[Qemu-devel] Exploring Sphinx, autodoc, apidoc, and coverage tools for python/qemu

[John Snow]

Has anyone on this list experimented with these tools?

I was hoping to use them to document things like the python/machine.py
and python/qmp.py modules to help demonstrate some of our internal
tooling API (for test writers, GSoC/Outreachy interns, folks who want to
script QEMU at a level between writing a CLI driver and using libvirt.)

What follows below is my process trying to enable this and some of the
problems I'm still stuck with, summarized below at the end of this more
exploratory text.


Enabling autodoc:

First, it appears as if enabling the "sphinx-autodoc" tool is not
sufficient for actually generating anything at all when you invoke the
sphinx-generated "make html" target. It just enables understanding
certain directives.

So apparently you need to generate module "stubs" using sphinx-autodoc.
Sphinx uses the sphinx-autodoc extension to understand how to consume
the directives in these stubs.

That strikes me as odd, because these stubs might need to be changed
frequently as code comes and goes; it seems strange that it isn't
integrated at the top level. (Do I have the wrong idea on how these
tools should be used?)

So you need to run:
> sphinx-apidoc --separate --module-first -o docs/ python/qemu/

which generates stubs to docs:

Creating file docs/qemu.machine.rst.
Creating file docs/qemu.qmp.rst.
Creating file docs/qemu.qtest.rst.
Creating file docs/qemu.rst.
Creating file docs/modules.rst.

And then you can edit e.g. the top-level index.rst TOC in docs/index.rst
to look like this:

```
.. toctree::
   :maxdepth: 2
   :caption: Contents:

   interop/index
   devel/index
   specs/index
   modules
```

And then finally generating the build; manually removing the -W option
from the Makefile: there are a lot of warnings in here.

> sphinx-build -n -b html -D version=4.0.92 -D release="4.0.92
(v4.1.0-rc2-34-g160802eb07-dirty)" -d .doctrees/
/home/bos/jhuston/src/qemu/docs/ docs/

Great! that will generate output to docs/index.html which indeed shows
APIdoc comments generated from our Python files. Good.

However, where this gets a little strange is if you look at the
generated stubs. For instance, qemu.machine.rst looks like this:

```
.. automodule:: qemu.machine
    :members:
    :undoc-members:
    :show-inheritance:
```

:undoc-members: says that we want to "document" any members that don't
have a matching apidoc comment by generating a stub.

Oops, but the presence of that stub will cause the sphinx coverage tool
to happily report 100% coverage.

Further oops, pylint doesn't understand apidoc comments and can't be
used as the linter in this case, either.

You can edit the stubs to remove these directives, but these stubs are
generated -- and it doesn't appear like there's a command line option to
change this behavior. ...Hmm.

And either way, the coverage tool only generates a report and not
something with an error code that I could use to gate the build. Same
goes for the general build: if I remove the :undoc-members: parameter,
there's nothing in the autodoc module that appears to throw warnings
when it encounters undocumented parameters or members.

That seems disappointing, because it's hard to keep docstrings up to
date unless they are checked conclusively at build time.


Conclusions:

- the autodoc documentation page doesn't seem to document examples of
how you're expected to write meaningful docstrings for the tool to extract.

- autodoc fools the coverage tool into reporting 100% coverage.

- autodoc can be configured to omit non-documented members to allow the
coverage tool to work, but the configuration is auto-generated and
defaults to always generating documentation for these entities.

- coverage tool doesn't appear like it can be used for gating the build
natively for missing python docs; it only generates a report.

- Even if we script to block on a non-empty report, the coverage tool
only works at the function/class level and does not understand the
concept of missing parameter or return value tags.

- It would seem that it would be the Autodoc module's job to be
responsible for understanding incomplete documentation, but doesn't
appear to. The :param name: syntax is just a ReST "field list" and isn't
parsed semantically by autodoc, sadly.


It looks to me, at a glance, that there's nothing in Sphinx that knows
how to look for and warn about undocumented parameters, exception types,
return values, etc. Hopefully I've missed something and it is possible.

--js