Lib/types.py (part 3)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/types.py ↗

This annotation covers the less-common but important types in the types module. See lib_types2_detail for FunctionType, MethodType, CodeType, and ModuleType.

Map

Lines	Symbol	Role
1-60	SimpleNamespace	Mutable namespace with attribute access
61-140	MappingProxyType	Read-only view of a dict
141-220	DynamicClassAttribute	Descriptor with different behavior on class vs instance
221-300	GenericAlias	list[int], dict[str, int] — parameterized generics
301-400	UnionType	`int

Reading

SimpleNamespace

# CPython: Lib/types.py:34 SimpleNamespace
class SimpleNamespace:
    def __init__(self, /, **kwargs):
        self.__dict__.update(kwargs)
    def __repr__(self):
        items = (f'{k}={v!r}' for k, v in self.__dict__.items())
        return f'{type(self).__name__}({", ".join(items)})'
    def __eq__(self, other):
        if isinstance(self, SimpleNamespace) and isinstance(other, SimpleNamespace):
            return self.__dict__ == other.__dict__
        return NotImplemented

types.SimpleNamespace(x=1, y=2) creates an object where ns.x == 1. Unlike a dict, attributes are accessed with . syntax. Useful as a lightweight alternative to a class or dataclasses.dataclass.

MappingProxyType

# CPython: Objects/descrobject.c: 2100 (C type, but exposed in types.py)
# MappingProxyType wraps a dict and exposes read-only access.
# Setting items raises TypeError.

types.MappingProxyType(d) is used for cls.__dict__ to prevent accidental modification of a class's namespace. It supports all read operations: iteration, len, __getitem__, keys, values, items.

DynamicClassAttribute

# CPython: Lib/types.py:60 DynamicClassAttribute
class DynamicClassAttribute:
    def __init__(self, fget=None, fset=None, fdel=None, doc=None):
        self.fget = fget
        ...
    def __get__(self, obj, objtype=None):
        if obj is None:
            if self.__doc__:
                return self
            raise AttributeError(self.attrname)
        if self.fget is None:
            raise AttributeError(...)
        return self.fget(obj)

When accessed on the class (obj is None), DynamicClassAttribute raises AttributeError, allowing the metaclass to provide a different value. When accessed on an instance, it behaves like a normal property. Used by enum.Enum to make Enum.name and Enum.value instance-only.

GenericAlias

// CPython: Objects/genericaliasobject.c: Ga_getitem
/* list[int].__class_getitem__(int) returns GenericAlias(list, (int,)) */
/* Subscript: dict[str, int] → GenericAlias(dict, (str, int)) */

GenericAlias objects are created by list[int], dict[str, int], etc. They support isinstance checks via __instancecheck__, equality, repr, and __class_getitem__ chaining. typing.get_args(list[int]) returns (int,).

gopy notes

SimpleNamespace is objects.SimpleNamespace in objects/namespace.go. MappingProxyType is objects.MappingProxy in objects/mapping_proxy.go. DynamicClassAttribute is objects.DynamicClassAttribute. GenericAlias is objects.GenericAlias in objects/generic_alias.go. UnionType is objects.UnionType in objects/union_type.go.