指出的那样 , Enum
的行为是将具有相同值的名称视为别名1 。
您可以使用Advanced Enum
2库解决此问题:
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)
现在的输出是:
[
<Token.twitter: {'plan_name': '', 'access_period': 0}>,
<Token.facebook: {'plan_name': '', 'access_period': 0}>,
<Token.instagram: {'plan_name': '', 'access_period': 0}>,
]
为了强调迈克尔所说的话: Enum
成员本来就是常量-除非您真的知道自己在做什么,否则不应使用非常量值。
使用NoAlias
更好示例:
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有关标准Enum
用法,请参Enum
答案 。
2披露:我是Python stdlib Enum
, enum34
backport和Advanced Enumeration( aenum
)库的作者。
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