Equivalent of sorted function with compare function in Python 3

Question

This works perfectly in Python 2.7

def some_compare_func(x, y):
  ....

a = sorted(some_list, lambda x, y: some_compare_func(x, y))

However, the same gives this error in Python 3.x.

TypeError: sorted expected 1 arguments, got 2

Is there a way to sort with a sorting function that works both in Python 2.7 and 3.x?

But then it doesn't work with Python 2.7 anymore. Man, this is annoying :( — MetallicPriest, Aug 09 '19 at 11:51
Yes, tried it, doesn't work in 2.7. Says TypeError: () takes exactly 1 argument (2 given) — MetallicPriest, Aug 09 '19 at 11:54
Can you show an actual minimal reproducible example? you also have to specify the key via a keyword argument like `key = lambda`... — Chris_Rands, Aug 09 '19 at 11:59
This how-to article describes a wrapper function that can be used to converted `some_compare_func` to be used in Python 3, although it is designed for porting, not for interoperability https://docs.python.org/3/howto/sorting.html — Deepstop, Aug 09 '19 at 12:01
the kwarg `cmp=some_compare_function` has been removed in py3. You can use `key`. — Diptangsu Goswami, Aug 09 '19 at 12:02
Ah- maybe i misunderstood in py3, you need `functools.cmp_to_key` to the old `cmp=` style sorting (it's very rarely useful though) — Chris_Rands, Aug 09 '19 at 12:03
You may want to have a look at the [official documentation](https://docs.python.org/3/howto/sorting.html#the-old-way-using-the-cmp-parameter) that explains how to port code that uses Python 2 sorting to Python 3. — GZ0, Aug 09 '19 at 12:01
It's much more efficient to sort using a `key` function, rather than a `cmp` function. The former only gets called once per item in the list, but the latter must be called every time a comparison is performed, so it's significantly slower. — PM 2Ring, Aug 09 '19 at 12:36

score 5 · Accepted Answer · answered Aug 09 '19 at 12:18

5

You can convert the python2 style cmp to key using functools.cmp_to_key()

The following code should work both in python2 and python3

def some_compare_func(x, y):
  ....

import functools
a = sorted(some_list, key=functools.cmp_to_key(some_compare_func))

answered Aug 09 '19 at 12:18

Sunitha

1 Answers1