[code-quality] Extending pylint to deal with template variables?

[Jonathan Eunice]

I originally asked the following question on Stack
Overflow<http://stackoverflow.com/questions/18500998/extending-pylint-to-deal-with-template-variables>.
It was suggested that this would be the better venue for discussion.

--

I wrote the say <https://pypi.python.org/pypi/say> module to make formatted
printing simpler and more straightforward. E.g.

say("{len(items)} retrieved; {n_errors} encountered")

 rather than:

print("{0} retrieved; {1} encountered".format(len(items), n_errors))

 That part is going great. But I like to run pylint to look for gotchas and
mistakes. Unfortunately, many data values are constructed solely for their
usefulness in output operations, and pylint cannot "see" that use in a say call
(or any other templates output mechanism) constitutes a genuine use of the
variable. Wave after wave of W0612 (unused-variable) warnings can result.

It's possible to simply put in ignore comments, but that seems retrograde.
I'd rather just extend pylint to understand that variables used in
say() templates
are, in fact, used.

.../pylint/checkers/variables.py appears to be the place to add this check,
but I'm unfamiliar with the codebase. Any hints or suggestions as to how to
register variables used in format-style strings are, indeed, properly used?

--

One SO commenter suggested that since pylint doesn't do much with strings,
this may be a fair amount of work. But I already know how to parse
format-style strings; I just use Python's own string.Formatter.parse.
Creating an AST from the contents of the format braces and walking it to
find what names are referenced--I've got that covered. What I most need is
insight into how to communicate to pylint that it should consider the
identifiers I've found to be used.

I'd also seen at least one comment online suggesting that pylint doesn't
really have an extensions/plugin mechanism. Is that true? Either way, what
would be the best way to provide extended functionality atop pylint? Is the
team receptive to code contributions?
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