Lib/enum.py (part 3)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/enum.py ↗

This annotation covers the Flag, IntFlag, StrEnum variants and the internal EnumMeta.__new__ class-creation machinery. See lib_enum_detail2 for Enum, IntEnum, and basic usage.

Map

Lines	Symbol	Role
1-200	_generate_next_value_, auto	Automatic value assignment
201-500	EnumMeta.__new__ internals	Class body namespace, member creation
501-800	Flag	Non-integer flag with `
801-1100	IntFlag	Integer-based flag (inherits from int)
1101-1300	StrEnum	String enum where members are also str
1301-1600	_missing_ hook	Custom lookup for missing values

Reading

auto and _generate_next_value_

# CPython: Lib/enum.py:290 auto
class auto:
    value = _auto_null

    def __new__(cls):
        obj = object.__new__(cls)
        obj.value = _auto_null
        return obj

# Used as:
class Color(Enum):
    RED = auto()   # calls Color._generate_next_value_('RED', 1, 0, [])

_generate_next_value_ defaults to returning max(last_values) + 1 for IntEnum and the member name (lowercase) for StrEnum.

EnumMeta.__new__ member collection

# CPython: Lib/enum.py:412 EnumMeta.__new__ (abridged)
def __new__(metacls, cls, bases, classdict, **kwds):
    # classdict is an _EnumDict that intercepts assignments
    members = {k: v for k, v in classdict.items()
               if not k.startswith('_') and k not in classdict._ignore}
    ...
    enum_class = super().__new__(metacls, cls, bases, classdict, **kwds)
    for member_name, value in members.items():
        if isinstance(value, auto):
            value = enum_class._generate_next_value_(member_name, ...)
        member = object.__new__(enum_class)
        member._name_ = member_name
        member._value_ = value
        enum_class._value2member_map_[value] = member
        setattr(enum_class, member_name, member)
    return enum_class

Flag composition

# CPython: Lib/enum.py:820 Flag.__or__
def __or__(self, other):
    if isinstance(other, self.__class__):
        return self.__class__(self._value_ | other._value_)
    return NotImplemented

Flag members created by | are "pseudo-members" (composites) that may not be in _value2member_map_. The _decompose helper breaks them back into named members.

_missing_ hook

# CPython: Lib/enum.py:680 EnumMeta.__call__
def __call__(cls, value, names=None, ...):
    if names is None:
        # Value lookup
        try:
            return cls._value2member_map_[value]
        except KeyError:
            pass
        # Try _missing_
        if cls._missing_ is not None:
            result = cls._missing_(value)
            if isinstance(result, cls):
                return result
        raise ValueError(f"{value!r} is not a valid {cls.__name__}")

_missing_ lets enum classes handle values not in the map (e.g., return a sentinel or raise a custom error).

gopy notes

enum is needed for http.HTTPStatus, socket constants, re.RegexFlag, and many stdlib modules. EnumMeta.__new__ requires a working metaclass system and _EnumDict (a custom dict subclass). Flag bit operations require __or__, __and__, __xor__, and __invert__ on the enum class.