Extend python enum in __new__ with auto()

Question

I have a question related to using auto() with python enum.

I have a base clase like this:

class TokenEnum(IntEnum):
    def __new__(cls, value):
        member = object.__new__(cls)

        # Always auto-generate the int enum value
        member._value_ = auto()  # << not working !!

        member.rule = value
        return member

and want to use it like this. The enum value should be an int and auto-generated. The string provided should go into the additional 'rule' variable.

class Tokens(TokenEnum):
    ID = r'[a-zA-Z_][a-zA-Z0-9_]*'
    ...

auto() doesn't seem to work in the place where I'm using it. And idea on how to get this working?

Answer 1

auto is resolved before __new__ is called -- it should only be used on the member assignment line (i.e. ID = auto()).

If you need TokenEnum members to be integers, you'll want:

def __new__(cls, value):
    int_value_ = len(cls.__members__) + 1   # numbering usually starts at 1
    member = int.__new__(cls, int_value)    # use int, not object
    member._value_ = int_value
    member.rule = value
    return member

If they don't need to be integers, but you want the value to be an integer, you can do:

def __new__(cls, value):
    int_value_ = len(cls.__members__) + 1   # numbering usually starts at 1
    member = object.__new__(cls)            # use object, not int
    member._value_ = int_value              # but assign the int value to _value_
    member.rule = value
    return member

If you don't need the value to be an integer, but you do need the rule attribute:

def __new__(cls, value):
    member = object.__new__(cls)            # use object, not int
    member._value_ = value
    member.rule = value
    return member

Finally, if you want to just use the enum as if it were a string:

class Tokens(str, Enum):
    ID = r'[a-zA-Z_][a-zA-Z0-9_]*'

re.match(ID, some_target_string)

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¹ Disclosure: I am the author of the Python stdlib Enum, the enum34 backport, and the Advanced Enumeration (aenum) library.