Python3.11 introduced StrEnum
and IntEnum
which inherit str
or int
respectively, and also inherit ReprEnum
, which in turn inherits Enum
.
ReprEnum
's implementation is actually empty.
>>> print(inspect.getsource(ReprEnum))
class ReprEnum(Enum):
"""
Only changes the repr(), leaving str() and format() to the mixed-in type.
"""
If I create a StrEnum
and check the MRO, I can see that str
comes first.
class Strings(StrEnum):
A = "a"
>>> Strings.__mro__
(<enum 'Strings'>, <enum 'StrEnum'>, <class 'str'>, <enum 'ReprEnum'>, <enum 'Enum'>, <class 'object'>)
Both str
and Enum
define a __str__
and a __repr__
>>> str.__repr__
<slot wrapper '__repr__' of 'str' objects>
>>> str.__str__
<slot wrapper '__str__' of 'str' objects>
>>> Enum.__repr__
<function Enum.__repr__ at 0x7ffff69f72e0>
>>> Enum.__str__
<function Enum.__str__ at 0x7ffff69f7380>
How then does __repr__
get inherited from Enum
and __str__
get inherited from str
?
The __repr__
method comes the normal way, inherited from Enum
(via StrEnum
)
>>> Strings.__repr__ is StrEnum.__repr__ is Enum.__repr__
True
For the __str__
method, the metaclass EnumType
checks for the presence of ReprEnum
and "hoists up" the str
and format
handling of the mixed-in data type into the class namespace at class definition time here:
class EnumType(type):
...
def __new__(metacls, cls, bases, classdict, *, boundary=None, _simple=False, **kwds):
...
# Also, special handling for ReprEnum
if ReprEnum is not None and ReprEnum in bases:
if member_type is object:
raise TypeError(
'ReprEnum subclasses must be mixed with a data type (i.e.'
' int, str, float, etc.)'
)
if '__format__' not in classdict:
enum_class.__format__ = member_type.__format__
classdict['__format__'] = enum_class.__format__
if '__str__' not in classdict:
method = member_type.__str__
if method is object.__str__:
# if member_type does not define __str__, object.__str__ will use
# its __repr__ instead, so we'll also use its __repr__
method = member_type.__repr__
enum_class.__str__ = method
classdict['__str__'] = enum_class.__str__
...
Now that a Strings.__str__
method may be found directly in the class namespace, the MRO needn't be traversed.