Support version ranges

[pglass]

• Update the README with the new feature
• Rewrite check-pip-version.sh in python, to support checking a version
  (19.0) is contained within a version set ('!=19.1,>18.0'). This has a
  dependency on packaging which must be installed in each tox env,
  unfortunately (note: test requirement only, no changes to the setup.py)
• Add tests/test-version-specifiers/ to test the new version ranges

Fixes #12
Fixes #15

[jayvdb]

Your PR intro comment sounds scary -- the dependency on packaging looks like it is only needed for the local tests here... -- or does it need to be installed into venvs in production usage of the plugin?

A dependency on packaging should only be in the outer environment. To do that, the parsing needs to be in an earlier phase -- tox_configure is a good hook for this -- all of the raw venv configs are available in tox_configure. Then inside the venv the plugin would trust that the value has already been validated.

[pglass]

@jayvdb The dependency on packaging is only for the tests. No changes to the requirements in setup.py.

[jayvdb]

Great. "unfortunately" in the commit message is what triggered me. If it is only a test dependency, it isnt unfortunate ;-)

[jayvdb]

Why allow a prefix of pip ?

Why should it be prefixed with only pip? Why not setuptools, etc :P

Does allowing pip prefix help with anything?

If not, I think you should prevent it in order to avoid misuse, even if it means more work to block that. Extra syntax which achieves nothing invites problems, and forward compatibility problems, in the future.

It does actually make sense to allow setuptools and wheel and anything else that could be needed before the testenv:deps are installed. But that should be a separate enhancement with a bit of design put into it, and potentially the plugin needs a rename then ;-) This plugin is doing a venv prep phase.

[pglass]

Yeah, agreed. Appreciate the good feedback!

@jayvdb I'm actually leaning in the opposite direction, I think. I prefer using the pip-ful (including the prefix) syntax, mainly because I think it's slightly more consistent:

pip command	pip-less	pip-ful
pip install -U pip>19.0	pip_version = >19.0	pip_version = pip>19.0
pip install -U pip<19,>18	pip_version = <19,>18	pip_version = pip<19,>18
pip install -U pip	pip_version = '' pip_version = latest ???	pip_version = pip

That last line in the table is where there was no exact equivalent to a pip install -U pip and I didn't really like either empty string or latest to support that (open to suggestions though). It's true that a user can always write >19.0 to get latest, so not a huge practical concern.

In the future, I might expand this to pip_version = <flags> <packages> and pass those arguments straight to pip install <flags> <packages> to support other pip flags/options. So this is one step in that direction (and definitely hear you on the package rename, and moving toward treating this as a generic "venv prep" plugin, which makes sense to me).

Anyway, I agree that either the "pip-less" or "pip-ful" syntax should be removed. I don't feel that strongly one or another, but am leaning toward removing the "pip-less" syntax.

[pglass]

One other point I missed:

Whichever syntax is settled on in this PR, I did want to keep the pip_version = 19.0 syntax (without any specifiers), for backwards-compatibility for now

[drewbrew]

+1 to removing pip-less

[jayvdb]

pip_version = >=19.0 is much more useful and expressive than pip_version = pip, or even pip_version = latest.

The former creates a log entry in the codebase history of what version was actually required at that time, without needing to see the build logs to know what version was being installed. People will likely only bump it occasionally, because the -U means they wont notice if the version specified isnt good enough for the usage of pip within the testenv, but it will encourage them to think about it occasionally.

[pglass]

I'm not sure this is an argument one way or another? You will still be able to write pip_version = pip>=19.0.

In any case, updated to remove the "pip-less" syntax.