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 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.


The build system relies heavily on virtualenvs. 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 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


There are numerous deficiencies with the way virtualenvs are handled in the build system.

  • mach reinvents the virtualenv.

    There is code in build/ 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 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:

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.