Move the code in clashing() in abc.py to the module level

[asmeurer]

This makes linters such as pyflakes not complain about undefined names in the
function. This way is preferable regardless because the doctests for the
function, which have been moved to the module docstring, can actually be run.
Previously, clashing was deleted at the end of the module, so it was
impossible for the doctester to run it.

See #17544 (comment).

References to other Issues or PRs

Brief description of what is fixed or changed

Other comments

Release Notes

• abc
  • document the _clash variables in the sympy.abc docstring

[sympy-bot]

✅

Hi, I am the SymPy bot (v148). I'm here to help you write a release notes entry. Please read the guide on how to write release notes.

Your release notes are in good order.

Here is what the release notes will look like:

• abc
  • document the _clash variables in the sympy.abc docstring (#17548 by @asmeurer)

This will be added to https://github.com/sympy/sympy/wiki/Release-Notes-for-1.5.

Note: This comment will be updated with the latest check if you edit the pull request. You need to reload the page to see it.

Click here to see the pull request description that was parsed.

This makes linters such as pyflakes not complain about undefined names in the
function. This way is preferable regardless because the doctests for the
function, which have been moved to the module docstring, can actually be run.
Previously, clashing was deleted at the end of the module, so it was
impossible for the doctester to run it.

See https://github.com/sympy/sympy/pull/17544#issuecomment-526373164.

<!-- Your title above should be a short description of what
was changed. Do not include the issue number in the title. -->

#### References to other Issues or PRs
<!-- If this pull request fixes an issue, write "Fixes #NNNN" in that exact
format, e.g. "Fixes #1234". See
https://github.com/blog/1506-closing-issues-via-pull-requests . Please also
write a comment on that issue linking back to this pull request once it is
open. -->


#### Brief description of what is fixed or changed


#### Other comments


#### Release Notes

<!-- Write the release notes for this release below. See
https://github.com/sympy/sympy/wiki/Writing-Release-Notes for more information
on how to write release notes. The bot will check your release notes
automatically to see if they are formatted correctly. -->

<!-- BEGIN RELEASE NOTES -->
* abc
  * document the `_clash` variables in the sympy.abc docstring
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[oscarbenjamin]

This seems to have broken doctests that import from abc somehow

[asmeurer]

Ah, that sort of thing is probably why this code was in a function.

[codecov]

Codecov Report

Merging #17548 into master will increase coverage by 0.025%.
The diff coverage is 100%.

@@              Coverage Diff              @@
##            master    #17548       +/-   ##
=============================================
+ Coverage   74.711%   74.736%   +0.025%     
=============================================
  Files          633       633               
  Lines       164617    164614        -3     
  Branches     38657     38657               
=============================================
+ Hits        122987    123026       +39     
+ Misses       36235     36192       -43     
- Partials      5395      5396        +1

[oscarbenjamin]

Looks good