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| -rw-r--r-- | Doc/howto/sorting.rst | 21 |
1 files changed, 17 insertions, 4 deletions
diff --git a/Doc/howto/sorting.rst b/Doc/howto/sorting.rst index 746e40c0bd..acdf3ab086 100644 --- a/Doc/howto/sorting.rst +++ b/Doc/howto/sorting.rst @@ -111,6 +111,15 @@ sort by *grade* then by *age*: >>> sorted(student_objects, key=attrgetter('grade', 'age')) [('john', 'A', 15), ('dave', 'B', 10), ('jane', 'B', 12)] +The :func:`operator.methodcaller` function makes method calls with fixed +parameters for each object being sorted. For example, the :meth:`str.count` +method could be used to compute message priority by counting the +number of exclamation marks in a message: + + >>> messages = ['critical!!!', 'hurry!', 'standby', 'immediate!!'] + >>> sorted(messages, key=methodcaller('count', '!')) + ['standby', 'hurry!', 'immediate!!', 'critical!!!'] + Ascending and Descending ======================== @@ -259,8 +268,8 @@ Odd and Ends * For locale aware sorting, use :func:`locale.strxfrm` for a key function or :func:`locale.strcoll` for a comparison function. -* The *reverse* parameter still maintains sort stability (i.e. records with - equal keys retain the original order). Interestingly, that effect can be +* The *reverse* parameter still maintains sort stability (so that records with + equal keys retain their original order). Interestingly, that effect can be simulated without the parameter by using the builtin :func:`reversed` function twice: @@ -275,12 +284,16 @@ Odd and Ends >>> sorted(student_objects) [('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)] + For general purpose comparisons, the recommended approach is to define all six + rich comparison operators. The :func:`functools.total_ordering` class + decorator makes this easy to implement. + * 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 are stored in a dictionary, they can be used to sort a separate list of student names: >>> students = ['dave', 'john', 'jane'] - >>> newgrades = {'john': 'F', 'jane':'A', 'dave': 'C'} - >>> sorted(students, key=newgrades.__getitem__) + >>> grades = {'john': 'F', 'jane':'A', 'dave': 'C'} + >>> sorted(students, key=grades.__getitem__) ['jane', 'dave', 'john'] |