collections.abc — Abstract Base Classes for Containers — Python 3.7.2 documentation

# `collections.abc` — Abstract Base Classes for Containers

New in version 3.3: Formerly, this module was part of the `collections` module.

Source code: Lib/_collections_abc.py

This module provides abstract base classes that can be used to test whether a class provides a particular interface; for example, whether it is hashable or whether it is a mapping.

## Collections Abstract Base Classes

The collections module offers the following ABCs:

ABC | Inherits from | Abstract Methods | Mixin Methods
--- | --- | --- | ---
`Container` |  | `__contains__` |
`Hashable` |  | `__hash__` |
`Iterable` |  | `__iter__` |
`Iterator` | `Iterable` | `__next__` | `__iter__`
`Reversible` | `Iterable` | `__reversed__` |
`Generator` | `Iterator` | `send`, `throw` | `close`, `__iter__`, `__next__`
`Sized` |  | `__len__` |
`Callable` |  | `__call__` |
`Collection` | `Sized`, `Iterable`, `Container` | `__contains__`, `__iter__`, `__len__` |
`Sequence` | `Reversible`, `Collection` | `__getitem__`, `__len__` | `__contains__`, `__iter__`, `__reversed__`, `index`, and `count`
`MutableSequence` | `Sequence` | `__getitem__`, `__setitem__`, `__delitem__`, `__len__`, `insert` | Inherited `Sequence` methods and `append`, `reverse`, `extend`, `pop`, `remove`, and `__iadd__`
`ByteString` | `Sequence` | `__getitem__`, `__len__` | Inherited `Sequence` methods
`Set` | `Collection` | `__contains__`, `__iter__`, `__len__` | `__le__`, `__lt__`, `__eq__`, `__ne__`, `__gt__`, `__ge__`, `__and__`, `__or__`, `__sub__`, `__xor__`, and `isdisjoint`
`MutableSet` | `Set` | `__contains__`, `__iter__`, `__len__`, `add`, `discard` | Inherited `Set` methods and `clear`, `pop`, `remove`, `__ior__`, `__iand__`, `__ixor__`, and `__isub__`
`Mapping` | `Collection` | `__getitem__`, `__iter__`, `__len__` | `__contains__`, `keys`, `items`, `values`, `get`, `__eq__`, and `__ne__`
`MutableMapping` | `Mapping` | `__getitem__`, `__setitem__`, `__delitem__`, `__iter__`, `__len__` | Inherited `Mapping` methods and `pop`, `popitem`, `clear`, `update`, and `setdefault`
`MappingView` | `Sized` |  | `__len__`
`ItemsView` | `MappingView`, `Set` |  | `__contains__`, `__iter__`
`KeysView` | `MappingView`, `Set` |  | `__contains__`, `__iter__`
`ValuesView` | `MappingView`, `Collection` |  | `__contains__`, `__iter__`
`Awaitable` |  | `__await__` |
`Coroutine` | `Awaitable` | `send`, `throw` | `close`
`AsyncIterable` |  | `__aiter__` |
`AsyncIterator` | `AsyncIterable` | `__anext__` | `__aiter__`
`AsyncGenerator` | `AsyncIterator` | `asend`, `athrow` | `aclose`, `__aiter__`, `__anext__`

class `collections.abc.` `Container`
class `collections.abc.` `Hashable`
class `collections.abc.` `Sized`
class `collections.abc.` `Callable`

  ABCs for classes that provide respectively the methods `__contains__()`, `__hash__()`, `__len__()`, and `__call__()`.

class `collections.abc.` `Iterable`

  ABC for classes that provide the `__iter__()` method.

  Checking `isinstance(obj, Iterable)` detects classes that are registered as `Iterable` or that have an `__iter__()` method, but it does not detect classes that iterate with the `__getitem__()` method. The only reliable way to determine whether an object is iterable is to call `iter(obj)`.

class `collections.abc.` `Collection`

  ABC for sized iterable container classes.

  New in version 3.6.

class `collections.abc.` `Iterator`

  ABC for classes that provide the `__iter__()` and `__next__()` methods. See also the definition of iterator.

class `collections.abc.` `Reversible`

  ABC for iterable classes that also provide the `__reversed__()` method.

  New in version 3.6.

class `collections.abc.` `Generator`

  ABC for generator classes that implement the protocol defined in PEP 342 that extends iterators with the `send()`, `throw()` and `close()` methods. See also the definition of generator.

  New in version 3.5.

class `collections.abc.` `Sequence`
class `collections.abc.` `MutableSequence`
class `collections.abc.` `ByteString`

  ABCs for read-only and mutable sequences.

  Implementation note: Some of the mixin methods, such as `__iter__()`, `__reversed__()` and `index()`, make repeated calls to the underlying `__getitem__()` method. Consequently, if `__getitem__()` is implemented with constant access speed, the mixin methods will have linear performance; however, if the underlying method is linear (as it would be with a linked list), the mixins will have quadratic performance and will likely need to be overridden.

  Changed in version 3.5: The index() method added support for stop and start arguments.

class `collections.abc.` `Set`
class `collections.abc.` `MutableSet`

  ABCs for read-only and mutable sets.

class `collections.abc.` `Mapping`
class `collections.abc.` `MutableMapping`

  ABCs for read-only and mutable mappings.

class `collections.abc.` `MappingView`
class `collections.abc.` `ItemsView`
class `collections.abc.` `KeysView`
class `collections.abc.` `ValuesView`

  ABCs for mapping, items, keys, and values views.

class `collections.abc.` `Awaitable`

  ABC for awaitable objects, which can be used in `await` expressions. Custom implementations must provide the `__await__()` method.

  Coroutine objects and instances of the `Coroutine` ABC are all instances of this ABC.

  Note

  In CPython, generator-based coroutines (generators decorated with `types.coroutine()` or `asyncio.coroutine()`) are awaitables, even though they do not have an `__await__()` method. Using `isinstance(gencoro, Awaitable)` for them will return `False`. Use `inspect.isawaitable()` to detect them.

  New in version 3.5.

class `collections.abc.` `Coroutine`

  ABC for coroutine compatible classes. These implement the following methods, defined in Coroutine Objects: `send()`, `throw()`, and `close()`. Custom implementations must also implement `__await__()`. All `Coroutine` instances are also instances of `Awaitable`. See also the definition of coroutine.

  Note

  In CPython, generator-based coroutines (generators decorated with `types.coroutine()` or `asyncio.coroutine()`) are awaitables, even though they do not have an `__await__()` method. Using `isinstance(gencoro, Coroutine)` for them will return `False`. Use `inspect.isawaitable()` to detect them.

  New in version 3.5.

class `collections.abc.` `AsyncIterable`

  ABC for classes that provide `__aiter__` method. See also the definition of asynchronous iterable.

  New in version 3.5.

class `collections.abc.` `AsyncIterator`

  ABC for classes that provide `__aiter__` and `__anext__` methods. See also the definition of asynchronous iterator.

  New in version 3.5.

class `collections.abc.` `AsyncGenerator`

  ABC for asynchronous generator classes that implement the protocol defined in PEP 525 and PEP 492.

  New in version 3.6.

These ABCs allow us to ask classes or instances if they provide particular functionality, for example:

    size = None
    if isinstance(myvar, collections.abc.Sized):
        size = len(myvar)

Several of the ABCs are also useful as mixins that make it easier to develop classes supporting container APIs. For example, to write a class supporting the full `Set` API, it is only necessary to supply the three underlying abstract methods: `__contains__()`, `__iter__()`, and `__len__()`. The ABC supplies the remaining methods such as `__and__()` and `isdisjoint()`:

    class ListBasedSet(collections.abc.Set):
        ''' Alternate set implementation favoring space over speed
            and not requiring the set elements to be hashable. '''
        def __init__(self, iterable):
            self.elements = lst = []
            for value in iterable:
                if value not in lst:
                    lst.append(value)

        def __iter__(self):
            return iter(self.elements)

        def __contains__(self, value):
            return value in self.elements

        def __len__(self):
            return len(self.elements)

    s1 = ListBasedSet('abcdef')
    s2 = ListBasedSet('defghi')
    overlap = s1 & s2            # The __and__() method is supported automatically

Notes on using `Set` and `MutableSet` as a mixin:

1. Since some set operations create new sets, the default mixin methods need a way to create new instances from an iterable. The class constructor is assumed to have a signature in the form `ClassName(iterable)`. That assumption is factored-out to an internal classmethod called `_from_iterable()` which calls `cls(iterable)` to produce a new set. If the `Set` mixin is being used in a class with a different constructor signature, you will need to override `_from_iterable()` with a classmethod that can construct new instances from an iterable argument.
2. To override the comparisons (presumably for speed, as the semantics are fixed), redefine `__le__()` and `__ge__()`, then the other operations will automatically follow suit.
3. The `Set` mixin provides a `_hash()` method to compute a hash value for the set; however, `__hash__()` is not defined because not all sets are hashable or immutable. To add set hashability using mixins, inherit from both `Set()` and `Hashable()`, then define `__hash__ = Set._hash`.

See also

- OrderedSet recipe for an example built on `MutableSet`.
- For more about ABCs, see the `abc` module and PEP 3119.