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Diffstat (limited to 'Doc/library/collections.rst')
| -rw-r--r-- | Doc/library/collections.rst | 350 |
1 files changed, 136 insertions, 214 deletions
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst index 4975f74..f344b5a 100644 --- a/Doc/library/collections.rst +++ b/Doc/library/collections.rst @@ -13,7 +13,7 @@ import itertools __name__ = '<doctest>' -**Source code:** :source:`Lib/collections.py` and :source:`Lib/_abcoll.py` +**Source code:** :source:`Lib/collections/__init__.py` -------------- @@ -24,6 +24,7 @@ Python's general purpose built-in containers, :class:`dict`, :class:`list`, ===================== ==================================================================== :func:`namedtuple` factory function for creating tuple subclasses with named fields :class:`deque` list-like container with fast appends and pops on either end +:class:`ChainMap` dict-like class for creating a single view of multiple mappings :class:`Counter` dict subclass for counting hashable objects :class:`OrderedDict` dict subclass that remembers the order entries were added :class:`defaultdict` dict subclass that calls a factory function to supply missing values @@ -32,12 +33,125 @@ Python's general purpose built-in containers, :class:`dict`, :class:`list`, :class:`UserString` wrapper around string objects for easier string subclassing ===================== ==================================================================== -In addition to the concrete container classes, the collections module provides -:ref:`abstract base classes <collections-abstract-base-classes>` that can be -used to test whether a class provides a particular interface, for example, -whether it is hashable or a mapping. +.. versionchanged:: 3.3 + Moved :ref:`collections-abstract-base-classes` to the :mod:`collections.abc` module. + For backwards compatibility, they continue to be visible in this module + as well. +:class:`ChainMap` objects +------------------------- + +A :class:`ChainMap` class is provided for quickly linking a number of mappings +so they can be treated as a single unit. It is often much faster than creating +a new dictionary and running multiple :meth:`~dict.update` calls. + +The class can be used to simulate nested scopes and is useful in templating. + +.. class:: ChainMap(*maps) + + A :class:`ChainMap` groups multiple dicts or other mappings together to + create a single, updateable view. If no *maps* are specified, a single empty + dictionary is provided so that a new chain always has at least one mapping. + + The underlying mappings are stored in a list. That list is public and can + accessed or updated using the *maps* attribute. There is no other state. + + Lookups search the underlying mappings successively until a key is found. In + contrast, writes, updates, and deletions only operate on the first mapping. + + A :class:`ChainMap` incorporates the underlying mappings by reference. So, if + one of the underlying mappings gets updated, those changes will be reflected + in :class:`ChainMap`. + + All of the usual dictionary methods are supported. In addition, there is a + *maps* attribute, a method for creating new subcontexts, and a property for + accessing all but the first mapping: + + .. attribute:: maps + + A user updateable list of mappings. The list is ordered from + first-searched to last-searched. It is the only stored state and can + modified to change which mappings are searched. The list should + always contain at least one mapping. + + .. method:: new_child() + + Returns a new :class:`ChainMap` containing a new :class:`dict` followed by + all of the maps in the current instance. A call to ``d.new_child()`` is + equivalent to: ``ChainMap({}, *d.maps)``. This method is used for + creating subcontexts that can be updated without altering values in any + of the parent mappings. + + .. method:: parents() + + Returns a new :class:`ChainMap` containing all of the maps in the current + instance except the first one. This is useful for skipping the first map + in the search. The use-cases are similar to those for the + :keyword:`nonlocal` keyword used in :term:`nested scopes <nested scope>`. + The use-cases also parallel those for the builtin :func:`super` function. + A reference to ``d.parents`` is equivalent to: ``ChainMap(*d.maps[1:])``. + + .. versionadded:: 3.3 + + Example of simulating Python's internal lookup chain:: + + import builtins + pylookup = ChainMap(locals(), globals(), vars(builtins)) + + Example of letting user specified values take precedence over environment + variables which in turn take precedence over default values:: + + import os, argparse + defaults = {'color': 'red', 'user': guest} + parser = argparse.ArgumentParser() + parser.add_argument('-u', '--user') + parser.add_argument('-c', '--color') + user_specified = vars(parser.parse_args()) + combined = ChainMap(user_specified, os.environ, defaults) + + Example patterns for using the :class:`ChainMap` class to simulate nested + contexts:: + + c = ChainMap() Create root context + d = c.new_child() Create nested child context + e = c.new_child() Child of c, independent from d + e.maps[0] Current context dictionary -- like Python's locals() + e.maps[-1] Root context -- like Python's globals() + e.parents Enclosing context chain -- like Python's nonlocals + + d['x'] Get first key in the chain of contexts + d['x'] = 1 Set value in current context + del['x'] Delete from current context + list(d) All nested values + k in d Check all nested values + len(d) Number of nested values + d.items() All nested items + dict(d) Flatten into a regular dictionary + + .. seealso:: + + * The `MultiContext class + <http://svn.enthought.com/svn/enthought/CodeTools/trunk/enthought/contexts/multi_context.py>`_ + in the Enthought `CodeTools package + <https://github.com/enthought/codetools>`_ has options to support + writing to any mapping in the chain. + + * Django's `Context class + <http://code.djangoproject.com/browser/django/trunk/django/template/context.py>`_ + for templating is a read-only chain of mappings. It also features + pushing and popping of contexts similar to the + :meth:`~collections.ChainMap.new_child` method and the + :meth:`~collections.ChainMap.parents` property. + + * The `Nested Contexts recipe + <http://code.activestate.com/recipes/577434/>`_ has options to control + whether writes and other mutations apply only to the first mapping or to + any mapping in the chain. + + * A `greatly simplified read-only version of Chainmap + <http://code.activestate.com/recipes/305268/>`_. + :class:`Counter` objects ------------------------ @@ -399,7 +513,8 @@ in Unix:: def tail(filename, n=10): 'Return the last n lines of a file' - return deque(open(filename), n) + with open(filename) as f: + return deque(f, n) Another approach to using deques is to maintain a sequence of recently added elements by appending to the right and popping to the left:: @@ -546,7 +661,7 @@ Setting the :attr:`default_factory` to :class:`set` makes the ... d[k].add(v) ... >>> list(d.items()) - [('blue', set([2, 4])), ('red', set([1, 3]))] + [('blue', {2, 4}), ('red', {1, 3})] :func:`namedtuple` Factory Function for Tuples with Named Fields @@ -579,7 +694,9 @@ they add the ability to access fields by name instead of position index. converted to ``['abc', '_1', 'ghi', '_3']``, eliminating the keyword ``def`` and the duplicate fieldname ``abc``. - If *verbose* is true, the class definition is printed just before being built. + If *verbose* is true, the class definition is printed after it is + built. This option is outdated; instead, it is simpler to print the + :attr:`_source` attribute. Named tuple instances do not have per-instance dictionaries, so they are lightweight and require no more memory than regular tuples. @@ -593,53 +710,6 @@ they add the ability to access fields by name instead of position index. >>> # Basic example >>> Point = namedtuple('Point', ['x', 'y']) - >>> p = Point(x=10, y=11) - - >>> # Example using the verbose option to print the class definition - >>> Point = namedtuple('Point', 'x y', verbose=True) - class Point(tuple): - 'Point(x, y)' - <BLANKLINE> - __slots__ = () - <BLANKLINE> - _fields = ('x', 'y') - <BLANKLINE> - def __new__(_cls, x, y): - 'Create a new instance of Point(x, y)' - return _tuple.__new__(_cls, (x, y)) - <BLANKLINE> - @classmethod - def _make(cls, iterable, new=tuple.__new__, len=len): - 'Make a new Point object from a sequence or iterable' - result = new(cls, iterable) - if len(result) != 2: - raise TypeError('Expected 2 arguments, got %d' % len(result)) - return result - <BLANKLINE> - def __repr__(self): - 'Return a nicely formatted representation string' - return self.__class__.__name__ + '(x=%r, y=%r)' % self - <BLANKLINE> - def _asdict(self): - 'Return a new OrderedDict which maps field names to their values' - return OrderedDict(zip(self._fields, self)) - <BLANKLINE> - __dict__ = property(_asdict) - <BLANKLINE> - def _replace(_self, **kwds): - 'Return a new Point object replacing specified fields with new values' - result = _self._make(map(kwds.pop, ('x', 'y'), _self)) - if kwds: - raise ValueError('Got unexpected field names: %r' % list(kwds.keys())) - return result - <BLANKLINE> - def __getnewargs__(self): - 'Return self as a plain tuple. Used by copy and pickle.' - return tuple(self) - <BLANKLINE> - x = _property(_itemgetter(0), doc='Alias for field number 0') - y = _property(_itemgetter(1), doc='Alias for field number 1') - >>> p = Point(11, y=22) # instantiate with positional or keyword arguments >>> p[0] + p[1] # indexable like the plain tuple (11, 22) 33 @@ -668,7 +738,7 @@ by the :mod:`csv` or :mod:`sqlite3` modules:: print(emp.name, emp.title) In addition to the methods inherited from tuples, named tuples support -three additional methods and one attribute. To prevent conflicts with +three additional methods and two attributes. To prevent conflicts with field names, the method and attribute names start with an underscore. .. classmethod:: somenamedtuple._make(iterable) @@ -706,6 +776,15 @@ field names, the method and attribute names start with an underscore. >>> for partnum, record in inventory.items(): ... inventory[partnum] = record._replace(price=newprices[partnum], timestamp=time.now()) +.. attribute:: somenamedtuple._source + + A string with the pure Python source code used to create the named + tuple class. The source makes the named tuple self-documenting. + It can be printed, executed using :func:`exec`, or saved to a file + and imported. + + .. versionadded:: 3.3 + .. attribute:: somenamedtuple._fields Tuple of strings listing the field names. Useful for introspection @@ -754,7 +833,6 @@ a fixed-width print format: The subclass shown above sets ``__slots__`` to an empty tuple. This helps keep memory requirements low by preventing the creation of instance dictionaries. - Subclassing is not useful for adding new, stored fields. Instead, simply create a new named tuple type from the :attr:`_fields` attribute: @@ -766,6 +844,7 @@ customize a prototype instance: >>> Account = namedtuple('Account', 'owner balance transaction_count') >>> default_account = Account('<owner name>', 0.0, 0) >>> johns_account = default_account._replace(owner='John') + >>> janes_account = default_account._replace(owner='Jane') Enumerated constants can be implemented with named tuples, but it is simpler and more efficient to use a simple class declaration: @@ -984,160 +1063,3 @@ attribute. be an instance of :class:`bytes`, :class:`str`, :class:`UserString` (or a subclass) or an arbitrary sequence which can be converted into a string using the built-in :func:`str` function. - -.. _collections-abstract-base-classes: - -ABCs - abstract base classes ----------------------------- - -The collections module offers the following :term:`ABCs <abstract base class>`: - -========================= ===================== ====================== ==================================================== -ABC Inherits from Abstract Methods Mixin Methods -========================= ===================== ====================== ==================================================== -:class:`Container` ``__contains__`` -:class:`Hashable` ``__hash__`` -:class:`Iterable` ``__iter__`` -:class:`Iterator` :class:`Iterable` ``__next__`` ``__iter__`` -:class:`Sized` ``__len__`` -:class:`Callable` ``__call__`` - -:class:`Sequence` :class:`Sized`, ``__getitem__`` ``__contains__``, ``__iter__``, ``__reversed__``, - :class:`Iterable`, ``index``, and ``count`` - :class:`Container` - -:class:`MutableSequence` :class:`Sequence` ``__setitem__``, Inherited :class:`Sequence` methods and - ``__delitem__``, ``append``, ``reverse``, ``extend``, ``pop``, - ``insert`` ``remove``, and ``__iadd__`` - -:class:`Set` :class:`Sized`, ``__le__``, ``__lt__``, ``__eq__``, ``__ne__``, - :class:`Iterable`, ``__gt__``, ``__ge__``, ``__and__``, ``__or__``, - :class:`Container` ``__sub__``, ``__xor__``, and ``isdisjoint`` - -:class:`MutableSet` :class:`Set` ``add``, Inherited :class:`Set` methods and - ``discard`` ``clear``, ``pop``, ``remove``, ``__ior__``, - ``__iand__``, ``__ixor__``, and ``__isub__`` - -:class:`Mapping` :class:`Sized`, ``__getitem__`` ``__contains__``, ``keys``, ``items``, ``values``, - :class:`Iterable`, ``get``, ``__eq__``, and ``__ne__`` - :class:`Container` - -:class:`MutableMapping` :class:`Mapping` ``__setitem__``, Inherited :class:`Mapping` methods and - ``__delitem__`` ``pop``, ``popitem``, ``clear``, ``update``, - and ``setdefault`` - - -:class:`MappingView` :class:`Sized` ``__len__`` -:class:`ItemsView` :class:`MappingView`, ``__contains__``, - :class:`Set` ``__iter__`` -:class:`KeysView` :class:`MappingView`, ``__contains__``, - :class:`Set` ``__iter__`` -:class:`ValuesView` :class:`MappingView` ``__contains__``, ``__iter__`` -========================= ===================== ====================== ==================================================== - - -.. class:: Container - Hashable - Sized - Callable - - ABCs for classes that provide respectively the methods :meth:`__contains__`, - :meth:`__hash__`, :meth:`__len__`, and :meth:`__call__`. - -.. class:: Iterable - - ABC for classes that provide the :meth:`__iter__` method. - See also the definition of :term:`iterable`. - -.. class:: Iterator - - ABC for classes that provide the :meth:`__iter__` and :meth:`next` methods. - See also the definition of :term:`iterator`. - -.. class:: Sequence - MutableSequence - - ABCs for read-only and mutable :term:`sequences <sequence>`. - -.. class:: Set - MutableSet - - ABCs for read-only and mutable sets. - -.. class:: Mapping - MutableMapping - - ABCs for read-only and mutable :term:`mappings <mapping>`. - -.. class:: MappingView - ItemsView - KeysView - ValuesView - - ABCs for mapping, items, keys, and values :term:`views <view>`. - - -These ABCs allow us to ask classes or instances if they provide -particular functionality, for example:: - - size = None - if isinstance(myvar, collections.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 :class:`Set` API, it only necessary to supply the three underlying -abstract methods: :meth:`__contains__`, :meth:`__iter__`, and :meth:`__len__`. -The ABC supplies the remaining methods such as :meth:`__and__` and -:meth:`isdisjoint` :: - - class ListBasedSet(collections.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 :class:`Set` and :class:`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 - :meth:`_from_iterable` which calls ``cls(iterable)`` to produce a new set. - If the :class:`Set` mixin is being used in a class with a different - constructor signature, you will need to override :meth:`_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 :meth:`__le__` and - then the other operations will automatically follow suit. - -(3) - The :class:`Set` mixin provides a :meth:`_hash` method to compute a hash value - for the set; however, :meth:`__hash__` is not defined because not all sets - are hashable or immutable. To add set hashabilty using mixins, - inherit from both :meth:`Set` and :meth:`Hashable`, then define - ``__hash__ = Set._hash``. - -.. seealso:: - - * `OrderedSet recipe <http://code.activestate.com/recipes/576694/>`_ for an - example built on :class:`MutableSet`. - - * For more about ABCs, see the :mod:`abc` module and :pep:`3119`. |