python.rst (6768B)
1 .. _python: 2 3 =========================== 4 Python and the Build System 5 =========================== 6 7 The Python programming language is used significantly in the build 8 system. If we need to write code for the build system or for a tool 9 related to the build system, Python is typically the first choice. 10 11 Python Requirements 12 =================== 13 14 The tree requires Python 3.9 or greater to build. 15 All Python packages not in the Python distribution are included in the 16 source tree. So all you should need is a vanilla Python install and you 17 should be good to go. 18 19 Only CPython (the Python distribution available from www.python.org) is 20 supported. 21 22 Compiled Python Packages 23 ======================== 24 25 There are some features of the build that rely on compiled Python packages 26 (packages containing C source). These features are currently all 27 optional because not every system contains the Python development 28 headers required to build these extensions. 29 30 We recommend you have the Python development headers installed (``mach 31 bootstrap`` should do this for you) so you can take advantage of these 32 features. 33 34 Issues with OS X System Python 35 ============================== 36 37 The Python that ships with OS X has historically been littered with 38 subtle bugs and suboptimalities. 39 40 OS X 10.8 and below users will be required to install a new Python 41 distribution. This may not be necessary for OS X 10.9+. However, we 42 still recommend installing a separate Python because of the history with 43 OS X's system Python issues. 44 45 We recommend installing Python through Homebrew or MacPorts. If you run 46 ``mach bootstrap``, this should be done for you. 47 48 Virtual Environments 49 ==================== 50 51 The build system relies heavily on 52 `venv <https://docs.python.org/3/library/venv.html>`_. Venv provides 53 standalone and isolated Python "virtual environments". The problem a venv 54 solves is that of dependencies across multiple Python components. If two 55 components on a system relied on different versions of a package, there 56 could be a conflict. Instead of managing multiple versions of a package 57 simultaneously, Python and venv take the route that it is easier 58 to just keep them separate so there is no potential for conflicts. 59 60 Very early in the build process, a venv is created inside the 61 :term:`object directory`. The venv is configured such that it can 62 find all the Python packages in the source tree. The code for this lives 63 in ``mach.site``. 64 65 Deficiencies 66 ------------ 67 68 There are numerous deficiencies with the way virtual environments are 69 handled in the build system. 70 71 * mach reinvents the venv. 72 73 There is code in ``build/mach_initialize.py`` that configures ``sys.path`` 74 much the same way the venv does. There are various bugs tracking 75 this. However, no clear solution has yet been devised. It's not a huge 76 problem and thus not a huge priority. 77 78 * They aren't preserved across copies and packaging. 79 80 If you attempt to copy an entire tree from one machine to another or 81 from one directory to another, chances are the venv will fall 82 apart. It would be nice if we could preserve it somehow. Instead of 83 actually solving portable venv, all we really need to solve is 84 encapsulating the logic for populating the venv along with all 85 dependent files in the appropriate place. 86 87 * .pyc files written to source directory. 88 89 We rely heavily on ``.pth`` files in our venv. A ``.pth`` file 90 is a special file that contains a list of paths. Python will take the 91 set of listed paths encountered in ``.pth`` files and add them to 92 ``sys.path``. 93 94 When Python compiles a ``.py`` file to bytecode, it writes out a 95 ``.pyc`` file so it doesn't have to perform this compilation again. 96 It puts these ``.pyc`` files alongside the ``.pyc`` file. Python 97 provides very little control for determining where these ``.pyc`` files 98 go, even in Python 3 (which offers customer importers). 99 100 With ``.pth`` files pointing back to directories in the source tree 101 and not the object directory, ``.pyc`` files are created in the source 102 tree. This is bad because when Python imports a module, it first looks 103 for a ``.pyc`` file before the ``.py`` file. If there is a ``.pyc`` 104 file but no ``.py`` file, it will happily import the module. This 105 wreaks havoc during file moves, refactoring, etc. 106 107 There are various proposals for fixing this. See bug 795995. 108 109 Installing Python Manually 110 ========================== 111 112 We highly recommend you use your system's package manager or a 113 well-supported 3rd party package manager to install Python for you. If 114 these are not available to you, we recommend the following tools for 115 installing Python: 116 117 * `buildout.python <https://github.com/collective/buildout.python>`_ 118 * `pyenv <https://github.com/yyuu/pyenv>`_ 119 * An official installer from http://www.python.org. 120 121 If all else fails, consider compiling Python from source manually. But this 122 should be viewed as the least desirable option. 123 124 Common Issues with Python 125 ========================= 126 127 Upgrading your Python distribution breaks the venv 128 -------------------------------------------------------- 129 130 If you upgrade the Python distribution (e.g. install Python 3.6.15 131 from 3.6.9), chances are parts of the venv will break. 132 This commonly manifests as a cryptic ``Cannot import XXX`` exception. 133 More often than not, the module being imported contains binary/compiled 134 components. 135 136 If you upgrade or reinstall your Python distribution, we recommend 137 clobbering your build. 138 139 Packages installed at the system level conflict with build system's 140 ------------------------------------------------------------------- 141 142 It is common for people to install Python packages using ``sudo`` (e.g. 143 ``sudo pip install psutil``) or with the system's package manager 144 (e.g. ``apt-get install python-mysql``. 145 146 A problem with this is that packages installed at the system level may 147 conflict with the package provided by the source tree. As of bug 907902 148 and changeset f18eae7c3b27 (September 16, 2013), this should no longer 149 be an issue since the venv created as part of the build doesn't 150 add the system's ``site-packages`` directory to ``sys.path``. However, 151 poorly installed packages may still find a way to creep into the mix and 152 interfere with our venv. 153 154 As a general principle, we recommend against using your system's package 155 manager or using ``sudo`` to install Python packages. Instead, create 156 virtual environments and isolated Python environments for all of your 157 Python projects. 158 159 Python on $PATH is not appropriate 160 ---------------------------------- 161 162 Tools like ``mach`` will look for Python by performing ``/usr/bin/env 163 python`` or equivalent. Please be sure the appropriate Python 2.7.3+ 164 path is on $PATH. On OS X, this likely means you'll need to modify your 165 shell's init script to put something ahead of ``/usr/bin``.