The Tor Browser / file revision

 .. _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 2.7.3 or greater but not Python 3 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.

 We require Python 2.7.3 (and not say 2.7.2) to build because Python

 2.7.3 contains numerous bug fixes, especially around the area of Unicode

 handling. These bug fixes are extremely annoying and have to be worked

 around. The build maintainers were tired of doing this, so the minimum

 version requirement was upped (bug 870420).

 We intend to eventually support Python 3. This will come by way of dual

 2.7/3.x compatibility because a single flag day conversion to 3.x will

 be too cumbersome given the amount of Python that would need converted.

 We will not know which 3.x minor release we are targeting until this

 effort is underway. This is tracked in bug 636155.

 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. Furthermore, OS X up through 10.8 don't

 ship with Python 2.7.3 (10.8 ships with 2.7.2).

 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.

 Virtualenvs

 ===========

 The build system relies heavily on

 `virtualenvs <http://www.virtualenv.org/en/latest/>`_. Virtualenvs are

 standalone and isolated Python environments. The problem a virtualenv

 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 virtualenvs 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 virtualenv is created inside the

 :term:`object directory`. The virtualenv is configured such that it can

 find all the Python packages in the source tree. The code for this lives

 in :py:mod:`mozbuild.virtualenv`.

 Deficiencies

 ------------

 There are numerous deficiencies with the way virtualenvs are handled in

 the build system.

 * mach reinvents the virtualenv.

   There is code in ``build/mach_bootstrap.py`` that configures ``sys.path``

   much the same way the virtualenv 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 virtualenv will fall

   apart. It would be nice if we could preserve it somehow. Instead of

   actually solving portable virtualenvs, all we really need to solve is

   encapsulating the logic for populating the virtualenv along with all

   dependent files in the appropriate place.

 * .pyc files written to source directory.

   We rely heavily on ``.pth`` files in our virtualenv. 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 determing 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 virtualenv

 --------------------------------------------------------

 If you upgrade the Python distribution (e.g. install Python 2.7.5

 from 2.7.3, chances are parts of the virtualenv 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 virtualenv 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 virtualenv.

 As a general principle, we recommend against using your system's package

 manager or using ``sudo`` to install Python packages. Instead, create

 virtualenvs 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``.