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