use abc stdlib module to mark abstract classes

[skarzi]

I have made things!

Checklist

• I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc)
• I have created at least one test case for the changes I have made
• I have updated the documentation for the changes I have made
• I have added my changes to the CHANGELOG.md

Related issues

Closes #1121

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[coveralls]

Pull Request Test Coverage Report for Build 2513

• 14 of 14 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 2506:	0.0%
Covered Lines:	5031
Relevant Lines:	5031

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💛 - Coveralls

[sobolevn]

Just a quick question (because I don't use abc.ABC generally, I prefer to use metaclass=ABCMeta): shouldn't we also mark these classes:

• https://github.com/wemake-services/wemake-python-styleguide/blob/master/wemake_python_styleguide/visitors/base.py#L140
• https://github.com/wemake-services/wemake-python-styleguide/blob/master/wemake_python_styleguide/visitors/base.py#L182
• https://github.com/wemake-services/wemake-python-styleguide/blob/master/wemake_python_styleguide/visitors/base.py#L218
• https://github.com/wemake-services/wemake-python-styleguide/blob/master/wemake_python_styleguide/violations/base.py#L123
• https://github.com/wemake-services/wemake-python-styleguide/blob/master/wemake_python_styleguide/violations/base.py#L135
• https://github.com/wemake-services/wemake-python-styleguide/blob/master/wemake_python_styleguide/violations/base.py#L141
• https://github.com/wemake-services/wemake-python-styleguide/blob/master/wemake_python_styleguide/violations/base.py#L154

What do you think?

[skarzi]

abc.ABC is just small helper - it is object's subclass that use abc.ABCMeta metaclass, so it just makes most common abstract classes definitions more concise.
Generally any class that subclass some ABC class and do NOT override all abstract methods is also ABC class, that's the reason I haven't add metaclass=abc.ABCMeta to BaseFilenameVisitor however I have used @abc.abstractmethod decorator on visit_filename().

We can add metaclass=abc.ABCMeta to other classes, however they aren't real ABC classes, because they already implement run() method (which is the only abstract method they need to implement), so it will be just an information to developers

[skarzi]

Similar story with all violations' base classes, but there even BaseViolation is not a real ABC class, because it doesn't declare any abstract method, however still we can make its class attributes abstract by using 2nd or 3rd solution that I have mentioned in issue #1121 comment

[sobolevn]

so it will be just an information to developers

Yes, please! That was the main idea behind this change.
Sometimes developers what to see what classes are base ones (that how this task was originally born). And this (I hope) is going to help them.

[sobolevn]

Thanks a lot! 😄