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Py3.0 transition aid. The sorted() builtin and the list.sort() method no longer accept a cmp function in Python 3.0. Most cases are easy to convert manually. This recipe handles the remaining cases.

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def cmp_to_key(mycmp):
    'Convert a cmp= function into a key= function'
    class K(object):
        def __init__(self, obj, *args):
            self.obj = obj
        def __lt__(self, other):
            return mycmp(self.obj, other.obj) < 0
        def __gt__(self, other):
            return mycmp(self.obj, other.obj) > 0
        def __eq__(self, other):
            return mycmp(self.obj, other.obj) == 0
        def __le__(self, other):
            return mycmp(self.obj, other.obj) <= 0  
        def __ge__(self, other):
            return mycmp(self.obj, other.obj) >= 0
        def __ne__(self, other):
            return mycmp(self.obj, other.obj) != 0
    return K

In most cases it is easy to convert old-style to new-style. For instance, l.sort(cmp=lambda x, y: cmp(x.lower(), y.lower) converts directly to l.sort(key=str.lower).

However, sometimes there are extensive existing tests that supply custom cmp functions or a user supplied cmp function (part of the exposed API). In those cases, this wrapper can ease the transition. For instance, l.sort(cmp=mycmp) converts to l.sort(key=cmp_to_key(mycmp)).

The functions above was added to Python 2.7 and Python3.2 in the functools module.

1 comment

Gabriel Genellina 11 years, 10 months ago  # | flag

Warning: the cmp function may return any integer, not just 0, 1, -1. What matters is whether it is greater than, lower than, or equal to 0. __lt__ should be written as:

mycmp(self.obj, other.obj) < 0

and so on.

See note (8) at http://docs.python.org/library/stdtypes.html#mutable-sequence-types

Created by Raymond Hettinger on Wed, 18 Feb 2009 (MIT)
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