[MapProxy-dev] RFC: Introduce linting

[Julien R]

Hello,

I would like to propose adding linting to the mapproxy codebase and enforce
it on the CI. The benefits of linting are mostly that they make reviews
easier because reviewers don't have to point stylistic issues.

Johannes Weskamm originally proposed to introduce linting in this github
issue:
https://github.com/mapproxy/mapproxy/issues/478

I personally have experience with using flake8 professionally for multiple
years and it works great. I started experimenting with setting up flake8 on
the CI here:
https://github.com/mapproxy/mapproxy/pull/486

If this is accepted, the plan would be to:
- Introduce flake8 on CI, but disable all errors at first
- Enable errors one by one in separate commits

This is to avoid submitting a massive PR touching everything. See the
attached 'mapproxy_flake8_stats.txt' file for statistics about the "errors"
found by flake8.

Best regards,
Julien

--
Julien Rebetez
Lead Software & Machine Learning Engineer
Picterra SA
Rue de la Mouline 8, 1022 Chavannes, Switzerland
www.picterra.ch
<https://picterra.ch/?utm_source=signature&utm_medium=email&utm_campaign=43989-en-pt-email%20signature&utm_content=link>
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9     E262 inline comment should start with '# '
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108   E301 expected 1 blank line, found 0
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29    E402 module level import not at top of file
2460  E501 line too long (97 > 79 characters)
1     E502 the backslash is redundant between brackets
57    E701 multiple statements on one line (colon)
5     E702 multiple statements on one line (semicolon)
2     E703 statement ends with a semicolon
55    E711 comparison to None should be 'if cond is not None:'
44    E712 comparison to False should be 'if cond is False:' or 'if not cond:'
5     E713 test for membership should be 'not in'
1     E721 do not compare types, use 'isinstance()'
9     E722 do not use bare 'except'
10    E731 do not assign a lambda expression, use a def
30    E741 ambiguous variable name 'l'
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