`cmp_to_key` gives an error and I don't know if this is a bug or just not supported

[secarica]

I want to adapt an IP address sorter from GeeksforGeeks - Sort the given IP addresses in ascending order and it gives an error related to cmp_to_key.

Attached is a test code, which on my system (Windows 10) gives this error:

""" C:\temp>python testcase_natsorted_ip_sort.py Traceback (most recent call last): File "C:\temp\testcase_natsorted_ip_sort.py", line 57, in ip_list_sorted = sortIPAddress(ip_list) ^^^^^^^^^^^^^^^^^^^^^^ File "C:\temp\testcase_natsorted_ip_sort.py", line 39, in sortIPAddress ip_list_sorted = natsorted(ip_list, key = cmp_to_key(customComparator)) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "C:\python31.164\Lib\site-packages\natsort\natsort.py", line 293, in natsorted return sorted(seq, reverse=reverse, key=natsort_keygen(key, alg)) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "C:\python31.164\Lib\site-packages\natsort\utils.py", line 350, in natsort_key return num_func(val) ^^^^^^^^^^^^^ File "C:\python31.164\Lib\site-packages\natsort\utils.py", line 433, in func elif val == _nan_replace: ^^^^^^^^^^^^^^^^^^^ TypeError: other argument must be K instance """

By changing back the natsorted with the original sorted, the code then runs fine.

Is this a bug related to natsorted or I have to find another way to get the key ?

[SethMMorton]

This isn't really supported. natsort works by transforming your input into something that can be sorted naturally, then passing that to the key argument to sorted. When you pass a key to natsort, it assumes that the output of that key is something that natsort can handle.

cmp_to_key returns an object that wraps your cmp function to make it behave like a key. natsort cannot transform this object into something useful, and so you get the weird error you see.

[SethMMorton]

I took a look at the link you provided, and that cmp function they define is the same as giving lambda x: x.split(".") as the key (e.g. sorted(arr, key=lambda x: x.split(".")) or natsorted(arr, key=lambda x: x.split(".")). That's the reason Python removed native support for cmp functions in favor of key - you can often express what you are trying to do WAY simpler with a key than with cmp.

Having said that, you should be able to pass those IP addresses to natsort directly and not need a key at all, and it will work as expected.

[secarica]

Tested -- it works fine with lambda etc. de-compactized version so to say, but I will try also a more direct approach.

Thank you very much -- both for the tip and mostly for natsorted :)