diff --git a/Doc/howto/sorting.rst b/Doc/howto/sorting.rst index b98f91e023b..70c34cde8a0 100644 --- a/Doc/howto/sorting.rst +++ b/Doc/howto/sorting.rst @@ -47,11 +47,14 @@ lists. In contrast, the :func:`sorted` function accepts any iterable. Key Functions ============= -Both :meth:`list.sort` and :func:`sorted` have a *key* parameter to specify a -function (or other callable) to be called on each list element prior to making +The :meth:`list.sort` method and the functions :func:`sorted`, +:func:`min`, :func:`max`, :func:`heapq.nsmallest`, and +:func:`heapq.nlargest` have a *key* parameter to specify a function (or +other callable) to be called on each list element prior to making comparisons. -For example, here's a case-insensitive string comparison: +For example, here's a case-insensitive string comparison using +:meth:`str.casefold`: .. doctest:: @@ -272,6 +275,70 @@ to make it usable as a key function:: sorted(words, key=cmp_to_key(strcoll)) # locale-aware sort order +Strategies For Unorderable Types and Values +=========================================== + +A number of type and value issues can arise when sorting. +Here are some strategies that can help: + +* Convert non-comparable input types to strings prior to sorting: + +.. doctest:: + + >>> data = ['twelve', '11', 10] + >>> sorted(map(str, data)) + ['10', '11', 'twelve'] + +This is needed because most cross-type comparisons raise a +:exc:`TypeError`. + +* Remove special values prior to sorting: + +.. doctest:: + + >>> from math import isnan + >>> from itertools import filterfalse + >>> data = [3.3, float('nan'), 1.1, 2.2] + >>> sorted(filterfalse(isnan, data)) + [1.1, 2.2, 3.3] + +This is needed because the `IEEE-754 standard +`_ specifies that, "Every NaN +shall compare unordered with everything, including itself." + +Likewise, ``None`` can be stripped from datasets as well: + +.. doctest:: + + >>> data = [3.3, None, 1.1, 2.2] + >>> sorted(x for x in data if x is not None) + [1.1, 2.2, 3.3] + +This is needed because ``None`` is not comparable to other types. + +* Convert mapping types into sorted item lists before sorting: + +.. doctest:: + + >>> data = [{'a': 1}, {'b': 2}] + >>> sorted(data, key=lambda d: sorted(d.items())) + [{'a': 1}, {'b': 2}] + +This is needed because dict-to-dict comparisons raise a +:exc:`TypeError`. + +* Convert set types into sorted lists before sorting: + +.. doctest:: + + >>> data = [{'a', 'b', 'c'}, {'b', 'c', 'd'}] + >>> sorted(map(sorted, data)) + [['a', 'b', 'c'], ['b', 'c', 'd']] + +This is needed because the elements contained in set types do not have a +deterministic order. For example, ``list({'a', 'b'})`` may produce +either ``['a', 'b']`` or ``['b', 'a']``. + Odds and Ends =============