Lib/enum.py (part 2)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/enum.py ↗

This annotation covers EnumType.__new__ (the metaclass that builds enum classes), Flag bitwise operations, and the newer helper classes. See lib_enum_detail for basic Enum, IntEnum, and auto().

Map

Lines	Symbol	Role
1-300	EnumType.__new__	Build the enum class from a namespace
301-500	EnumType.__call__, _missing_	Lookup by value with missing hook
501-700	Flag	Bitwise flag enum; __and__, __or__, __xor__, __invert__
701-850	IntFlag	Flag that also behaves as int
851-950	StrEnum	Enum that also behaves as str
951-1050	ReprEnum	Mixin for enums whose repr shows the value
1051-1200	auto	Auto-generate values via _generate_next_value_
1201-1400	_EnumDict	Custom namespace dict that captures definitions in order
1401-3000	Helper functions	unique, verify, global_enum, show_flag_values

Reading

EnumType.__new__: building enum members

When Python executes class Color(Enum): RED = 1, EnumType.__new__ is called. It:

1. Separates the class body into descriptors, enum member definitions, and dunder methods
2. Creates an _EnumDict that tracks member names in definition order
3. For each member value, creates an instance of the enum class (calling __new__ and optionally __init__)
4. Stores instances in cls._value2member_map_ for reverse lookup

# CPython: Lib/enum.py:255 EnumType.__new__
class EnumType(type):
    def __new__(metacls, cls, bases, classdict, *, boundary=None, _simple=False, **kwds):
        ...
        # Create the actual enum members
        for name in classdict._member_names:
            value = classdict[name]
            ...
            enum_member = __new__(enum_class, *args)
            enum_member._name_ = name
            enum_member._value_ = value
            ...
            enum_class._member_map_[name] = enum_member
            enum_class._value2member_map_[value] = enum_member

_missing_ hook

If Enum(value) doesn't find value in _value2member_map_, it calls cls._missing_(value). The default raises ValueError; subclasses can override to return a sentinel or a dynamically created member.

Flag bitwise operations

# CPython: Lib/enum.py:896 Flag.__or__
def __or__(self, other):
    if isinstance(other, self.__class__):
        return self.__class__(self._value_ | other._value_)
    return NotImplemented

Combined flags that don't correspond to a named member are represented as a pseudo-member with a combined value. Flag.__contains__ tests if a flag is a subset of a composite.

StrEnum

# CPython: Lib/enum.py:1140 StrEnum
class StrEnum(str, Enum):
    def __new__(cls, value, *args, **kwargs):
        if not isinstance(value, str):
            raise TypeError(f'{value!r} is not a string')
        return str.__new__(cls, value)
    def _generate_next_value_(name, start, count, last_values):
        return name.lower()

Members are also str instances, so MyEnum.VALUE == 'value' (useful for config keys, HTTP methods, etc.).

gopy notes

EnumType is a metaclass; gopy needs to support metaclasses before enum can be fully ported. The member creation loop maps to runtime Type construction with custom __new__. _value2member_map_ maps to a Go map[interface{}]*EnumMember.