Is sort unstable in Python when using cmp_to_key?

Question

I want to sort the positive values in a list of numbers, but leave the negative values where they are.

I tried to create a custom comparison function that says that negative numbers are equal to any other number, but this does not seem to work as I expected. Here is the code:

import functools

def my_cmp(a, b):
    if (a < 0) or (b < 0):
        return 0
    elif a < b:
        return -1
    else:
        return 1

x = [2, 7, 5.2, -7, -2, 10, 30, 13.1, -3, 14, 9, 15]

print(sorted(x, key=functools.cmp_to_key(my_cmp)))

The code returns:

[2, 5.2, 7, -7, -2, 9, 10, 13.1, 14, 30, -3, 15]

But I expected

[2, 5.2, 7, -7, -2, 10, 13.1, 30, -3, 9, 14, 15]

since Python's sort is supposed to be stable. Is this not working because the negative numbers are equal to any other number?

Is there a better way to this without using loops? I can do this by breaking up the list and sorting each part corresponding to the positive numbers, but I would like a more elegant way to this.

I guess the problem is that my comparison function is not transitive, since 9 < 10, but 9 == -3 and 10 == -3. — marcm
– marcm, Commented May 4, 2019 at 23:55

sophros · Accepted Answer · 2020-05-23 05:37:26Z

1

Splitting the list into discrete parts based on sign and sorting those is probably easiest. Here's a way using itertools.groupby

from itertools import chain, groupby

x = [2, 7, 5.2, -7, -2, 10, 30, 13.1, -3, 14, 9, 15]

groups = groupby(x, key=lambda a: a > 0)
res = list(chain.from_iterable(sorted(g) if k else g for k, g in groups))

print(res)
# [2, 5.2, 7, -7, -2, 10, 13.1, 30, -3, 9, 14, 15]

edited May 23, 2020 at 5:37

sophros

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answered May 4, 2019 at 23:56

Patrick Haugh

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