[英]Python Enum shows weird behavior when using same dictionary for member values
當我將一個dict分配為每個成員的值時,我不明白為什么此Enum沒有我定義的所有成員:
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))
輸出為:
[<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}>]
為什么不顯示所有成員?
枚舉強制成員的唯一值。 與其他定義具有相同值的成員定義將被視為別名。
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
定義的名稱確實存在,但是它們都映射到同一成員。
如果您感興趣,請看一下源代碼 :
# [...]
# 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,在我看來,您似乎想利用Enum類在運行時修改值(字典)。 不鼓勵這樣做,對於其他閱讀/使用您的代碼的人來說也很不直觀。 枚舉應由常量組成。
正如@MichaelHoff指出的那樣 , Enum
的行為是將具有相同值的名稱視為別名1 。
您可以使用Advanced Enum
2庫解決此問題:
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)
現在的輸出是:
[
<Token.twitter: {'plan_name': '', 'access_period': 0}>,
<Token.facebook: {'plan_name': '', 'access_period': 0}>,
<Token.instagram: {'plan_name': '', 'access_period': 0}>,
]
為了強調邁克爾所說的話: Enum
成員本來就是常量-除非您真的知道自己在做什么,否則不應使用非常量值。
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有關標准Enum
用法,請參Enum
答案 。
2披露:我是Python stdlib Enum
, enum34
backport和Advanced Enumeration( aenum
)庫的作者。
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