mirror of
https://github.com/mozilla/gecko-dev.git
synced 2024-12-01 00:32:11 +00:00
166 lines
6.6 KiB
ReStructuredText
166 lines
6.6 KiB
ReStructuredText
.. _python:
|
|
|
|
===========================
|
|
Python and the Build System
|
|
===========================
|
|
|
|
The Python programming language is used significantly in the build
|
|
system. If we need to write code for the build system or for a tool
|
|
related to the build system, Python is typically the first choice.
|
|
|
|
Python Requirements
|
|
===================
|
|
|
|
The tree requires Python 3.8 or greater to build.
|
|
All Python packages not in the Python distribution are included in the
|
|
source tree. So all you should need is a vanilla Python install and you
|
|
should be good to go.
|
|
|
|
Only CPython (the Python distribution available from www.python.org) is
|
|
supported.
|
|
|
|
Compiled Python Packages
|
|
========================
|
|
|
|
There are some features of the build that rely on compiled Python packages
|
|
(packages containing C source). These features are currently all
|
|
optional because not every system contains the Python development
|
|
headers required to build these extensions.
|
|
|
|
We recommend you have the Python development headers installed (``mach
|
|
bootstrap`` should do this for you) so you can take advantage of these
|
|
features.
|
|
|
|
Issues with OS X System Python
|
|
==============================
|
|
|
|
The Python that ships with OS X has historically been littered with
|
|
subtle bugs and suboptimalities.
|
|
|
|
OS X 10.8 and below users will be required to install a new Python
|
|
distribution. This may not be necessary for OS X 10.9+. However, we
|
|
still recommend installing a separate Python because of the history with
|
|
OS X's system Python issues.
|
|
|
|
We recommend installing Python through Homebrew or MacPorts. If you run
|
|
``mach bootstrap``, this should be done for you.
|
|
|
|
Virtual Environments
|
|
====================
|
|
|
|
The build system relies heavily on
|
|
`venv <https://docs.python.org/3/library/venv.html>`_. Venv provides
|
|
standalone and isolated Python "virtual environments". The problem a venv
|
|
solves is that of dependencies across multiple Python components. If two
|
|
components on a system relied on different versions of a package, there
|
|
could be a conflict. Instead of managing multiple versions of a package
|
|
simultaneously, Python and venv take the route that it is easier
|
|
to just keep them separate so there is no potential for conflicts.
|
|
|
|
Very early in the build process, a venv is created inside the
|
|
:term:`object directory`. The venv is configured such that it can
|
|
find all the Python packages in the source tree. The code for this lives
|
|
in ``mach.site``.
|
|
|
|
Deficiencies
|
|
------------
|
|
|
|
There are numerous deficiencies with the way virtual environments are
|
|
handled in the build system.
|
|
|
|
* mach reinvents the venv.
|
|
|
|
There is code in ``build/mach_initialize.py`` that configures ``sys.path``
|
|
much the same way the venv does. There are various bugs tracking
|
|
this. However, no clear solution has yet been devised. It's not a huge
|
|
problem and thus not a huge priority.
|
|
|
|
* They aren't preserved across copies and packaging.
|
|
|
|
If you attempt to copy an entire tree from one machine to another or
|
|
from one directory to another, chances are the venv will fall
|
|
apart. It would be nice if we could preserve it somehow. Instead of
|
|
actually solving portable venv, all we really need to solve is
|
|
encapsulating the logic for populating the venv along with all
|
|
dependent files in the appropriate place.
|
|
|
|
* .pyc files written to source directory.
|
|
|
|
We rely heavily on ``.pth`` files in our venv. A ``.pth`` file
|
|
is a special file that contains a list of paths. Python will take the
|
|
set of listed paths encountered in ``.pth`` files and add them to
|
|
``sys.path``.
|
|
|
|
When Python compiles a ``.py`` file to bytecode, it writes out a
|
|
``.pyc`` file so it doesn't have to perform this compilation again.
|
|
It puts these ``.pyc`` files alongside the ``.pyc`` file. Python
|
|
provides very little control for determining where these ``.pyc`` files
|
|
go, even in Python 3 (which offers customer importers).
|
|
|
|
With ``.pth`` files pointing back to directories in the source tree
|
|
and not the object directory, ``.pyc`` files are created in the source
|
|
tree. This is bad because when Python imports a module, it first looks
|
|
for a ``.pyc`` file before the ``.py`` file. If there is a ``.pyc``
|
|
file but no ``.py`` file, it will happily import the module. This
|
|
wreaks havoc during file moves, refactoring, etc.
|
|
|
|
There are various proposals for fixing this. See bug 795995.
|
|
|
|
Installing Python Manually
|
|
==========================
|
|
|
|
We highly recommend you use your system's package manager or a
|
|
well-supported 3rd party package manager to install Python for you. If
|
|
these are not available to you, we recommend the following tools for
|
|
installing Python:
|
|
|
|
* `buildout.python <https://github.com/collective/buildout.python>`_
|
|
* `pyenv <https://github.com/yyuu/pyenv>`_
|
|
* An official installer from http://www.python.org.
|
|
|
|
If all else fails, consider compiling Python from source manually. But this
|
|
should be viewed as the least desirable option.
|
|
|
|
Common Issues with Python
|
|
=========================
|
|
|
|
Upgrading your Python distribution breaks the venv
|
|
--------------------------------------------------------
|
|
|
|
If you upgrade the Python distribution (e.g. install Python 3.6.15
|
|
from 3.6.9), chances are parts of the venv will break.
|
|
This commonly manifests as a cryptic ``Cannot import XXX`` exception.
|
|
More often than not, the module being imported contains binary/compiled
|
|
components.
|
|
|
|
If you upgrade or reinstall your Python distribution, we recommend
|
|
clobbering your build.
|
|
|
|
Packages installed at the system level conflict with build system's
|
|
-------------------------------------------------------------------
|
|
|
|
It is common for people to install Python packages using ``sudo`` (e.g.
|
|
``sudo pip install psutil``) or with the system's package manager
|
|
(e.g. ``apt-get install python-mysql``.
|
|
|
|
A problem with this is that packages installed at the system level may
|
|
conflict with the package provided by the source tree. As of bug 907902
|
|
and changeset f18eae7c3b27 (September 16, 2013), this should no longer
|
|
be an issue since the venv created as part of the build doesn't
|
|
add the system's ``site-packages`` directory to ``sys.path``. However,
|
|
poorly installed packages may still find a way to creep into the mix and
|
|
interfere with our venv.
|
|
|
|
As a general principle, we recommend against using your system's package
|
|
manager or using ``sudo`` to install Python packages. Instead, create
|
|
virtual environments and isolated Python environments for all of your
|
|
Python projects.
|
|
|
|
Python on $PATH is not appropriate
|
|
----------------------------------
|
|
|
|
Tools like ``mach`` will look for Python by performing ``/usr/bin/env
|
|
python`` or equivalent. Please be sure the appropriate Python 2.7.3+
|
|
path is on $PATH. On OS X, this likely means you'll need to modify your
|
|
shell's init script to put something ahead of ``/usr/bin``.
|