300 lines
11 KiB
ReStructuredText
300 lines
11 KiB
ReStructuredText
.. _sortinghowto:
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Sorting HOW TO
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**************
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:Author: Andrew Dalke and Raymond Hettinger
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:Release: 0.1
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Python lists have a built-in :meth:`list.sort` method that modifies the list
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in-place. There is also a :func:`sorted` built-in function that builds a new
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sorted list from an iterable.
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In this document, we explore the various techniques for sorting data using Python.
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Sorting Basics
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==============
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A simple ascending sort is very easy: just call the :func:`sorted` function. It
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returns a new sorted list::
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>>> sorted([5, 2, 3, 1, 4])
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[1, 2, 3, 4, 5]
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You can also use the :meth:`list.sort` method of a list. It modifies the list
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in-place (and returns *None* to avoid confusion). Usually it's less convenient
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than :func:`sorted` - but if you don't need the original list, it's slightly
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more efficient.
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>>> a = [5, 2, 3, 1, 4]
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>>> a.sort()
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>>> a
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[1, 2, 3, 4, 5]
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Another difference is that the :meth:`list.sort` method is only defined for
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lists. In contrast, the :func:`sorted` function accepts any iterable.
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>>> sorted({1: 'D', 2: 'B', 3: 'B', 4: 'E', 5: 'A'})
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[1, 2, 3, 4, 5]
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Key Functions
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=============
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Starting with Python 2.4, both :meth:`list.sort` and :func:`sorted` added a
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*key* parameter to specify a function to be called on each list element prior to
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making comparisons.
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For example, here's a case-insensitive string comparison:
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>>> sorted("This is a test string from Andrew".split(), key=str.lower)
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['a', 'Andrew', 'from', 'is', 'string', 'test', 'This']
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The value of the *key* parameter should be a function that takes a single argument
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and returns a key to use for sorting purposes. This technique is fast because
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the key function is called exactly once for each input record.
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A common pattern is to sort complex objects using some of the object's indices
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as keys. For example:
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>>> student_tuples = [
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('john', 'A', 15),
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('jane', 'B', 12),
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('dave', 'B', 10),
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]
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>>> sorted(student_tuples, key=lambda student: student[2]) # sort by age
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[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
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The same technique works for objects with named attributes. For example:
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>>> class Student:
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def __init__(self, name, grade, age):
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self.name = name
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self.grade = grade
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self.age = age
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def __repr__(self):
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return repr((self.name, self.grade, self.age))
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>>> student_objects = [
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Student('john', 'A', 15),
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Student('jane', 'B', 12),
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Student('dave', 'B', 10),
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]
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>>> sorted(student_objects, key=lambda student: student.age) # sort by age
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[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
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Operator Module Functions
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=========================
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The key-function patterns shown above are very common, so Python provides
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convenience functions to make accessor functions easier and faster. The operator
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module has :func:`operator.itemgetter`, :func:`operator.attrgetter`, and
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starting in Python 2.5 a :func:`operator.methodcaller` function.
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Using those functions, the above examples become simpler and faster:
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>>> from operator import itemgetter, attrgetter
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>>> sorted(student_tuples, key=itemgetter(2))
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[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
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>>> sorted(student_objects, key=attrgetter('age'))
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[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
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The operator module functions allow multiple levels of sorting. For example, to
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sort by *grade* then by *age*:
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>>> sorted(student_tuples, key=itemgetter(1,2))
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[('john', 'A', 15), ('dave', 'B', 10), ('jane', 'B', 12)]
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>>> sorted(student_objects, key=attrgetter('grade', 'age'))
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[('john', 'A', 15), ('dave', 'B', 10), ('jane', 'B', 12)]
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The :func:`operator.methodcaller` function makes method calls with fixed
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parameters for each object being sorted. For example, the :meth:`str.count`
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method could be used to compute message priority by counting the
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number of exclamation marks in a message:
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>>> messages = ['critical!!!', 'hurry!', 'standby', 'immediate!!']
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>>> sorted(messages, key=methodcaller('count', '!'))
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['standby', 'hurry!', 'immediate!!', 'critical!!!']
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Ascending and Descending
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========================
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Both :meth:`list.sort` and :func:`sorted` accept a *reverse* parameter with a
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boolean value. This is using to flag descending sorts. For example, to get the
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student data in reverse *age* order:
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>>> sorted(student_tuples, key=itemgetter(2), reverse=True)
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[('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]
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>>> sorted(student_objects, key=attrgetter('age'), reverse=True)
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[('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]
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Sort Stability and Complex Sorts
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================================
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Starting with Python 2.2, sorts are guaranteed to be `stable
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<http://en.wikipedia.org/wiki/Sorting_algorithm#Stability>`_\. That means that
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when multiple records have the same key, their original order is preserved.
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>>> data = [('red', 1), ('blue', 1), ('red', 2), ('blue', 2)]
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>>> sorted(data, key=itemgetter(0))
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[('blue', 1), ('blue', 2), ('red', 1), ('red', 2)]
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Notice how the two records for *blue* retain their original order so that
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``('blue', 1)`` is guaranteed to precede ``('blue', 2)``.
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This wonderful property lets you build complex sorts in a series of sorting
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steps. For example, to sort the student data by descending *grade* and then
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ascending *age*, do the *age* sort first and then sort again using *grade*:
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>>> s = sorted(student_objects, key=attrgetter('age')) # sort on secondary key
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>>> sorted(s, key=attrgetter('grade'), reverse=True) # now sort on primary key, descending
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[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
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The `Timsort <http://en.wikipedia.org/wiki/Timsort>`_ algorithm used in Python
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does multiple sorts efficiently because it can take advantage of any ordering
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already present in a dataset.
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The Old Way Using Decorate-Sort-Undecorate
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==========================================
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This idiom is called Decorate-Sort-Undecorate after its three steps:
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* First, the initial list is decorated with new values that control the sort order.
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* Second, the decorated list is sorted.
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* Finally, the decorations are removed, creating a list that contains only the
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initial values in the new order.
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For example, to sort the student data by *grade* using the DSU approach:
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>>> decorated = [(student.grade, i, student) for i, student in enumerate(student_objects)]
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>>> decorated.sort()
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>>> [student for grade, i, student in decorated] # undecorate
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[('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]
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This idiom works because tuples are compared lexicographically; the first items
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are compared; if they are the same then the second items are compared, and so
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on.
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It is not strictly necessary in all cases to include the index *i* in the
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decorated list, but including it gives two benefits:
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* The sort is stable -- if two items have the same key, their order will be
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preserved in the sorted list.
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* The original items do not have to be comparable because the ordering of the
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decorated tuples will be determined by at most the first two items. So for
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example the original list could contain complex numbers which cannot be sorted
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directly.
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Another name for this idiom is
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`Schwartzian transform <http://en.wikipedia.org/wiki/Schwartzian_transform>`_\,
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after Randal L. Schwartz, who popularized it among Perl programmers.
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For large lists and lists where the comparison information is expensive to
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calculate, and Python versions before 2.4, DSU is likely to be the fastest way
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to sort the list. For 2.4 and later, key functions provide the same
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functionality.
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The Old Way Using the *cmp* Parameter
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=====================================
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Many constructs given in this HOWTO assume Python 2.4 or later. Before that,
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there was no :func:`sorted` builtin and :meth:`list.sort` took no keyword
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arguments. Instead, all of the Py2.x versions supported a *cmp* parameter to
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handle user specified comparison functions.
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In Py3.0, the *cmp* parameter was removed entirely (as part of a larger effort to
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simplify and unify the language, eliminating the conflict between rich
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comparisons and the :meth:`__cmp__` magic method).
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In Py2.x, sort allowed an optional function which can be called for doing the
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comparisons. That function should take two arguments to be compared and then
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return a negative value for less-than, return zero if they are equal, or return
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a positive value for greater-than. For example, we can do:
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>>> def numeric_compare(x, y):
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return x - y
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>>> sorted([5, 2, 4, 1, 3], cmp=numeric_compare)
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[1, 2, 3, 4, 5]
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Or you can reverse the order of comparison with:
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>>> def reverse_numeric(x, y):
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return y - x
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>>> sorted([5, 2, 4, 1, 3], cmp=reverse_numeric)
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[5, 4, 3, 2, 1]
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When porting code from Python 2.x to 3.x, the situation can arise when you have
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the user supplying a comparison function and you need to convert that to a key
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function. The following wrapper makes that easy to do::
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def cmp_to_key(mycmp):
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'Convert a cmp= function into a key= function'
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class K(object):
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def __init__(self, obj, *args):
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self.obj = obj
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def __lt__(self, other):
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return mycmp(self.obj, other.obj) < 0
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def __gt__(self, other):
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return mycmp(self.obj, other.obj) > 0
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def __eq__(self, other):
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return mycmp(self.obj, other.obj) == 0
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def __le__(self, other):
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return mycmp(self.obj, other.obj) <= 0
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def __ge__(self, other):
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return mycmp(self.obj, other.obj) >= 0
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def __ne__(self, other):
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return mycmp(self.obj, other.obj) != 0
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return K
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To convert to a key function, just wrap the old comparison function:
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>>> sorted([5, 2, 4, 1, 3], key=cmp_to_key(reverse_numeric))
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[5, 4, 3, 2, 1]
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In Python 2.7, the :func:`functools.cmp_to_key` function was added to the
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functools module.
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Odd and Ends
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============
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* For locale aware sorting, use :func:`locale.strxfrm` for a key function or
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:func:`locale.strcoll` for a comparison function.
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* The *reverse* parameter still maintains sort stability (so that records with
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equal keys retain their original order). Interestingly, that effect can be
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simulated without the parameter by using the builtin :func:`reversed` function
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twice:
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>>> data = [('red', 1), ('blue', 1), ('red', 2), ('blue', 2)]
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>>> assert sorted(data, reverse=True) == list(reversed(sorted(reversed(data))))
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* The sort routines are guaranteed to use :meth:`__lt__` when making comparisons
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between two objects. So, it is easy to add a standard sort order to a class by
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defining an :meth:`__lt__` method::
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>>> Student.__lt__ = lambda self, other: self.age < other.age
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>>> sorted(student_objects)
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[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
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For general purpose comparisons, the recommended approach is to define all six
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rich comparison operators. The :func:`functools.total_ordering` class
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decorator makes this easy to implement.
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* Key functions need not depend directly on the objects being sorted. A key
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function can also access external resources. For instance, if the student grades
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are stored in a dictionary, they can be used to sort a separate list of student
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names:
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>>> students = ['dave', 'john', 'jane']
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>>> grades = {'john': 'F', 'jane':'A', 'dave': 'C'}
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>>> sorted(students, key=grades.__getitem__)
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['jane', 'dave', 'john']
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