bpo-36778: Avoid functools in encodings.cp65001

[vstinner]

Replace functools.partial() with lambda to reduce the number of
imports at startup. Avoid the following imports at startup:

• _collections
• _functools
• _heapq
• _operator
• collections
• functools
• heapq
• itertools
• keyword
• operator
• reprlib

https://bugs.python.org/issue36778

[serhiy-storchaka]

Could you please measure how much overhead it adds?

[vstinner]

Oh, my PR looks wrong. Most CP65001Test tests fail...

[paulmon]

Oh, my PR looks wrong. Most CP65001Test tests fail...

I don't know if it helps, but test_startup_imports passes on IoT Core using this change with a default codepage of cp65001. So it looks like this strategy could work if there's a way to remove functools without breaking other things.

[methane]

staticmethod is required, maybe.

[vstinner]

staticmethod is required, maybe.

Right. I fixed my PR with that. I also rebased my PR and added a NEWS entry.

[vstinner]

@serhiy-storchaka: "Could you please measure how much overhead it adds?"

Benchmark in release mode:

>python -m venv env
>env\scripts\python_d.exe -m pip install perf
>env\scripts\python_d.exe -m perf timeit -s "import codecs; encode=codecs.lookup('cp65001').encode" "encode('')" -v -o ref.json
>git co master
>env\scripts\python_d.exe -m perf timeit -s "import codecs; encode=codecs.lookup('cp65001').encode" "encode('')" -v -o patch.json
env\scripts\python_d.exe -m perf compare_to ref.json patch.json
Mean +- std dev: [ref] 156 ns +- 3 ns -> [patch] 417 ns +- 17 ns: 2.68x slower (+168%)

[vstinner]

I merged my PR #13230 instead.