python enum.Enum _value_ vs value

Question

from enum import Enum
class Type(Enum):
  a = 1
  b = 2

print Type.a.value, Type.a._value_

这打印

1 1

_ value_和value有什么区别？

Answer 1

不同之处在于，它.value是property-backed 版本并且无法更改：

>>> from enum import Enum
>>> class Color(Enum):
...   red = 1
...   green = 2
...   blue = 3
... 
>>> Color.green
<Color.green: 2>
>>> Color.green.value
2
>>> Color(2)
<Color.green: 2>
>>> Color.green.value = 3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.5/types.py", line 141, in __set__
    raise AttributeError("can't set attribute")
AttributeError: can't set attribute

但._value_实际值存储在实例字典中，并且可以更改：

>>> Color.green._value_ = 4
>>> Color.green
<Color.green: 4>

正如Tobias 所解释的那样，除非有充分的理由，否则应避免以下划线开头的名称，因为使用它们可能会破坏事物：

>>> Color(4)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.5/enum.py", line 241, in __call__
    return cls.__new__(cls, value)
  File "/usr/local/lib/python3.5/enum.py", line 476, in __new__
    raise ValueError("%r is not a valid %s" % (value, cls.__name__))
ValueError: 4 is not a valid Color