How can I get 2.x-like sorting behaviour in Python 3.x?

Stupid idea: make a first pass to divide all the different items in groups that can be compared between each other, sort the individual groups and finally concatenate them. I assume that an item is comparable to all members of a group, if it is comparable with the first member of a group. Something like this (Python3):

import itertoolsdef python2sort(x):    it = iter(x)    groups = [[next(it)]]    for item in it:        for group in groups:            try:                item < group[0]  # exception if not comparable                group.append(item)                break            except TypeError:                continue        else:  # did not break, make new group            groups.append([item])    print(groups)  # for debugging    return itertools.chain.from_iterable(sorted(group) for group in groups)

This will have quadratic running time in the pathetic case that none of the items are comparable, but I guess the only way to know that for sure is to check all possible combinations. See the quadratic behavior as a deserved punishment for anyone trying to sort a long list of unsortable items, like complex numbers. In a more common case of a mix of some strings and some integers, the speed should be similar to the speed of a normal sort. Quick test:

In [19]: x = [0, 'one', 2.3, 'four', -5, 1j, 2j,  -5.5, 13 , 15.3, 'aa', 'zz']In [20]: list(python2sort(x))[[0, 2.3, -5, -5.5, 13, 15.3], ['one', 'four', 'aa', 'zz'], [1j], [2j]]Out[20]: [-5.5, -5, 0, 2.3, 13, 15.3, 'aa', 'four', 'one', 'zz', 1j, 2j]

It seems to be a 'stable sort' as well, since the groups are formed in the order the incomparable items are encountered.

This answer aims to faithfully re-create the Python 2 sort order, in Python 3, in every detail.

The actual Python 2 implementation is quite involved, but object.c's default_3way_compare does the final fallback after instances have been given a chance to implement normal comparison rules. This is after individual types have been given a chance to compare (via the __cmp__ or __lt__ hooks).

Implementing that function as pure Python in a wrapper, plus emulating the exceptions to the rules (dict and complex numbers specifically) gives us the same Python 2 sorting semantics in Python 3:

from numbers import Number# decorator for type to function mapping special casesdef per_type_cmp(type_):    try:        mapping = per_type_cmp.mapping    except AttributeError:        mapping = per_type_cmp.mapping = {}    def decorator(cmpfunc):        mapping[type_] = cmpfunc        return cmpfunc    return decoratorclass python2_sort_key(object):    _unhandled_types = {complex}    def __init__(self, ob):       self._ob = ob    def __lt__(self, other):        _unhandled_types = self._unhandled_types        self, other = self._ob, other._ob  # we don't care about the wrapper        # default_3way_compare is used only if direct comparison failed        try:            return self < other        except TypeError:            pass        # hooks to implement special casing for types, dict in Py2 has        # a dedicated __cmp__ method that is gone in Py3 for example.        for type_, special_cmp in per_type_cmp.mapping.items():            if isinstance(self, type_) and isinstance(other, type_):                return special_cmp(self, other)        # explicitly raise again for types that won't sort in Python 2 either        if type(self) in _unhandled_types:            raise TypeError('no ordering relation is defined for {}'.format(                type(self).__name__))        if type(other) in _unhandled_types:            raise TypeError('no ordering relation is defined for {}'.format(                type(other).__name__))        # default_3way_compare from Python 2 as Python code        # same type but no ordering defined, go by id        if type(self) is type(other):            return id(self) < id(other)        # None always comes first        if self is None:            return True        if other is None:            return False        # Sort by typename, but numbers are sorted before other types        self_tname = '' if isinstance(self, Number) else type(self).__name__        other_tname = '' if isinstance(other, Number) else type(other).__name__        if self_tname != other_tname:            return self_tname < other_tname        # same typename, or both numbers, but different type objects, order        # by the id of the type object        return id(type(self)) < id(type(other))@per_type_cmp(dict)def dict_cmp(a, b, _s=object()):    if len(a) != len(b):        return len(a) < len(b)    adiff = min((k for k in a if a[k] != b.get(k, _s)), key=python2_sort_key, default=_s)    if adiff is _s:        # All keys in a have a matching value in b, so the dicts are equal        return False    bdiff = min((k for k in b if b[k] != a.get(k, _s)), key=python2_sort_key)    if adiff != bdiff:        return python2_sort_key(adiff) < python2_sort_key(bdiff)    return python2_sort_key(a[adiff]) < python2_sort_key(b[bdiff])

I incorporated handling dictionary sorting as implemented in Python 2, since that'd be supported by the type itself via a __cmp__ hook. I've stuck to the Python 2 ordering for the keys and values as well, naturally.

I've also added special casing for complex numbers, as Python 2 raises an exception when you try sort to these:

>>> sorted([0.0, 1, (1+0j), False, (2+3j)])Traceback (most recent call last):  File "<stdin>", line 1, in <module>TypeError: no ordering relation is defined for complex numbers

You may have to add more special cases if you want to emulate Python 2 behaviour exactly.

If you wanted to sort complex numbers anyway you'll need to consistently put them with the non-numbers group; e.g.:

# Sort by typename, but numbers are sorted before other typesif isinstance(self, Number) and not isinstance(self, complex):    self_tname = ''else:    self_tname = type(self).__name__if isinstance(other, Number) and not isinstance(other, complex):    other_tname = ''else:    other_tname = type(other).__name__

Some test cases:

>>> sorted([0, 'one', 2.3, 'four', -5], key=python2_sort_key)[-5, 0, 2.3, 'four', 'one']>>> sorted([0, 123.4, 5, -6, 7.89], key=python2_sort_key)[-6, 0, 5, 7.89, 123.4]>>> sorted([{1:2}, {3:4}], key=python2_sort_key)[{1: 2}, {3: 4}]>>> sorted([{1:2}, None, {3:4}], key=python2_sort_key)[None, {1: 2}, {3: 4}]

Not running Python 3 here, but maybe something like this would work. Test to see if doing a "less than" compare on "value" creates an exception and then do "something" to handle that case, like convert it to a string.

Of course you'd still need more special handling if there are other types in your list that are not the same type but are mutually orderable.

from numbers import Realfrom decimal import Decimaldef motley(value):    numeric = Real, Decimal    if isinstance(value, numeric):        typeinfo = numeric    else:        typeinfo = type(value)    try:        x = value < value    except TypeError:        value = repr(value)    return repr(typeinfo), value>>> print sorted([0, 'one', 2.3, 'four', -5, (2+3j), (1-3j)], key=motley)[-5, 0, 2.3, (1-3j), (2+3j), 'four', 'one']