Re: [PATCH 0/7] python: create installable package

[Cleber Rosa]

On Tue, Jun 02, 2020 at 08:15:16PM -0400, John Snow wrote:
> Based-on: 20200602214528.12107-1-jsnow@redhat.com
> 
> This series factors the python/qemu directory as an installable
> module. As a developer, you can install this to your virtual environment
> and then always have access to the classes contained within without
> needing to wrangle python import path problems.
>

First of all, major kudos for picking up this task.  It's so high in
importance to so many users (myself included) that I feel like I owe
you many truck loads of beers now. :)

> When developing, you could go to qemu/python/ and invoke `pipenv shell`
> to activate a virtual environment within which you could type `pip
> install -e .` to install a special development version of this package
> to your virtual environment. This package will always reflect the most
> recent version of the source files in the tree.
> 
> When not developing, you could install a version of this package to your
> environment outright to gain access to the QMP and QEMUMachine classes
> for lightweight scripting and testing.
> 
> This package is formatted in such a way that it COULD be uploaded to
> https://pypi.org/project/qemu and installed independently of qemu.git
> with `pip install qemu`, but of course that button remains unpushed.
> 
> There are a few major questions to answer first:
> 
> - What versioning scheme should we use? See patch 2.
> 
> - Should we use a namespace package or not?
>   - Namespaced: 'qemu.machine', 'qemu.monitor' etc may be separately
>     versioned, packaged and distributed packages. Third party authors
>     may register 'qemu.xxx' to create plugins within the namespace as
>     they see fit.
> 
>   - Non-namespaced: 'qemu' is one giant glob package, packaged and
>     versioned in unison. We control this package exclusively.
>

For simplicity sake, I'd suggest starting with non-namespaced
approach.  It should be easier to move to a namespaced package if the
need arises.  Also, there are many ways to extend Python code without
necessarily requiring third party authors to register their packages
according to a namespace.

In the Avocado project, we have been using setuptools entrypoints with
a reasonable level of success.  Anyone can have code under any
namespace whatsoever extending Avocado, as long as it register their
entrypoints.

> - How do we eliminate sys.path hacks from the rest of the QEMU tree?
>   (Background: sys.path hacks generally impede the function of static
>   code quality analysis tools like mypy and pylint.)
> 
>   - Simplest: parent scripts (or developer) needs to set PYTHONPATH.
> 
>   - Harder: Python scripts should all be written to assume package form,
>     all tests and CI that use Python should execute within a VENV.
>

Having a venv is desirable, but it's not really necessary.  As long as
"python setup.py develop --user" is called, that user can access this
code without sys.path hacks.  And if the user chooses to use a venv,
it's just an extra step.

In the Avocado project, we have a `make develop` rule that does that
for the main setup.py file, and for all plugins we carry on the same
tree, which is similar in some regards to the "not at the project root
directory" situation here with "qemu/python/setup.py".

>   In either case, we lose the ability (for many scripts) to "just run" a
>   script out of the source tree if it depends on other QEMU Python
>   files. This is annoying, but as the complexity of the Python lib
>   grows, it is unavoidable.
>

Like I said before, we may introduce a "make develop"-like
requirement, but after that, I don't think we'll loose anything.
Also, I think this is just a sign of maturity.  We should be using
Python as it's inteded to be used, and sys.path hacks is not among
those.

>   In the VENV case, we at least establish a simple paradigm: the scripts
>   should work in their "installed" forms; and the rest of the build and
>   test infrastructure should use this VENV to automatically handle
>   dependencies and path requirements. This should allow us to move many
>   of our existing python scripts with "hidden" dependencies into a
>   proper python module hierarchy and test for regressions with mypy,
>   flake8, pylint, etc.
> 
>   (We could even establish e.g. Sphinx versions as a dependency for our
>   build kit here and make sure it's installed to the VENV.)
> 
>   Pros: Almost all scripts can be moved under python/qemu/* and checked
>   with CQA tools. imports are written the same no matter where you are
>   (Use the fully qualified names, e.g. qemu.core.qmp.QMPMessage).
>   Regressions in scripts are caught *much* faster.
> 
>   Downsides: Kind of annoying; most scripts now require you to install a
>   devkit forwarder (pip3 install --user .) or be inside of an activated
>   venv. Not too bad if you know python at all, but it's certainly less
>   plug-n-play.
> 
> - What's our backwards compatibility policy if we start shipping this?
> 
>   Proposed: Attempt to maintain API compatibility (after stabilizing the
>   library). Incompatible changes should probably cause a semver bump.
>
>   Each published release makes no attempt to support any version of QEMU
>   other than the one it was released against. We publish this on the tin
>   in big red letters.

It may be too early to tell, but it's not clear to me how we'll keep
both the QEMU version supported by a given release, and its API
"level".

Are you proposing that we have, say, "python-qemu" version 10, being
the 10th API version, without any regard to the QEMU version
supported?  Or version 10.5.3 would mean 10th API version, intended
to support QEMU 5.3?

> 
> TESTING THIS PACKAGE OUT:
> 
> 1. You can install to your local user's environment normally by
> navigating to qemu/python/ and typing "pip3 install --user ."
>

s/install/develop/ should be a better option here.  I mean, I'm
not aware of any reason to user install while developing.

- Cleber.

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