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.. _descriptorhowto:

:Author: Raymond Hettinger

:Contact: <python at rcn dot com>

.. _sortinghowto:

:Author: Andrew Dalke and Raymond Hettinger

Python lists have a built-in :meth:`list.sort` method that modifies the list

.. doctest::

['a', 'Andrew', 'from', 'is', 'string', 'test', 'This']

The value of the *key* parameter should be a function (or other callable) that

>>> sorted(student_objects, key=lambda student: student.age) # sort by age

[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]

convenience functions to make accessor functions easier and faster. The

:mod:`operator` module has :func:`~operator.itemgetter`,

:func:`~operator.attrgetter`, and a :func:`~operator.methodcaller` function.

>>> sorted(student_objects, key=attrgetter('grade', 'age'))

[('john', 'A', 15), ('dave', 'B', 10), ('jane', 'B', 12)]

Ascending and Descending

For example, to sort the student data by *grade* using the DSU approach:

>>> decorated = [(student.grade, i, student) for i, student in enumerate(student_objects)]

>>> decorated.sort()

>>> [student for grade, i, student in decorated] # undecorate

[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]

However, note that ``<`` can fall back to using :meth:`~object.__gt__` if

* Key functions need not depend directly on the objects being sorted. A key

function can also access external resources. For instance, if the student grades

>>> newgrades = {'john': 'F', 'jane':'A', 'dave': 'C'}

>>> sorted(students, key=newgrades.__getitem__)