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diff --git a/build/docs/python.rst b/build/docs/python.rst new file mode 100644 index 0000000000..4edef1244d --- /dev/null +++ b/build/docs/python.rst @@ -0,0 +1,165 @@ +.. _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``. |