Python 3 preparation: cmp parameter of sorted() function and L.sort() method are gone

[wluebbe]

In Py2 the standard function sorted() and the mutable list method sort() both have an optional parameter cmp= that takes a custom comparison function of two arguments.

The parameter is no more available in Py3.

The recommended way is to use the key= which specifies a function of one argument that is used to extract a comparison key from each list element. This has also better performance.

While there is a function functools.cmp_to_key() to convert an old-style cmp function to a key function, it is preferable to directly use an appropriate key function.

This ticket is tracked as a dependency of meta-ticket ticket:15980.

Depends on #16853
Depends on #17200
Depends on #17229

CC: @tscrim @embray @jdemeyer

Component: python3

Issue created by migration from https://trac.sagemath.org/ticket/16536

[wluebbe]

Demonstrate behavior of cmp= and key= in Py2 and Py3

[wluebbe]

comment:1

Attachment: exercise-cmp-key-parameters.py.gz

The workings of functools.cmp_to_key is nicely explained in stackoverflow.

This is also demonstrated in the attached script.

It is also shown that it is not a good idea the use "cmp=" and "key=" together even in Py2!

[a-andre]

Dependencies: #16853

[a-andre]

Changed dependencies from #16853 to #16853, #17200

[a-andre]

Changed dependencies from #16853, #17200 to #16853, #17200, #17229

[fchapoton]

comment:7

see #21035 for a step in that direction

[fchapoton]

comment:9

This is now mostly done. In some places, cmp was only deprecated.

[fchapoton]

comment:12

another step in #23698

[fchapoton]

comment:13

One should soon take care of removing what was deprecated in #21766.

See #24409 for that

[fchapoton]

comment:14

Maybe we could now close this one ?

[fchapoton]

comment:15

ping ? any objection to close as invalid ?