collectionsabc

collections.abc is a module in the Python standard library that defines abstract base classes (ABCs) for container types. It provides a formal set of interfaces that objects can expose, allowing type checks with isinstance and issubclass to verify whether an object supports a given protocol. The ABCs also enable virtual subclassing, so existing classes can be registered as conforming to an interface without explicit inheritance.

The module includes several core interfaces that describe common container behaviors. Prominent examples are Iterable, Iterator, Sized, Container, Reversible, and Collection, which combine basic iteration, size, containment, and ordering capabilities. More specialized interfaces include Mapping and MutableMapping for dictionary-like objects, Sequence and MutableSequence for ordered collections supporting index-based access, and Set and MutableSet for unordered collections. These ABCs define the methods that concrete implementations should provide, such as __iter__, __len__, __contains__, __getitem__, and others, guiding library authors in designing compatible APIs.

Usage typically involves importing the module and performing isinstance checks to confirm an object’s capabilities, or registering non-ABC types as virtual subclasses. For example, isinstance([1, 2, 3], abc.Sequence) is True, and a custom class that implements the required methods can be registered as a virtual subclass of abc.Sequence. The collections.abc interfaces help promote duck-typing while providing a stable, documented contract for container-like behavior.

Historically,Collections.abc serves as a formalization of common protocols that the old collections module lacked, enabling clearer API design and interoperability across Python codebases.