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diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst index 57e0481..45da4e5 100644 --- a/Doc/library/collections.rst +++ b/Doc/library/collections.rst @@ -2,17 +2,17 @@ ========================================== .. module:: collections - :synopsis: Container datatypes + :synopsis: Container datatypes .. moduleauthor:: Raymond Hettinger <python@rcn.com> .. sectionauthor:: Raymond Hettinger <python@rcn.com> .. testsetup:: * - from collections import * - import itertools - __name__ = '<doctest>' + from collections import * + import itertools + __name__ = '<doctest>' -**Source code:** :source:`Lib/collections.py` and :source:`Lib/_abcoll.py` +**Source code:** :source:`Lib/collections/__init__.py` -------------- @@ -23,6 +23,7 @@ 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 @@ -31,10 +32,167 @@ 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 +------------------------- + +.. versionadded:: 3.3 + +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 + be 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. + + .. attribute:: parents + + Property returning 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. 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 built-in + :func:`super` function. A reference to ``d.parents`` is equivalent to: + ``ChainMap(*d.maps[1:])``. + + +.. seealso:: + + * The `MultiContext class + <https://github.com/enthought/codetools/blob/4.0.0/codetools/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:`ChainMap` Examples and Recipes +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +This section shows various approaches to working with chained maps. + + +Example of simulating Python's internal lookup chain:: + + import builtins + pylookup = ChainMap(locals(), globals(), vars(builtins)) + +Example of letting user specified command-line arguments 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') + namespace = parser.parse_args() + command_line_args = {k:v for k, v in vars(namespace).items() if v} + + combined = ChainMap(command_line_args, os.environ, defaults) + print(combined['color']) + print(combined['user']) + +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 d['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 + +The :class:`ChainMap` class only makes updates (writes and deletions) to the +first mapping in the chain while lookups will search the full chain. However, +if deep writes and deletions are desired, it is easy to make a subclass that +updates keys found deeper in the chain:: + + class DeepChainMap(ChainMap): + 'Variant of ChainMap that allows direct updates to inner scopes' + + def __setitem__(self, key, value): + for mapping in self.maps: + if key in mapping: + mapping[key] = value + return + self.maps[0][key] = value + + def __delitem__(self, key): + for mapping in self.maps: + if key in mapping: + del mapping[key] + return + raise KeyError(key) + + >>> d = DeepChainMap({'zebra': 'black'}, {'elephant' : 'blue'}, {'lion' : 'yellow'}) + >>> d['lion'] = 'orange' # update an existing key two levels down + >>> d['snake'] = 'red' # new keys get added to the topmost dict + >>> del d['elephant'] # remove an existing key one level down + DeepChainMap({'zebra': 'black', 'snake': 'red'}, {}, {'lion': 'orange'}) :class:`Counter` objects @@ -59,85 +217,85 @@ For example:: .. class:: Counter([iterable-or-mapping]) - A :class:`Counter` is a :class:`dict` subclass for counting hashable objects. - It is an unordered collection where elements are stored as dictionary keys - and their counts are stored as dictionary values. Counts are allowed to be - any integer value including zero or negative counts. The :class:`Counter` - class is similar to bags or multisets in other languages. + A :class:`Counter` is a :class:`dict` subclass for counting hashable objects. + It is an unordered collection where elements are stored as dictionary keys + and their counts are stored as dictionary values. Counts are allowed to be + any integer value including zero or negative counts. The :class:`Counter` + class is similar to bags or multisets in other languages. - Elements are counted from an *iterable* or initialized from another - *mapping* (or counter): + Elements are counted from an *iterable* or initialized from another + *mapping* (or counter): >>> c = Counter() # a new, empty counter >>> c = Counter('gallahad') # a new counter from an iterable >>> c = Counter({'red': 4, 'blue': 2}) # a new counter from a mapping >>> c = Counter(cats=4, dogs=8) # a new counter from keyword args - Counter objects have a dictionary interface except that they return a zero - count for missing items instead of raising a :exc:`KeyError`: + Counter objects have a dictionary interface except that they return a zero + count for missing items instead of raising a :exc:`KeyError`: >>> c = Counter(['eggs', 'ham']) >>> c['bacon'] # count of a missing element is zero 0 - Setting a count to zero does not remove an element from a counter. - Use ``del`` to remove it entirely: + Setting a count to zero does not remove an element from a counter. + Use ``del`` to remove it entirely: >>> c['sausage'] = 0 # counter entry with a zero count >>> del c['sausage'] # del actually removes the entry - .. versionadded:: 3.1 + .. versionadded:: 3.1 - Counter objects support three methods beyond those available for all - dictionaries: + Counter objects support three methods beyond those available for all + dictionaries: - .. method:: elements() + .. method:: elements() - Return an iterator over elements repeating each as many times as its - count. Elements are returned in arbitrary order. If an element's count - is less than one, :meth:`elements` will ignore it. + Return an iterator over elements repeating each as many times as its + count. Elements are returned in arbitrary order. If an element's count + is less than one, :meth:`elements` will ignore it. >>> c = Counter(a=4, b=2, c=0, d=-2) >>> list(c.elements()) ['a', 'a', 'a', 'a', 'b', 'b'] - .. method:: most_common([n]) + .. method:: most_common([n]) - Return a list of the *n* most common elements and their counts from the - most common to the least. If *n* is not specified, :func:`most_common` - returns *all* elements in the counter. Elements with equal counts are - ordered arbitrarily: + Return a list of the *n* most common elements and their counts from the + most common to the least. If *n* is not specified, :func:`most_common` + returns *all* elements in the counter. Elements with equal counts are + ordered arbitrarily: >>> Counter('abracadabra').most_common(3) [('a', 5), ('r', 2), ('b', 2)] - .. method:: subtract([iterable-or-mapping]) + .. method:: subtract([iterable-or-mapping]) - Elements are subtracted from an *iterable* or from another *mapping* - (or counter). Like :meth:`dict.update` but subtracts counts instead - of replacing them. Both inputs and outputs may be zero or negative. + Elements are subtracted from an *iterable* or from another *mapping* + (or counter). Like :meth:`dict.update` but subtracts counts instead + of replacing them. Both inputs and outputs may be zero or negative. >>> c = Counter(a=4, b=2, c=0, d=-2) >>> d = Counter(a=1, b=2, c=3, d=4) >>> c.subtract(d) Counter({'a': 3, 'b': 0, 'c': -3, 'd': -6}) - .. versionadded:: 3.2 + .. versionadded:: 3.2 - The usual dictionary methods are available for :class:`Counter` objects - except for two which work differently for counters. + The usual dictionary methods are available for :class:`Counter` objects + except for two which work differently for counters. - .. method:: fromkeys(iterable) + .. method:: fromkeys(iterable) - This class method is not implemented for :class:`Counter` objects. + This class method is not implemented for :class:`Counter` objects. - .. method:: update([iterable-or-mapping]) + .. method:: update([iterable-or-mapping]) - Elements are counted from an *iterable* or added-in from another - *mapping* (or counter). Like :meth:`dict.update` but adds counts - instead of replacing them. Also, the *iterable* is expected to be a - sequence of elements, not a sequence of ``(key, value)`` pairs. + Elements are counted from an *iterable* or added-in from another + *mapping* (or counter). Like :meth:`dict.update` but adds counts + instead of replacing them. Also, the *iterable* is expected to be a + sequence of elements, not a sequence of ``(key, value)`` pairs. Common patterns for working with :class:`Counter` objects:: @@ -149,7 +307,7 @@ Common patterns for working with :class:`Counter` objects:: c.items() # convert to a list of (elem, cnt) pairs Counter(dict(list_of_pairs)) # convert from a list of (elem, cnt) pairs c.most_common()[:-n:-1] # n least common elements - c += Counter() # remove zero and negative counts + +c # remove zero and negative counts Several mathematical operations are provided for combining :class:`Counter` objects to produce multisets (counters that have counts greater than zero). @@ -169,32 +327,44 @@ counts, but the output will exclude results with counts of zero or less. >>> c | d # union: max(c[x], d[x]) Counter({'a': 3, 'b': 2}) +Unary addition and substraction are shortcuts for adding an empty counter +or subtracting from an empty counter. + + >>> c = Counter(a=2, b=-4) + >>> +c + Counter({'a': 2}) + >>> -c + Counter({'b': 4}) + +.. versionadded:: 3.3 + Added support for unary plus, unary minus, and in-place multiset operations. + .. note:: - Counters were primarily designed to work with positive integers to represent - running counts; however, care was taken to not unnecessarily preclude use - cases needing other types or negative values. To help with those use cases, - this section documents the minimum range and type restrictions. + Counters were primarily designed to work with positive integers to represent + running counts; however, care was taken to not unnecessarily preclude use + cases needing other types or negative values. To help with those use cases, + this section documents the minimum range and type restrictions. - * The :class:`Counter` class itself is a dictionary subclass with no - restrictions on its keys and values. The values are intended to be numbers - representing counts, but you *could* store anything in the value field. + * The :class:`Counter` class itself is a dictionary subclass with no + restrictions on its keys and values. The values are intended to be numbers + representing counts, but you *could* store anything in the value field. - * The :meth:`most_common` method requires only that the values be orderable. + * The :meth:`most_common` method requires only that the values be orderable. - * For in-place operations such as ``c[key] += 1``, the value type need only - support addition and subtraction. So fractions, floats, and decimals would - work and negative values are supported. The same is also true for - :meth:`update` and :meth:`subtract` which allow negative and zero values - for both inputs and outputs. + * For in-place operations such as ``c[key] += 1``, the value type need only + support addition and subtraction. So fractions, floats, and decimals would + work and negative values are supported. The same is also true for + :meth:`update` and :meth:`subtract` which allow negative and zero values + for both inputs and outputs. - * The multiset methods are designed only for use cases with positive values. - The inputs may be negative or zero, but only outputs with positive values - are created. There are no type restrictions, but the value type needs to - support addition, subtraction, and comparison. + * The multiset methods are designed only for use cases with positive values. + The inputs may be negative or zero, but only outputs with positive values + are created. There are no type restrictions, but the value type needs to + support addition, subtraction, and comparison. - * The :meth:`elements` method requires integer counts. It ignores zero and - negative counts. + * The :meth:`elements` method requires integer counts. It ignores zero and + negative counts. .. seealso:: @@ -217,7 +387,7 @@ counts, but the output will exclude results with counts of zero or less. * To enumerate all distinct multisets of a given size over a given set of elements, see :func:`itertools.combinations_with_replacement`. - map(Counter, combinations_with_replacement('ABC', 2)) --> AA AB AC BB BC CC + map(Counter, combinations_with_replacement('ABC', 2)) --> AA AB AC BB BC CC :class:`deque` objects @@ -225,105 +395,105 @@ counts, but the output will exclude results with counts of zero or less. .. class:: deque([iterable, [maxlen]]) - Returns a new deque object initialized left-to-right (using :meth:`append`) with - data from *iterable*. If *iterable* is not specified, the new deque is empty. + Returns a new deque object initialized left-to-right (using :meth:`append`) with + data from *iterable*. If *iterable* is not specified, the new deque is empty. - Deques are a generalization of stacks and queues (the name is pronounced "deck" - and is short for "double-ended queue"). Deques support thread-safe, memory - efficient appends and pops from either side of the deque with approximately the - same O(1) performance in either direction. + Deques are a generalization of stacks and queues (the name is pronounced "deck" + and is short for "double-ended queue"). Deques support thread-safe, memory + efficient appends and pops from either side of the deque with approximately the + same O(1) performance in either direction. - Though :class:`list` objects support similar operations, they are optimized for - fast fixed-length operations and incur O(n) memory movement costs for - ``pop(0)`` and ``insert(0, v)`` operations which change both the size and - position of the underlying data representation. + Though :class:`list` objects support similar operations, they are optimized for + fast fixed-length operations and incur O(n) memory movement costs for + ``pop(0)`` and ``insert(0, v)`` operations which change both the size and + position of the underlying data representation. - If *maxlen* is not specified or is *None*, deques may grow to an - arbitrary length. Otherwise, the deque is bounded to the specified maximum - length. Once a bounded length deque is full, when new items are added, a - corresponding number of items are discarded from the opposite end. Bounded - length deques provide functionality similar to the ``tail`` filter in - Unix. They are also useful for tracking transactions and other pools of data - where only the most recent activity is of interest. + If *maxlen* is not specified or is *None*, deques may grow to an + arbitrary length. Otherwise, the deque is bounded to the specified maximum + length. Once a bounded length deque is full, when new items are added, a + corresponding number of items are discarded from the opposite end. Bounded + length deques provide functionality similar to the ``tail`` filter in + Unix. They are also useful for tracking transactions and other pools of data + where only the most recent activity is of interest. - Deque objects support the following methods: + Deque objects support the following methods: - .. method:: append(x) + .. method:: append(x) - Add *x* to the right side of the deque. + Add *x* to the right side of the deque. - .. method:: appendleft(x) + .. method:: appendleft(x) - Add *x* to the left side of the deque. + Add *x* to the left side of the deque. - .. method:: clear() + .. method:: clear() - Remove all elements from the deque leaving it with length 0. + Remove all elements from the deque leaving it with length 0. - .. method:: count(x) + .. method:: count(x) - Count the number of deque elements equal to *x*. + Count the number of deque elements equal to *x*. - .. versionadded:: 3.2 + .. versionadded:: 3.2 - .. method:: extend(iterable) + .. method:: extend(iterable) - Extend the right side of the deque by appending elements from the iterable - argument. + Extend the right side of the deque by appending elements from the iterable + argument. - .. method:: extendleft(iterable) + .. method:: extendleft(iterable) - Extend the left side of the deque by appending elements from *iterable*. - Note, the series of left appends results in reversing the order of - elements in the iterable argument. + Extend the left side of the deque by appending elements from *iterable*. + Note, the series of left appends results in reversing the order of + elements in the iterable argument. - .. method:: pop() + .. method:: pop() - Remove and return an element from the right side of the deque. If no - elements are present, raises an :exc:`IndexError`. + Remove and return an element from the right side of the deque. If no + elements are present, raises an :exc:`IndexError`. - .. method:: popleft() + .. method:: popleft() - Remove and return an element from the left side of the deque. If no - elements are present, raises an :exc:`IndexError`. + Remove and return an element from the left side of the deque. If no + elements are present, raises an :exc:`IndexError`. - .. method:: remove(value) + .. method:: remove(value) - Removed the first occurrence of *value*. If not found, raises a - :exc:`ValueError`. + Removed the first occurrence of *value*. If not found, raises a + :exc:`ValueError`. - .. method:: reverse() + .. method:: reverse() - Reverse the elements of the deque in-place and then return ``None``. + Reverse the elements of the deque in-place and then return ``None``. - .. versionadded:: 3.2 + .. versionadded:: 3.2 - .. method:: rotate(n) + .. method:: rotate(n) - Rotate the deque *n* steps to the right. If *n* is negative, rotate to - the left. Rotating one step to the right is equivalent to: - ``d.appendleft(d.pop())``. + Rotate the deque *n* steps to the right. If *n* is negative, rotate to + the left. Rotating one step to the right is equivalent to: + ``d.appendleft(d.pop())``. - Deque objects also provide one read-only attribute: + Deque objects also provide one read-only attribute: - .. attribute:: maxlen + .. attribute:: maxlen - Maximum size of a deque or *None* if unbounded. + Maximum size of a deque or *None* if unbounded. - .. versionadded:: 3.1 + .. versionadded:: 3.1 In addition to the above, deques support iteration, pickling, ``len(d)``, @@ -336,56 +506,56 @@ Example: .. doctest:: - >>> from collections import deque - >>> d = deque('ghi') # make a new deque with three items - >>> for elem in d: # iterate over the deque's elements - ... print(elem.upper()) - G - H - I - - >>> d.append('j') # add a new entry to the right side - >>> d.appendleft('f') # add a new entry to the left side - >>> d # show the representation of the deque - deque(['f', 'g', 'h', 'i', 'j']) - - >>> d.pop() # return and remove the rightmost item - 'j' - >>> d.popleft() # return and remove the leftmost item - 'f' - >>> list(d) # list the contents of the deque - ['g', 'h', 'i'] - >>> d[0] # peek at leftmost item - 'g' - >>> d[-1] # peek at rightmost item - 'i' - - >>> list(reversed(d)) # list the contents of a deque in reverse - ['i', 'h', 'g'] - >>> 'h' in d # search the deque - True - >>> d.extend('jkl') # add multiple elements at once - >>> d - deque(['g', 'h', 'i', 'j', 'k', 'l']) - >>> d.rotate(1) # right rotation - >>> d - deque(['l', 'g', 'h', 'i', 'j', 'k']) - >>> d.rotate(-1) # left rotation - >>> d - deque(['g', 'h', 'i', 'j', 'k', 'l']) - - >>> deque(reversed(d)) # make a new deque in reverse order - deque(['l', 'k', 'j', 'i', 'h', 'g']) - >>> d.clear() # empty the deque - >>> d.pop() # cannot pop from an empty deque - Traceback (most recent call last): - File "<pyshell#6>", line 1, in -toplevel- - d.pop() - IndexError: pop from an empty deque - - >>> d.extendleft('abc') # extendleft() reverses the input order - >>> d - deque(['c', 'b', 'a']) + >>> from collections import deque + >>> d = deque('ghi') # make a new deque with three items + >>> for elem in d: # iterate over the deque's elements + ... print(elem.upper()) + G + H + I + + >>> d.append('j') # add a new entry to the right side + >>> d.appendleft('f') # add a new entry to the left side + >>> d # show the representation of the deque + deque(['f', 'g', 'h', 'i', 'j']) + + >>> d.pop() # return and remove the rightmost item + 'j' + >>> d.popleft() # return and remove the leftmost item + 'f' + >>> list(d) # list the contents of the deque + ['g', 'h', 'i'] + >>> d[0] # peek at leftmost item + 'g' + >>> d[-1] # peek at rightmost item + 'i' + + >>> list(reversed(d)) # list the contents of a deque in reverse + ['i', 'h', 'g'] + >>> 'h' in d # search the deque + True + >>> d.extend('jkl') # add multiple elements at once + >>> d + deque(['g', 'h', 'i', 'j', 'k', 'l']) + >>> d.rotate(1) # right rotation + >>> d + deque(['l', 'g', 'h', 'i', 'j', 'k']) + >>> d.rotate(-1) # left rotation + >>> d + deque(['g', 'h', 'i', 'j', 'k', 'l']) + + >>> deque(reversed(d)) # make a new deque in reverse order + deque(['l', 'k', 'j', 'i', 'h', 'g']) + >>> d.clear() # empty the deque + >>> d.pop() # cannot pop from an empty deque + Traceback (most recent call last): + File "<pyshell#6>", line 1, in -toplevel- + d.pop() + IndexError: pop from an empty deque + + >>> d.extendleft('abc') # extendleft() reverses the input order + >>> d + deque(['c', 'b', 'a']) :class:`deque` Recipes @@ -396,9 +566,10 @@ This section shows various approaches to working with deques. Bounded length deques provide functionality similar to the ``tail`` filter in Unix:: - def tail(filename, n=10): - 'Return the last n lines of a file' - return deque(open(filename), n) + def tail(filename, n=10): + 'Return the last n lines of a file' + 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:: @@ -419,10 +590,10 @@ The :meth:`rotate` method provides a way to implement :class:`deque` slicing and deletion. For example, a pure Python implementation of ``del d[n]`` relies on the :meth:`rotate` method to position elements to be popped:: - def delete_nth(d, n): - d.rotate(-n) - d.popleft() - d.rotate(n) + def delete_nth(d, n): + d.rotate(-n) + d.popleft() + d.rotate(n) To implement :class:`deque` slicing, use a similar approach applying :meth:`rotate` to bring a target element to the left side of the deque. Remove @@ -438,50 +609,50 @@ stack manipulations such as ``dup``, ``drop``, ``swap``, ``over``, ``pick``, .. class:: defaultdict([default_factory[, ...]]) - Returns a new dictionary-like object. :class:`defaultdict` is a subclass of the - built-in :class:`dict` class. It overrides one method and adds one writable - instance variable. The remaining functionality is the same as for the - :class:`dict` class and is not documented here. + Returns a new dictionary-like object. :class:`defaultdict` is a subclass of the + built-in :class:`dict` class. It overrides one method and adds one writable + instance variable. The remaining functionality is the same as for the + :class:`dict` class and is not documented here. - The first argument provides the initial value for the :attr:`default_factory` - attribute; it defaults to ``None``. All remaining arguments are treated the same - as if they were passed to the :class:`dict` constructor, including keyword - arguments. + The first argument provides the initial value for the :attr:`default_factory` + attribute; it defaults to ``None``. All remaining arguments are treated the same + as if they were passed to the :class:`dict` constructor, including keyword + arguments. - :class:`defaultdict` objects support the following method in addition to the - standard :class:`dict` operations: + :class:`defaultdict` objects support the following method in addition to the + standard :class:`dict` operations: - .. method:: __missing__(key) + .. method:: __missing__(key) - If the :attr:`default_factory` attribute is ``None``, this raises a - :exc:`KeyError` exception with the *key* as argument. + If the :attr:`default_factory` attribute is ``None``, this raises a + :exc:`KeyError` exception with the *key* as argument. - If :attr:`default_factory` is not ``None``, it is called without arguments - to provide a default value for the given *key*, this value is inserted in - the dictionary for the *key*, and returned. + If :attr:`default_factory` is not ``None``, it is called without arguments + to provide a default value for the given *key*, this value is inserted in + the dictionary for the *key*, and returned. - If calling :attr:`default_factory` raises an exception this exception is - propagated unchanged. + If calling :attr:`default_factory` raises an exception this exception is + propagated unchanged. - This method is called by the :meth:`__getitem__` method of the - :class:`dict` class when the requested key is not found; whatever it - returns or raises is then returned or raised by :meth:`__getitem__`. + This method is called by the :meth:`__getitem__` method of the + :class:`dict` class when the requested key is not found; whatever it + returns or raises is then returned or raised by :meth:`__getitem__`. - Note that :meth:`__missing__` is *not* called for any operations besides - :meth:`__getitem__`. This means that :meth:`get` will, like normal - dictionaries, return ``None`` as a default rather than using - :attr:`default_factory`. + Note that :meth:`__missing__` is *not* called for any operations besides + :meth:`__getitem__`. This means that :meth:`get` will, like normal + dictionaries, return ``None`` as a default rather than using + :attr:`default_factory`. - :class:`defaultdict` objects support the following instance variable: + :class:`defaultdict` objects support the following instance variable: - .. attribute:: default_factory + .. attribute:: default_factory - This attribute is used by the :meth:`__missing__` method; it is - initialized from the first argument to the constructor, if present, or to - ``None``, if absent. + This attribute is used by the :meth:`__missing__` method; it is + initialized from the first argument to the constructor, if present, or to + ``None``, if absent. :class:`defaultdict` Examples @@ -490,13 +661,13 @@ stack manipulations such as ``dup``, ``drop``, ``swap``, ``over``, ``pick``, Using :class:`list` as the :attr:`default_factory`, it is easy to group a sequence of key-value pairs into a dictionary of lists: - >>> s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)] - >>> d = defaultdict(list) - >>> for k, v in s: - ... d[k].append(v) - ... - >>> list(d.items()) - [('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])] + >>> s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)] + >>> d = defaultdict(list) + >>> for k, v in s: + ... d[k].append(v) + ... + >>> list(d.items()) + [('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])] When each key is encountered for the first time, it is not already in the mapping; so an entry is automatically created using the :attr:`default_factory` @@ -506,24 +677,24 @@ again, the look-up proceeds normally (returning the list for that key) and the :meth:`list.append` operation adds another value to the list. This technique is simpler and faster than an equivalent technique using :meth:`dict.setdefault`: - >>> d = {} - >>> for k, v in s: - ... d.setdefault(k, []).append(v) - ... - >>> list(d.items()) - [('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])] + >>> d = {} + >>> for k, v in s: + ... d.setdefault(k, []).append(v) + ... + >>> list(d.items()) + [('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])] Setting the :attr:`default_factory` to :class:`int` makes the :class:`defaultdict` useful for counting (like a bag or multiset in other languages): - >>> s = 'mississippi' - >>> d = defaultdict(int) - >>> for k in s: - ... d[k] += 1 - ... - >>> list(d.items()) - [('i', 4), ('p', 2), ('s', 4), ('m', 1)] + >>> s = 'mississippi' + >>> d = defaultdict(int) + >>> for k in s: + ... d[k] += 1 + ... + >>> list(d.items()) + [('i', 4), ('p', 2), ('s', 4), ('m', 1)] When a letter is first encountered, it is missing from the mapping, so the :attr:`default_factory` function calls :func:`int` to supply a default count of @@ -534,23 +705,23 @@ constant functions. A faster and more flexible way to create constant functions is to use a lambda function which can supply any constant value (not just zero): - >>> def constant_factory(value): - ... return lambda: value - >>> d = defaultdict(constant_factory('<missing>')) - >>> d.update(name='John', action='ran') - >>> '%(name)s %(action)s to %(object)s' % d - 'John ran to <missing>' + >>> def constant_factory(value): + ... return lambda: value + >>> d = defaultdict(constant_factory('<missing>')) + >>> d.update(name='John', action='ran') + >>> '%(name)s %(action)s to %(object)s' % d + 'John ran to <missing>' Setting the :attr:`default_factory` to :class:`set` makes the :class:`defaultdict` useful for building a dictionary of sets: - >>> s = [('red', 1), ('blue', 2), ('red', 3), ('blue', 4), ('red', 1), ('blue', 4)] - >>> d = defaultdict(set) - >>> for k, v in s: - ... d[k].add(v) - ... - >>> list(d.items()) - [('blue', set([2, 4])), ('red', set([1, 3]))] + >>> s = [('red', 1), ('blue', 2), ('red', 3), ('blue', 4), ('red', 1), ('blue', 4)] + >>> d = defaultdict(set) + >>> for k, v in s: + ... d[k].add(v) + ... + >>> list(d.items()) + [('blue', {2, 4}), ('red', {1, 3})] :func:`namedtuple` Factory Function for Tuples with Named Fields @@ -562,168 +733,131 @@ they add the ability to access fields by name instead of position index. .. function:: namedtuple(typename, field_names, verbose=False, rename=False) - Returns a new tuple subclass named *typename*. The new subclass is used to - create tuple-like objects that have fields accessible by attribute lookup as - well as being indexable and iterable. Instances of the subclass also have a - helpful docstring (with typename and field_names) and a helpful :meth:`__repr__` - method which lists the tuple contents in a ``name=value`` format. + Returns a new tuple subclass named *typename*. The new subclass is used to + create tuple-like objects that have fields accessible by attribute lookup as + well as being indexable and iterable. Instances of the subclass also have a + helpful docstring (with typename and field_names) and a helpful :meth:`__repr__` + method which lists the tuple contents in a ``name=value`` format. - The *field_names* are a single string with each fieldname separated by whitespace - and/or commas, for example ``'x y'`` or ``'x, y'``. Alternatively, *field_names* - can be a sequence of strings such as ``['x', 'y']``. + The *field_names* are a single string with each fieldname separated by whitespace + and/or commas, for example ``'x y'`` or ``'x, y'``. Alternatively, *field_names* + can be a sequence of strings such as ``['x', 'y']``. - Any valid Python identifier may be used for a fieldname except for names - starting with an underscore. Valid identifiers consist of letters, digits, - and underscores but do not start with a digit or underscore and cannot be - a :mod:`keyword` such as *class*, *for*, *return*, *global*, *pass*, - or *raise*. + Any valid Python identifier may be used for a fieldname except for names + starting with an underscore. Valid identifiers consist of letters, digits, + and underscores but do not start with a digit or underscore and cannot be + a :mod:`keyword` such as *class*, *for*, *return*, *global*, *pass*, + or *raise*. - If *rename* is true, invalid fieldnames are automatically replaced - with positional names. For example, ``['abc', 'def', 'ghi', 'abc']`` is - converted to ``['abc', '_1', 'ghi', '_3']``, eliminating the keyword - ``def`` and the duplicate fieldname ``abc``. + If *rename* is true, invalid fieldnames are automatically replaced + with positional names. For example, ``['abc', 'def', 'ghi', 'abc']`` is + 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. + Named tuple instances do not have per-instance dictionaries, so they are + lightweight and require no more memory than regular tuples. - .. versionchanged:: 3.1 - Added support for *rename*. + .. versionchanged:: 3.1 + Added support for *rename*. .. doctest:: - :options: +NORMALIZE_WHITESPACE - - >>> # 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 - >>> x, y = p # unpack like a regular tuple - >>> x, y - (11, 22) - >>> p.x + p.y # fields also accessible by name - 33 - >>> p # readable __repr__ with a name=value style - Point(x=11, y=22) + :options: +NORMALIZE_WHITESPACE + + >>> # Basic example + >>> Point = namedtuple('Point', ['x', 'y']) + >>> p = Point(11, y=22) # instantiate with positional or keyword arguments + >>> p[0] + p[1] # indexable like the plain tuple (11, 22) + 33 + >>> x, y = p # unpack like a regular tuple + >>> x, y + (11, 22) + >>> p.x + p.y # fields also accessible by name + 33 + >>> p # readable __repr__ with a name=value style + Point(x=11, y=22) Named tuples are especially useful for assigning field names to result tuples returned by the :mod:`csv` or :mod:`sqlite3` modules:: - EmployeeRecord = namedtuple('EmployeeRecord', 'name, age, title, department, paygrade') + EmployeeRecord = namedtuple('EmployeeRecord', 'name, age, title, department, paygrade') - import csv - for emp in map(EmployeeRecord._make, csv.reader(open("employees.csv", "rb"))): - print(emp.name, emp.title) + import csv + for emp in map(EmployeeRecord._make, csv.reader(open("employees.csv", "rb"))): + print(emp.name, emp.title) - import sqlite3 - conn = sqlite3.connect('/companydata') - cursor = conn.cursor() - cursor.execute('SELECT name, age, title, department, paygrade FROM employees') - for emp in map(EmployeeRecord._make, cursor.fetchall()): - print(emp.name, emp.title) + import sqlite3 + conn = sqlite3.connect('/companydata') + cursor = conn.cursor() + cursor.execute('SELECT name, age, title, department, paygrade FROM employees') + for emp in map(EmployeeRecord._make, cursor.fetchall()): + 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) - Class method that makes a new instance from an existing sequence or iterable. + Class method that makes a new instance from an existing sequence or iterable. -.. doctest:: + .. doctest:: - >>> t = [11, 22] - >>> Point._make(t) - Point(x=11, y=22) + >>> t = [11, 22] + >>> Point._make(t) + Point(x=11, y=22) .. method:: somenamedtuple._asdict() - Return a new :class:`OrderedDict` which maps field names to their corresponding - values:: + Return a new :class:`OrderedDict` which maps field names to their corresponding + values. Note, this method is no longer needed now that the same effect can + be achieved by using the built-in :func:`vars` function:: - >>> p._asdict() - OrderedDict([('x', 11), ('y', 22)]) + >>> vars(p) + OrderedDict([('x', 11), ('y', 22)]) - .. versionchanged:: 3.1 - Returns an :class:`OrderedDict` instead of a regular :class:`dict`. + .. versionchanged:: 3.1 + Returns an :class:`OrderedDict` instead of a regular :class:`dict`. .. method:: somenamedtuple._replace(kwargs) - Return a new instance of the named tuple replacing specified fields with new - values: + Return a new instance of the named tuple replacing specified fields with new + values:: + + >>> p = Point(x=11, y=22) + >>> p._replace(x=33) + Point(x=33, y=22) + + >>> for partnum, record in inventory.items(): + ... inventory[partnum] = record._replace(price=newprices[partnum], timestamp=time.now()) -:: +.. attribute:: somenamedtuple._source - >>> p = Point(x=11, y=22) - >>> p._replace(x=33) - Point(x=33, y=22) + 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. - >>> for partnum, record in inventory.items(): - ... inventory[partnum] = record._replace(price=newprices[partnum], timestamp=time.now()) + .. versionadded:: 3.3 .. attribute:: somenamedtuple._fields - Tuple of strings listing the field names. Useful for introspection - and for creating new named tuple types from existing named tuples. + Tuple of strings listing the field names. Useful for introspection + and for creating new named tuple types from existing named tuples. -.. doctest:: + .. doctest:: - >>> p._fields # view the field names - ('x', 'y') + >>> p._fields # view the field names + ('x', 'y') - >>> Color = namedtuple('Color', 'red green blue') - >>> Pixel = namedtuple('Pixel', Point._fields + Color._fields) - >>> Pixel(11, 22, 128, 255, 0) - Pixel(x=11, y=22, red=128, green=255, blue=0) + >>> Color = namedtuple('Color', 'red green blue') + >>> Pixel = namedtuple('Pixel', Point._fields + Color._fields) + >>> Pixel(11, 22, 128, 255, 0) + Pixel(x=11, y=22, red=128, green=255, blue=0) To retrieve a field whose name is stored in a string, use the :func:`getattr` function: @@ -734,31 +868,30 @@ function: To convert a dictionary to a named tuple, use the double-star-operator (as described in :ref:`tut-unpacking-arguments`): - >>> d = {'x': 11, 'y': 22} - >>> Point(**d) - Point(x=11, y=22) + >>> d = {'x': 11, 'y': 22} + >>> Point(**d) + Point(x=11, y=22) Since a named tuple is a regular Python class, it is easy to add or change functionality with a subclass. Here is how to add a calculated field and a fixed-width print format: >>> class Point(namedtuple('Point', 'x y')): - __slots__ = () - @property - def hypot(self): - return (self.x ** 2 + self.y ** 2) ** 0.5 - def __str__(self): - return 'Point: x=%6.3f y=%6.3f hypot=%6.3f' % (self.x, self.y, self.hypot) + __slots__ = () + @property + def hypot(self): + return (self.x ** 2 + self.y ** 2) ** 0.5 + def __str__(self): + return 'Point: x=%6.3f y=%6.3f hypot=%6.3f' % (self.x, self.y, self.hypot) >>> for p in Point(3, 4), Point(14, 5/7): - print(p) + print(p) Point: x= 3.000 y= 4.000 hypot= 5.000 Point: x=14.000 y= 0.714 hypot=14.018 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: @@ -770,6 +903,7 @@ 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: @@ -778,19 +912,19 @@ and more efficient to use a simple class declaration: >>> Status.open, Status.pending, Status.closed (0, 1, 2) >>> class Status: - open, pending, closed = range(3) + open, pending, closed = range(3) .. seealso:: - * `Named tuple recipe <http://code.activestate.com/recipes/500261/>`_ - adapted for Python 2.4. + * `Named tuple recipe <http://code.activestate.com/recipes/500261/>`_ + adapted for Python 2.4. - * `Recipe for named tuple abstract base class with a metaclass mix-in - <http://code.activestate.com/recipes/577629-namedtupleabc-abstract-base-class-mix-in-for-named/>`_ - by Jan Kaliszewski. Besides providing an :term:`abstract base class` for - named tuples, it also supports an alternate :term:`metaclass`-based - constructor that is convenient for use cases where named tuples are being - subclassed. + * `Recipe for named tuple abstract base class with a metaclass mix-in + <http://code.activestate.com/recipes/577629-namedtupleabc-abstract-base-class-mix-in-for-named/>`_ + by Jan Kaliszewski. Besides providing an :term:`abstract base class` for + named tuples, it also supports an alternate :term:`metaclass`-based + constructor that is convenient for use cases where named tuples are being + subclassed. :class:`OrderedDict` objects @@ -802,36 +936,36 @@ the items are returned in the order their keys were first added. .. class:: OrderedDict([items]) - Return an instance of a dict subclass, supporting the usual :class:`dict` - methods. An *OrderedDict* is a dict that remembers the order that keys - were first inserted. If a new entry overwrites an existing entry, the - original insertion position is left unchanged. Deleting an entry and - reinserting it will move it to the end. + Return an instance of a dict subclass, supporting the usual :class:`dict` + methods. An *OrderedDict* is a dict that remembers the order that keys + were first inserted. If a new entry overwrites an existing entry, the + original insertion position is left unchanged. Deleting an entry and + reinserting it will move it to the end. - .. versionadded:: 3.1 + .. versionadded:: 3.1 - .. method:: popitem(last=True) + .. method:: popitem(last=True) - The :meth:`popitem` method for ordered dictionaries returns and removes a - (key, value) pair. The pairs are returned in LIFO order if *last* is true - or FIFO order if false. + The :meth:`popitem` method for ordered dictionaries returns and removes a + (key, value) pair. The pairs are returned in LIFO order if *last* is true + or FIFO order if false. - .. method:: move_to_end(key, last=True) + .. method:: move_to_end(key, last=True) - Move an existing *key* to either end of an ordered dictionary. The item - is moved to the right end if *last* is true (the default) or to the - beginning if *last* is false. Raises :exc:`KeyError` if the *key* does - not exist:: + Move an existing *key* to either end of an ordered dictionary. The item + is moved to the right end if *last* is true (the default) or to the + beginning if *last* is false. Raises :exc:`KeyError` if the *key* does + not exist:: - >>> d = OrderedDict.fromkeys('abcde') - >>> d.move_to_end('b') - >>> ''.join(d.keys()) - 'acdeb' - >>> d.move_to_end('b', last=False) - >>> ''.join(d.keys()) - 'bacde' + >>> d = OrderedDict.fromkeys('abcde') + >>> d.move_to_end('b') + >>> ''.join(d.keys()) + 'acdeb' + >>> d.move_to_end('b', last=False) + >>> ''.join(d.keys()) + 'bacde' - .. versionadded:: 3.2 + .. versionadded:: 3.2 In addition to the usual mapping methods, ordered dictionaries also support reverse iteration using :func:`reversed`. @@ -849,8 +983,8 @@ semantics pass-in keyword arguments using a regular unordered dictionary. .. seealso:: - `Equivalent OrderedDict recipe <http://code.activestate.com/recipes/576693/>`_ - that runs on Python 2.4 or later. + `Equivalent OrderedDict recipe <http://code.activestate.com/recipes/576693/>`_ + that runs on Python 2.4 or later. :class:`OrderedDict` Examples and Recipes ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ @@ -893,7 +1027,7 @@ original insertion position is changed and moved to the end:: An ordered dictionary can be combined with the :class:`Counter` class so that the counter remembers the order elements are first encountered:: - class OrderedCounter(Counter, OrderedDict): + class OrderedCounter(Counter, OrderedDict): 'Counter that remembers the order elements are first encountered' def __repr__(self): @@ -914,19 +1048,19 @@ attribute. .. class:: UserDict([initialdata]) - Class that simulates a dictionary. The instance's contents are kept in a - regular dictionary, which is accessible via the :attr:`data` attribute of - :class:`UserDict` instances. If *initialdata* is provided, :attr:`data` is - initialized with its contents; note that a reference to *initialdata* will not - be kept, allowing it be used for other purposes. + Class that simulates a dictionary. The instance's contents are kept in a + regular dictionary, which is accessible via the :attr:`data` attribute of + :class:`UserDict` instances. If *initialdata* is provided, :attr:`data` is + initialized with its contents; note that a reference to *initialdata* will not + be kept, allowing it be used for other purposes. - In addition to supporting the methods and operations of mappings, - :class:`UserDict` instances provide the following attribute: + In addition to supporting the methods and operations of mappings, + :class:`UserDict` instances provide the following attribute: - .. attribute:: data + .. attribute:: data - A real dictionary used to store the contents of the :class:`UserDict` - class. + A real dictionary used to store the contents of the :class:`UserDict` + class. @@ -944,19 +1078,19 @@ to work with because the underlying list is accessible as an attribute. .. class:: UserList([list]) - Class that simulates a list. The instance's contents are kept in a regular - list, which is accessible via the :attr:`data` attribute of :class:`UserList` - instances. The instance's contents are initially set to a copy of *list*, - defaulting to the empty list ``[]``. *list* can be any iterable, for - example a real Python list or a :class:`UserList` object. + Class that simulates a list. The instance's contents are kept in a regular + list, which is accessible via the :attr:`data` attribute of :class:`UserList` + instances. The instance's contents are initially set to a copy of *list*, + defaulting to the empty list ``[]``. *list* can be any iterable, for + example a real Python list or a :class:`UserList` object. - In addition to supporting the methods and operations of mutable sequences, - :class:`UserList` instances provide the following attribute: + In addition to supporting the methods and operations of mutable sequences, + :class:`UserList` instances provide the following attribute: - .. attribute:: data + .. attribute:: data - A real :class:`list` object used to store the contents of the - :class:`UserList` class. + A real :class:`list` object used to store the contents of the + :class:`UserList` class. **Subclassing requirements:** Subclasses of :class:`UserList` are expect to offer a constructor which can be called with either no arguments or one @@ -981,168 +1115,10 @@ attribute. .. class:: UserString([sequence]) - Class that simulates a string or a Unicode string object. The instance's - content is kept in a regular string object, which is accessible via the - :attr:`data` attribute of :class:`UserString` instances. The instance's - contents are initially set to a copy of *sequence*. The *sequence* can - 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`. + Class that simulates a string or a Unicode string object. The instance's + content is kept in a regular string object, which is accessible via the + :attr:`data` attribute of :class:`UserString` instances. The instance's + contents are initially set to a copy of *sequence*. The *sequence* can + 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. |