8.4. collections.abc — Abstract Base Classes for Containers — Python 3.4.0 documentation

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# 8.4. `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.

## 8.4.1. 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__`
`Sized` |  | `__len__` |
`Callable` |  | `__call__` |
`Sequence` | `Sized`, `Iterable`, `Container` | `__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__`
`Set` | `Sized`, `Iterable`, `Container` | `__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` | `Sized`, `Iterable`, `Container` | `__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` |  | `__contains__`, `__iter__`

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. See also the definition of iterable.

class `collections.abc.` `Iterator`

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

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

  ABCs for read-only and mutable sequences.

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.

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 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 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 hashabilty 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.

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