Python usage notes - iterable stuff

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semi-sorted

Sorting

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)
sorted()
for a sorted copy
object.sort()
for in-place


This is stable sorting, so you can do secondary sorting in multiple passes.

You can also do it in one pass, using key


key argument (py2, py3)

The key parameter should be a callable that fetches the value to sort on from an object.

This allows:

  • normalization, e.g. case-insensitive sort via:
sorted(['foo','BAR'], key=lambda x: x.lower()) == ['BAR', 'foo']
  • specific columns
sorted( [('b',2,),('a',1)], key=lambda l: l[1] ) == [('a', 1), ('b', 2)]
  • multiple columns, or other aspects, when you make the key function return a tuple, for example:
e.g. empty strings last, via:
sorted( ['b',,'A'], key=lambda x: (x==, x.lower())) == ['A', 'b', ]



cmp argument (removed in py3)

You can write an arbitrary comparison function, like

sorted( [[1,2], [2,1]], lambda a,b: cmp(b[1], a[1])) # second column descending

Almost all cmp functions I've seen can be easily rewritten with key (and reverse), possibly decorate-sort-undecorate. In this case:

sorted( [[1,2], [2,1]], key=lambda x: x[1], reverse=True)

As key tends to be faster, cmp is sort of redundant, and python3 removed cmp.


If you've got some cmp functions that are awkward to rewrite, you can use functools.cmp_to_key as a stopgap

Iteratable-related tools

See also: