bpo-35838: Fix duplicate optionxform calls on a section's option

[tirkarthi]

optionxform is called to check for duplicates and the transformed key is passed to set which again calls optionxform making two transformations to the key. This can be a problem when optionxform is not idempotent.

https://bugs.python.org/issue35838

[tirkarthi]

Friendly ping @methane . It would be helpful if this can be reviewed, I think it's an issue or this can be documented as in https://bugs.python.org/issue35838#msg334452

[methane]

Did you confirm non-idempotent optionxform is supported all other cases?

I fixed the issue locally by way similar to you. But I was not sure about it.
So I haven't sent a PR.

[tirkarthi]

From my reading of the code I couldn't come across any place where the optionxform is called twice causing a problem with respect to being idempotent. My initial guess was about template interpolation but they also apply optionxform only once and using optionxform = lambda x: f'{(x)}' didn't cause problem. There were also no docs or git log information where it was mentioned that optionxform should be idempotent. So I assumed it to be a bug. Sorry, I was not aware about the PR. Maybe @ambv might be able to have additional info about this.

[methane]

I couldn't come across any place where the optionxform is called twice

Even if all APIs doesn't call optionxform twice, it can be called twice while using some APIs.
For example:

>>> import configparser
>>> cfg = configparser.ConfigParser()
>>> cfg.optionxform = lambda s: "#"+s
>>> cfg.add_section("sec")
>>> cfg.set("sec", "foo", "1")
>>> cfg["sec2"] = cfg["sec"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/inada-n/work/python/cpython/Lib/configparser.py", line 974, in __setitem__
    self.read_dict({key: value})
  File "/Users/inada-n/work/python/cpython/Lib/configparser.py", line 747, in read_dict
    for key, value in keys.items():
  File "/Users/inada-n/work/python/cpython/Lib/_collections_abc.py", line 744, in __iter__
    yield (key, self._mapping[key])
  File "/Users/inada-n/work/python/cpython/Lib/configparser.py", line 1254, in __getitem__
    raise KeyError(key)
KeyError: '#foo'

(Note that I checked out this PR and tested above case)

[tirkarthi]

Closing this PR as there is no consensus on optionxform to be idempotent. I will reopen it if needed. Thanks @methane for the details.