[code-quality] [feature request] .gitignore option for flake8

[Ian Cordasco]

On Mon, Jan 12, 2015 at 5:28 PM, Laurent Bachelier
<laurent at bachelier.name> wrote:
> On 2015-01-12 23:20, Ben Finney wrote:
>> Austin Macdonald <asmacdo at gmail.com>
>> writes:
>>
>>> I would like if flake8 could ignore all files that match the
>>> .gitignore.
>>
>> Rather than making an option specific to one particular tool, I think
>> the correct way to do this would be to break it in two:
>>
>> * Query $SOME_TOOL for a set of file names. In your case, this would be
>>   something like ‘git clean --dry-run -dX’.
>
> I use something like
> flake8 $(git ls-files|grep '^scripts\|\.py$'|grep ...)
>
> ls-files seems to have a lot of options, you might not even need grep.

So I'm personally -1 on adding this feature to Flake8. There are a few reasons:

1. Already we support per-project and global configuration files for
Flake8. In either (or both) of these we allow the user to specify
things to ignore via --exclude. Ignoring things in git works similarly
(i.e., per-repository and globally) and handling both cases (as would
eventually be demanded) would be untenable, unmaintainable, and rather
complex.

2. We can not (and will not) only support this feature for git.
Supporting it for mercurial would be enough to satisfy me, but then we
would sacrifice the ability to simply read a file for patterns (since
mercurial allows for at least 2 ways (that I know of) to ignore
files).

2. a. Shelling out to git and mercurial is extremely sub-optimal and
subprocesses introduce yet another group of hacks to support on both
Windows, OS X, and Linux (already complicated by using
multiprocessing).

2. b. Git has no elegant API for this

2. c. Using Mercurial's API (according to Mercurial) would probably
made Flake8 would have to be re-licensed as a GPL-family tool, which
is unacceptable.

3. This can already quite easily be configured (globally or
per-project) but would require maintaining a .gitignore and a second
configuration file.

4. People may expect that some file which should not be checked in is
still compliant. Automatically ignoring it because it is ignored by a
*separate* tool will lead to confusion and likely bug reports.

5. Explicit is better than magic. If you want Flake8 to ignore
something, there are already well documented ways of doing so. Flake8
doesn't automatically check commit diffs for you either, nor will it.
It /will/ install a hook for you but it won't do it without your
consent. Adding this would violate internal consistency. There may be
value in a separate extension to /suggest/ patterns or files to pass
to --exclude based on VCS ignore files though.

I encourage others to continue offering solutions for their various
tools. We can turn them into snippets over on
https://gitlab.com/pycqa/flake8/snippets

Cheers,
Ian