Python enum _missing_ handler overwrites values on singleton for forward-compatible enums

[karimkhaleel]

Issue Description

The configuration options for pydantic models allow users to set forward_compatible_python_enums as an option which will generate a _missing_ handler in enum definitions that will get called on unrecognized values.

class BasePydanticModelCustomConfig(pydantic.BaseModel):
    ...
    enum_type: EnumTypes = "literals"
    """
    The type of enums to use in the generated models. Options are:
    - 'literals': Use Python Literal types, e.g. `MyEnum = Literal["foo", "bar"]`
    - 'forward_compatible_python_enums': Use Python Enum classes, with an `MyEnum._UNKNOWN member for forward compatibility, `MyEnum._UNKNOWN.value` contains the raw unrecognized value.
    - 'python_enums': Your vanilla Python enum class, with the members defined within your API.
    """

This missing handler returns the default instance of the enum value with a _value_ attribute pointing to the raw, unrecognized original value.

    @classmethod
    def _missing_(cls, value: object) -> Any:
        unknown = cls._UNKNOWN
        unknown._value_ = value
        return unknown

However, the _UNKNOWN instance of the enum is a singleton and the _value_ attribute on _UNKNOWN is overwritten each time an unrecognized value is converted into an enum representation.

This can lead to confusing behaviors like this:

import enum

class Color(enum.Enum):
    RED = "red"
    BLUE = "blue"
    _UNKNOWN = "__COLOR_UNKNOWN__"

    @classmethod
    def _missing_(cls, value: object) -> Any:
        unknown = cls._UNKNOWN
        unknown._value_ = value
        return unknown


colors = ["red", "blue", "green", "yellow", "purple"]
enum_colors: list[Color] = []

for c in colors:
    enum_colors.append(Color(c))

# Expected output:
# red
# blue
# green
# yellow
# purple

# Actual output:
# red
# blue
# purple
# purple
# purple

for c in enum_colors:
    if c == Color._UNKNOWN:
        print(c._value_)
    else:
        print(c.value)

The _missing_ hook should at least alert the user that an unexpected value was passed in.

import warnings

class Color(enum.Enum):
    RED = "red"
    BLUE = "blue"
    _UNKNOWN = "__COLOR_UNKNOWN__"

    @classmethod
    def _missing_(cls, value: object) -> Any:
        warnings.warn(f"Got an unexpected value: {value} for {cls.__name__}")
        unknown = cls._UNKNOWN
        # left for backwards compatibility, but perhaps the warning should state that there is potential for corrupted
        # data being present when checking the value
        unknown._value_ = value 
        return unknown

Additional Context (Optional)

No response