A perhaps pythonic application of `functools.reduce()`

readable_reduce.py

import enum, functools, operator
from typing import Self

class Color(enum.IntFlag):
    RED = enum.auto()
    GREEN = enum.auto()
    BLUE = enum.auto()

    @classmethod
    def from_string_pythonic(cls, string: str) -> Self:
        colors = (cls[s] for s in string.split("|") if s)
        return functools.reduce(operator.or_, colors, cls(0))

    @classmethod
    def from_string_less_pythonic(cls, string: str) -> Self:
        result = cls(0)
        if string:
            for s in string.split("|"):
                result |= cls[s]
        return result

Color.from_string_pythonic("RED|BLUE")
# <Color.RED|BLUE: 5>
Color.from_string_less_pythonic("GREEN|BLUE")
# <Color.GREEN|BLUE: 6>
Color.from_string_pythonic("")
# <Color: 0>
Color.from_string_less_pythonic("")
# <Color: 0>