Source

packaging_cpython / Doc / packaging / enduser / installpackages.rst

Full commit

Installing Python Projects

Author: Greg Ward and Packaging contributors
Release:|version|
Date: |today|

Abstract

This document describes the Python Distribution Utilities ("Packaging") from the end-user's point of view, and explains how to extend the functionality of a standard Python installation by building and installing third-party Python modules and extensions.

Introduction

Although Python's extensive standard library covers many programming needs, there often comes a time when you need to add new functionality to your Python installation in the form of third-party modules. This might be necessary to support your own programming, or to support an application that you want to use and that happens to be written in Python.

In the past, there was little support for adding third-party modules to an existing Python installation. End-users had to rely on easy_install or pip to install third-party modules from PyPI. With the introduction of the new Python Distribution Utilities (Packaging) in Python 3.3, this has changed.

This document is aimed primarily at people who need to install third-party Python modules: end-users and system administrators who just need to get some Python application running, and existing Python programmers who want to add new goodies to their toolbox. You don't need to know Python to read this document; there will be some brief forays into using Python's interactive mode to explore your installation, but that's it. If you're looking for information on how to distribute your own Python modules so that others may use them, see the :ref:`packaging-index` manual.

Best case: trivial installation

In the best case, someone will have prepared a special version of the module distribution you want to install that is targeted specifically at your platform and can be installed just like any other software on your platform. For example, the module's developer might make an executable installer available for Windows users, an RPM package for users of RPM-based Linux systems (Red Hat, SuSE, Mandrake, and many others), a Debian package for users of Debian-based Linux systems, and so forth.

In that case, you would download the specific installer for your platform and do the obvious thing with it: run it if it's an executable installer, rpm --install it if it's an RPM, etc. No need to run Python or a setup script, much less to compile anything ---you might not even need to read any instructions, although it's always a good idea to do so anyways.

Of course, things will not always be that easy. You might be interested in a module whose distribution doesn't have an easy-to-use installer for your platform. In that case, you'll have to start with the source distribution released by the module's author/maintainer. Installing from a source distribution is not too hard, as long as the modules are packaged in the standard way. The bulk of this document addresses the building and installing of modules from standard source distributions.

The new standard: Packaging

If you download a source distribution of a module, it will be obvious whether it was packaged and distributed in the standard way, i.e. using Packaging. First, the distribution's name and version number will be featured prominently in the name of the downloaded archive, e.g. :file:`foo-1.0.tar.gz` or :file:`widget-0.9.7.zip`. Next, the archive will unpack into a similarly-named directory: :file:`foo-1.0` or :file:`widget-0.9.7`. Additionally, the distribution will contain a metadata :file:`setup.cfg`, and a file named :file:`README.txt` ---or possibly just :file:`README`--- explaining that building and installing the module distribution is a simple matter of issuing the following command at your shell's prompt:

pysetup install

If all these things are true, then you already know how to build and install the modules you've just downloaded: by running the command above. If fact, unless you have to perform non-standard installations or customize the build process, you can stop reading this manual ---the above command is everything you need to get out of it.

Standard build and install

As described in section :ref:`packaging-new-standard`, building and installing a module distribution using Packaging usually comes down to one simple command:

pysetup install

How you actually run this command depends on the platform and the command line interface it provides:

Platform variations

The setup command is meant to be run from the root directory of the source distribution, i.e. the top-level subdirectory that the module source distribution unpacks into. For example, if you've just downloaded a module source distribution :file:`foo-1.0.tar.gz` onto a Unix system, the normal steps to follow are these:

gunzip -c foo-1.0.tar.gz | tar xf -    # unpacks into directory foo-1.0
cd foo-1.0
pysetup install

On Windows, you'd probably download :file:`foo-1.0.zip`. If you downloaded the archive file to :file:`C:\\Temp`, then it would unpack into :file:`C:\\Temp\\foo-1.0`. To actually unpack the archive, you can use either an archive manipulator with a graphical user interface (such as WinZip or 7-Zip) or a command-line tool (such as :program:`unzip`, :program:`pkunzip` or, again, :program:`7z`). Then, open a command prompt window ("DOS box" or Powershell), and run:

cd c:\Temp\foo-1.0
pysetup install

Splitting the job up

Running pysetup install builds and installs all modules in one go. If you prefer to work incrementally ---especially useful if you want to customize the build process, or if things are going wrong--- you can use the setup script to do one thing at a time. This is a valuable tool when different users will perform separately the build and install steps. For example, you might want to build a module distribution and hand it off to a system administrator for installation (or do it yourself, but with super-user or admin privileges).

For example, to build everything in one step and then install everything in a second step, you aptly invoke two distinct Packaging commands:

python -m packaging.run build
python -m packaging.run install

If you do this, you will notice that invoking the :command:`install` command first runs the :command:`build` command, which ---in this case--- quickly notices it can spare itself the work, since everything in the :file:`build` directory is up-to-date. Also, note that now we're using Packaging as a command line tool instead of pysetup.

You may often ignore this ability to divide the process in steps if all you do is installing modules downloaded from the Internet, but it's very handy for more advanced tasks. If you find yourself in the need for distributing your own Python modules and extensions, though, you'll most likely run many individual Packaging commands.

How building works

As implied above, the :command:`build` command is responsible for collecting and placing the files to be installed into a build directory. By default, this is :file:`build`, under the distribution root. If you're excessively concerned with speed, or want to keep the source tree pristine, you can specify a different build directory with the :option:`--build-base` option. For example:

python -m packaging.run build --build-base /tmp/pybuild/foo-1.0

(Or you could do this permanently with a directive in your system or personal Packaging configuration file; see section :ref:`packaging-config-files`.) In the usual case, however, all this is unnecessary.

The build tree's default layout looks like so:

--- build/ --- lib/
or
--- build/ --- lib.<plat>/
               temp.<plat>/

where <plat> expands to a brief description of the current OS/hardware platform and Python version. The first form, with just a :file:`lib` directory, is used for pure module distributions (module distributions that include only pure Python modules). If a module distribution contains any extensions (modules written in C/C++), then the second form, with two <plat> directories, is used. In that case, the :file:`temp.{plat}` directory holds temporary files generated during the compile/link process which are not intended to be installed. In either case, the :file:`lib` (or :file:`lib.{plat}`) directory contains all Python modules (pure Python and extensions) to be installed.

In the future, more directories will be added to handle Python scripts, documentation, binary executables, and whatever else is required to install Python modules and applications.

How installation works

After the :command:`build` command is run (whether explicitly or by the :command:`install` command on your behalf), the work of the :command:`install` command is relatively simple: all it has to do is copy the contents of :file:`build/lib` (or :file:`build/lib.{plat}`) to the installation directory of your choice.

If you don't choose an installation directory ---i.e., if you just run python -m packaging.run install--- then the :command:`install` command installs to the standard location for third-party Python modules. This location varies by platform and depending on how you built/installed Python itself. On Unix (and Mac OS X, which is also Unix-based), it also depends on whether the module distribution being installed is pure Python or contains extensions ("non-pure"):

Platform Standard installation location Default value Notes
Unix (pure) :file:`{prefix}/lib/python{X.Y}/site-packages` :file:`/usr/local/lib/python{X.Y}/site-packages` (1)
Unix (non-pure) :file:`{exec-prefix}/lib/python{X.Y}/site-packages` :file:`/usr/local/lib/python{X.Y}/site-packages` (1)
Windows :file:`{prefix}\\Lib\\site-packages` :file:`C:\\Python{XY}\\Lib\\site-packages` (2)

Notes:

  1. Most Linux distributions include Python as a standard part of the system, so :file:`{prefix}` and :file:`{exec-prefix}` are usually both :file:`/usr` on Linux. If you build Python yourself on Linux (or any Unix-like system), the default :file:`{prefix}` and :file:`{exec-prefix}` are :file:`/usr/local`.
  2. The default installation directory on Windows was :file:`C:\\Program Files\\Python` under Python 1.6a1, 1.5.2, and earlier.

:file:`{prefix}` and :file:`{exec-prefix}` stand for the directories that Python is installed to, and where it finds its libraries at run-time. They are always the same under Windows, and very often the same under Unix and Mac OS X. You can find out what your Python installation uses for :file:`{prefix}` and :file:`{exec-prefix}` by running Python in interactive mode and typing a few simple commands.

To start the interactive Python interpreter, you need to follow a slightly different recipe for each platform. Under Unix, just type :command:`python` at the shell prompt. Under Windows (assuming the Python executable is on your :envvar:`PATH`, which is the usual case), you can choose :menuselection:`Start --> Run`, type python and press enter. Alternatively, you can simply execute :command:`python` at a command prompt ("DOS console" or Powershell).

Once the interpreter is started, you type Python code at the prompt. For example, on my Linux system, I type the three Python statements shown below, and get the output as shown, to find out my :file:`{prefix}` and :file:`{exec-prefix}`:

Python 3.3 (r32:88445, Apr  2 2011, 10:43:54)
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.prefix
'/usr'
>>> sys.exec_prefix
'/usr'

If you don't want to install modules to the standard location, or if you don't have permission to write there, then you need to read about alternate installations in section :ref:`packaging-alt-install`. If you want to customize your installation directories more heavily, see section :ref:`packaging-custom-install`.

Alternate installation

Often, it is necessary or desirable to install modules to a location other than the standard location for third-party Python modules. For example, on a Unix system you might not have permission to write to the standard third-party module directory. Or you might wish to try out a module before making it a standard part of your local Python installation. This is especially true when upgrading a distribution already present: you want to make sure your existing base of scripts still works with the new version before actually upgrading.

The Packaging :command:`install` command is designed to make installing module distributions to an alternate location simple and painless. The basic idea is that you supply a base directory for the installation, and the :command:`install` command picks a set of directories (called an installation scheme) under this base directory in which to install files. The details differ across platforms, so read whichever of the following sections applies to you.

Alternate installation: the home scheme

The idea behind the "home scheme" is that you build and maintain a personal stash of Python modules. This scheme's name is derived from the concept of a "home" directory on Unix, since it's not unusual for a Unix user to make their home directory have a layout similar to :file:`/usr/` or :file:`/usr/local/`. In spite of its name's origin, this scheme can be used by anyone, regardless of the operating system.

Installing a new module distribution in this way is as simple as

python -m packaging.run install --home <dir>

where you can supply any directory you like for the :option:`--home` option. On Unix, lazy typists can just type a tilde (~); the :command:`install` command will expand this to your home directory:

python -m packaging.run install --home ~

The :option:`--home` option defines the base directory for the installation. Under it, files are installed to the following directories:

Type of file Installation Directory Override option
pure module distribution :file:`{home}/lib/python` :option:`--install-purelib`
non-pure module distribution :file:`{home}/lib/python` :option:`--install-platlib`
scripts :file:`{home}/bin` :option:`--install-scripts`
data :file:`{home}/share` :option:`--install-data`

Alternate installation: Unix (the prefix scheme)

The "prefix scheme" is useful when you wish to use one Python installation to run the build command, but install modules into the third-party module directory of a different Python installation (or something that looks like a different Python installation). If this sounds a trifle unusual, it is ---that's why the "home scheme" comes first. However, there are at least two known cases where the prefix scheme will be useful.

First, consider that many Linux distributions put Python in :file:`/usr`, rather than the more traditional :file:`/usr/local`. This is entirely appropriate, since in those cases Python is part of "the system" rather than a local add-on. However, if you are installing Python modules from source, you probably want them to go in :file:`/usr/local/lib/python2.{X}` rather than :file:`/usr/lib/python2.{X}`. This can be done with

/usr/bin/python -m packaging.run install --prefix /usr/local

Another possibility is a network filesystem where the name used to write to a remote directory is different from the name used to read it: for example, the Python interpreter accessed as :file:`/usr/local/bin/python` might search for modules in :file:`/usr/local/lib/python2.{X}`, but those modules would have to be installed to, say, :file:`/mnt/{@server}/export/lib/python2.{X}`. This could be done with

/usr/local/bin/python -m packaging.run install --prefix=/mnt/@server/export

In either case, the :option:`--prefix` option defines the installation base, and the :option:`--exec-prefix` option defines the platform-specific installation base, which is used for platform-specific files. (Currently, this just means non-pure module distributions, but could be expanded to C libraries, binary executables, etc.) If :option:`--exec-prefix` is not supplied, it defaults to :option:`--prefix`. Files are installed as follows:

Type of file Installation Directory Override option
pure module distribution :file:`{prefix}/lib/python{X.Y}/site-packages` :option:`--install-purelib`
non-pure module distribution :file:`{exec-prefix}/lib/python{X.Y}/site-packages` :option:`--install-platlib`
scripts :file:`{prefix}/bin` :option:`--install-scripts`
data :file:`{prefix}/share` :option:`--install-data`

There is no requirement that :option:`--prefix` or :option:`--exec-prefix` actually point to an alternate Python installation; if the directories listed above do not already exist, they are created at installation time.

Incidentally, the real reason the prefix scheme is important is simply that a standard Unix installation uses the prefix scheme, but with :option:`--prefix` and :option:`--exec-prefix` supplied by Python itself as sys.prefix and sys.exec_prefix. Thus, you might think you'll never use the prefix scheme, but every time you run python -m packaging.run install without any other options, you're using it.

Note that installing extensions to an alternate Python installation doesn't have anything to do with how those extensions are built: in particular, extensions will be compiled using the Python header files (:file:`Python.h` and friends) installed with the Python interpreter used to run the build command. It is therefore your responsibility to ensure compatibility between the interpreter intended to run extensions installed in this way and the interpreter used to build these same extensions. To avoid problems, it is best to make sure that the two interpreters are the same version of Python (possibly different builds, or possibly copies of the same build). (Of course, if your :option:`--prefix` and :option:`--exec-prefix` don't even point to an alternate Python installation, this is immaterial.)

Alternate installation: Windows (the prefix scheme)

Windows has a different and vaguer notion of home directories than Unix, and since its standard Python installation is simpler, the :option:`--prefix` option has traditionally been used to install additional packages to arbitrary locations.

python -m packaging.run install --prefix "\Temp\Python"

to install modules to the :file:`\\Temp\\Python` directory on the current drive.

The installation base is defined by the :option:`--prefix` option; the :option:`--exec-prefix` option is unsupported under Windows. Files are installed as follows:

Type of file Installation Directory Override option
pure module distribution :file:`{prefix}` :option:`--install-purelib`
non-pure module distribution :file:`{prefix}` :option:`--install-platlib`
scripts :file:`{prefix}\\Scripts` :option:`--install-scripts`
data :file:`{prefix}\\Data` :option:`--install-data`

Custom installation

Sometimes, the alternate installation schemes described in section :ref:`packaging-alt-install` just don't do what you want. You might want to tweak just one or two directories while keeping everything under the same base directory, or you might want to completely redefine the installation scheme. In either case, you're creating a custom installation scheme.

You probably noticed the column of "override options" in the tables describing the alternate installation schemes above. Those options are how you define a custom installation scheme. These override options can be relative, absolute, or explicitly defined in terms of one of the installation base directories. (There are two installation base directories, and they are normally the same ---they only differ when you use the Unix "prefix scheme" and supply different :option:`--prefix` and :option:`--exec-prefix` options.)

For example, say you're installing a module distribution to your home directory under Unix, but you want scripts to go in :file:`~/scripts` rather than :file:`~/bin`. As you might expect, you can override this directory with the :option:`--install-scripts` option and, in this case, it makes most sense to supply a relative path, which will be interpreted relative to the installation base directory (in our example, your home directory):

python -m packaging.run install --home ~ --install-scripts scripts

Another Unix example: suppose your Python installation was built and installed with a prefix of :file:`/usr/local/python`. Thus, in a standard installation, scripts will wind up in :file:`/usr/local/python/bin`. If you want them in :file:`/usr/local/bin` instead, you would supply this absolute directory for the :option:`--install-scripts` option:

python -m packaging.run install --install-scripts /usr/local/bin

This command performs an installation using the "prefix scheme", where the prefix is whatever your Python interpreter was installed with ---in this case, :file:`/usr/local/python`.

If you maintain Python on Windows, you might want third-party modules to live in a subdirectory of :file:`{prefix}`, rather than right in :file:`{prefix}` itself. This is almost as easy as customizing the script installation directory ---you just have to remember that there are two types of modules to worry about, pure modules and non-pure modules (i.e., modules from a non-pure distribution). For example:

python -m packaging.run install --install-purelib Site --install-platlib Site

The specified installation directories are relative to :file:`{prefix}`. Of course, you also have to ensure that these directories are in Python's module search path, such as by putting a :file:`.pth` file in :file:`{prefix}`. See section :ref:`packaging-search-path` to find out how to modify Python's search path.

If you want to define an entire installation scheme, you just have to supply all of the installation directory options. Using relative paths is recommended here. For example, if you want to maintain all Python module-related files under :file:`python` in your home directory, and you want a separate directory for each platform that you use your home directory from, you might define the following installation scheme:

python -m packaging.run install --home ~ \
                                --install-purelib python/lib \
                                --install-platlib python/'lib.$PLAT' \
                                --install-scripts python/scripts
                                --install-data python/data

or, equivalently,

python -m packaging.run install --home ~/python \
                                --install-purelib lib \
                                --install-platlib 'lib.$PLAT' \
                                --install-scripts scripts
                                --install-data data

$PLAT doesn't need to be defined as an environment variable ---it will also be expanded by Packaging as it parses your command line options, just as it does when parsing your configuration file(s). (More on that later.)

Obviously, specifying the entire installation scheme every time you install a new module distribution would be very tedious. To spare you all that work, you can store it in a Packaging config file instead (see section :ref:`packaging-config-files`), like so:

[install]
install-base = $HOME
install-purelib = python/lib
install-platlib = python/lib.$PLAT
install-scripts = python/scripts
install-data = python/data

or, equivalently,

[install]
install-base = $HOME/python
install-purelib = lib
install-platlib = lib.$PLAT
install-scripts = scripts
install-data = data

Note that these two are not equivalent if you override their installation base directory when running the setup script. For example,

python -m packaging.run install --install-base /tmp

would install pure modules to :file:`{/tmp/python/lib}` in the first case, and to :file:`{/tmp/lib}` in the second case. (For the second case, you'd probably want to supply an installation base of :file:`/tmp/python`.)

You may have noticed the use of $HOME and $PLAT in the sample configuration file. These are Packaging configuration variables, which bear a strong resemblance to environment variables. In fact, you can use environment variables in config files on platforms that have such a notion, but Packaging additionally defines a few extra variables that may not be in your environment, such as $PLAT. Of course, on systems that don't have environment variables, such as Mac OS 9, the configuration variables supplied by the Packaging are the only ones you can use. See section :ref:`packaging-config-files` for details.

Modifying Python's search path

When the Python interpreter executes an :keyword:`import` statement, it searches for both Python code and extension modules along a search path. A default value for this path is configured into the Python binary when the interpreter is built. You can obtain the search path by importing the :mod:`sys` module and printing the value of sys.path.

$ python
Python 2.2 (#11, Oct  3 2002, 13:31:27)
[GCC 2.96 20000731 (Red Hat Linux 7.3 2.96-112)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.path
['', '/usr/local/lib/python2.3', '/usr/local/lib/python2.3/plat-linux2',
 '/usr/local/lib/python2.3/lib-tk', '/usr/local/lib/python2.3/lib-dynload',
 '/usr/local/lib/python2.3/site-packages']
>>>

The null string in sys.path represents the current working directory.

The expected convention for locally installed packages is to put them in the :file:`{...}/site-packages/` directory, but you may want to choose a different location for some reason. For example, if your site kept by convention all web server-related software under :file:`/www`. Add-on Python modules might then belong in :file:`/www/python`, and in order to import them, this directory would have to be added to sys.path. There are several ways to solve this problem.

The most convenient way is to add a path configuration file to a directory that's already on Python's path, usually to the :file:`.../site-packages/` directory. Path configuration files have an extension of :file:`.pth`, and each line must contain a single path that will be appended to sys.path. (Because the new paths are appended to sys.path, modules in the added directories will not override standard modules. This means you can't use this mechanism for installing fixed versions of standard modules.)

Paths can be absolute or relative, in which case they're relative to the directory containing the :file:`.pth` file. See the documentation of the :mod:`site` module for more information.

A slightly less convenient way is to edit the :file:`site.py` file in Python's standard library, and modify sys.path. :file:`site.py` is automatically imported when the Python interpreter is executed, unless the :option:`-S` switch is supplied to suppress this behaviour. So you could simply edit :file:`site.py` and add two lines to it:

import sys
sys.path.append('/www/python/')

However, if you reinstall the same major version of Python (perhaps when upgrading from 3.3 to 3.3.1, for example) :file:`site.py` will be overwritten by the stock version. You'd have to remember that it was modified and save a copy before doing the installation.

Alternatively, there are two environment variables that can modify sys.path. :envvar:`PYTHONHOME` sets an alternate value for the prefix of the Python installation. For example, if :envvar:`PYTHONHOME` is set to /www/python, the search path will be set to ['', '/www/python/lib/pythonX.Y/', '/www/python/lib/pythonX.Y/plat-linux2', ...].

The :envvar:`PYTHONPATH` variable can be set to a list of paths that will be added to the beginning of sys.path. For example, if :envvar:`PYTHONPATH` is set to /www/python:/opt/py, the search path will begin with ['/www/python', '/opt/py']. (Note that directories must exist in order to be added to sys.path; the :mod:`site` module removes non-existent paths.)

Finally, sys.path is just a regular Python list, so any Python application can modify it by adding or removing entries.

Configuration files for Packaging

As mentioned above, you can use configuration files to store personal or site preferences for any option supported by any Packaging command. Depending on your platform, you can use one of two or three possible configuration files. These files will be read before parsing the command-line, so they take precedence over default values. In turn, the command-line will override configuration files. Lastly, if there are multiple configuration files, values from files read earlier will be overridden by values from files read later.

Location and names of config files

The name and location of the configuration files vary slightly across platforms. On Unix and Mac OS X, these are the three configuration files listed in the order they are processed:

Type of file Location and filename Notes
system :file:`{prefix}/lib/python{ver}/packaging/packaging.cfg` (1)
personal :file:`$HOME/.pypackaging.cfg` (2)
local :file:`packaging.cfg` (3)

Similarly, the configuration files on Windows ---also listed in the order they are processed--- are these:

Type of file Location and filename Notes
system :file:`{prefix}\\Lib\\packaging\\packaging.cfg` (4)
personal :file:`%HOME%\\pypackaging.cfg` (5)
local :file:`packaging.cfg` (3)

On all platforms, the personal file can be temporarily disabled by means of the --no-user-cfg option.

Notes:

  1. Strictly speaking, the system-wide configuration file lives in the directory where Packaging is installed.
  2. On Unix, if the :envvar:`HOME` environment variable is not defined, the user's home directory will be determined with the :func:`getpwuid` function from the standard :mod:`pwd` module. Packaging uses the :func:`os.path.expanduser` function to do this.
  3. I.e., in the current directory (usually the location from which the Packaging command is issued).
  4. (See also note (1).) Python's default installation prefix is :file:`C:\\Python`, so the system configuration file is normally :file:`C:\\Python\\Lib\\packaging\\packaging.cfg`.
  5. On Windows, if the :envvar:`HOME` environment variable is not defined, :envvar:`USERPROFILE` then :envvar:`HOMEDRIVE` and :envvar:`HOMEPATH` will be tried. Packaging uses the :func:`os.path.expanduser` function to do this.

Syntax of configuration files

All Packaging configuration files share the same syntax. Options defined in them are grouped into sections, and each Packaging command gets its own section. Additionally, there's a global section for options that affect every command. Sections consist of one or more lines containing a single option specified as option = value.

For example, here's a complete configuration file that forces all commands to run quietly by default:

[global]
verbose = 0

If this was the system configuration file, it would affect all processing of any Python module distribution by any user on the current system. If it was installed as your personal configuration file (on systems that support them), it would affect only module distributions processed by you. Lastly, if it was used as the :file:`setup.cfg` for a particular module distribution, it would affect that distribution only.

If you wanted to, you could override the default "build base" directory and make the :command:`build\*` commands always forcibly rebuild all files with the following:

[build]
build-base = blib
force = 1

which corresponds to the command-line arguments:

python -m packaging.run build --build-base blib --force

except that including the :command:`build` command on the command-line means that command will be run. Including a particular command in configuration files has no such implication; it only means that if the command is run, the options for it in the configuration file will apply. (This is also true if you run other commands that derive values from it.)

You can find out the complete list of options for any command using the :option:`--help` option, e.g.:

python -m packaging.run build --help

and you can find out the complete list of global options by using :option:`--help` without a command:

python -m packaging.run --help

See also the "Reference" section of the "Distributing Python Modules" manual.

Building extensions: tips and tricks

Whenever possible, Packaging tries to use the configuration information made available by the Python interpreter used to run python -m packaging.run. For example, the same compiler and linker flags used to compile Python will also be used for compiling extensions. Usually this will work well, but in complicated situations this might be inappropriate. This section discusses how to override the usual Packaging behaviour.

Tweaking compiler/linker flags

Compiling a Python extension written in C or C++ will sometimes require specifying custom flags for the compiler and linker in order to use a particular library or produce a special kind of object code. This is especially true if the extension hasn't been tested on your platform, or if you're trying to cross-compile Python.

In the most general case, the extension author might have foreseen that compiling the extensions would be complicated, and provided a :file:`Setup` file for you to edit. This will likely only be done if the module distribution contains many separate extension modules, or if they often require elaborate sets of compiler flags in order to work.

A :file:`Setup` file, if present, is parsed in order to get a list of extensions to build. Each line in a :file:`Setup` describes a single module. Lines have the following structure:

module ... [sourcefile ...] [cpparg ...] [library ...]

Let's examine each of the fields in turn.

  • module is the name of the extension module to be built, and should be a valid Python identifier. You can't just change this in order to rename a module (edits to the source code would also be needed), so this should be left alone.
  • sourcefile is anything that's likely to be a source code file, at least judging by the filename. Filenames ending in :file:`.c` are assumed to be written in C, filenames ending in :file:`.C`, :file:`.cc`, and :file:`.c++` are assumed to be C++, and filenames ending in :file:`.m` or :file:`.mm` are assumed to be in Objective C.
  • cpparg is an argument for the C preprocessor, and is anything starting with :option:`-I`, :option:`-D`, :option:`-U` or :option:`-C`.
  • library is anything ending in :file:`.a` or beginning with :option:`-l` or :option:`-L`.

If a particular platform requires a special library on your platform, you can add it by editing the :file:`Setup` file and running python setup.py build. For example, if the module defined by the line

foo foomodule.c

must be linked with the math library :file:`libm.a` on your platform, simply add :option:`-lm` to the line:

foo foomodule.c -lm

Arbitrary switches intended for the compiler or the linker can be supplied with the :option:`-Xcompiler` arg and :option:`-Xlinker` arg options:

foo foomodule.c -Xcompiler -o32 -Xlinker -shared -lm

The next option after :option:`-Xcompiler` and :option:`-Xlinker` will be appended to the proper command line, so in the above example the compiler will be passed the :option:`-o32` option, and the linker will be passed :option:`-shared`. If a compiler option requires an argument, you'll have to supply multiple :option:`-Xcompiler` options; for example, to pass -x c++ the :file:`Setup` file would have to contain -Xcompiler -x -Xcompiler c++.

Compiler flags can also be supplied through setting the :envvar:`CFLAGS` environment variable. If set, the contents of :envvar:`CFLAGS` will be added to the compiler flags specified in the :file:`Setup` file.

Using non-Microsoft compilers on Windows

Borland/CodeGear C++

This subsection describes the necessary steps to use Packaging with the Borland C++ compiler version 5.5. First you have to know that Borland's object file format (OMF) is different from the format used by the Python version you can download from the Python or ActiveState Web site. (Python is built with Microsoft Visual C++, which uses COFF as the object file format.) For this reason you have to convert Python's library :file:`python25.lib` into the Borland format. You can do this as follows:

coff2omf python25.lib python25_bcpp.lib

The :file:`coff2omf` program comes with the Borland compiler. The file :file:`python25.lib` is in the :file:`Libs` directory of your Python installation. If your extension uses other libraries (zlib, ...) you have to convert them too.

The converted files have to reside in the same directories as the normal libraries.

How does Packaging manage to use these libraries with their changed names? If the extension needs a library (eg. :file:`foo`) Packaging checks first if it finds a library with suffix :file:`_bcpp` (eg. :file:`foo_bcpp.lib`) and then uses this library. In the case it doesn't find such a special library it uses the default name (:file:`foo.lib`.) [1]

To let Packaging compile your extension with Borland C++ you now have to type:

python setup.py build --compiler bcpp

If you want to use the Borland C++ compiler as the default, you could specify this in your personal or system-wide configuration file for Packaging (see section :ref:`packaging-config-files`.)

GNU C / Cygwin / MinGW

This section describes the necessary steps to use Packaging with the GNU C/C++ compilers in their Cygwin and MinGW distributions. [2] For a Python interpreter that was built with Cygwin, everything should work without any of these following steps.

Not all extensions can be built with MinGW or Cygwin, but many can. Extensions most likely to not work are those that use C++ or depend on Microsoft Visual C extensions.

To let Packaging compile your extension with Cygwin you have to type:

python setup.py build --compiler=cygwin

and for Cygwin in no-cygwin mode [3] or for MinGW type:

python setup.py build --compiler=mingw32

If you want to use any of these options/compilers as default, you should consider writing it in your personal or system-wide configuration file for Packaging (see section :ref:`packaging-config-files`.)

Older versions of Python and MinGW

The following instructions only apply if you're using a version of Python inferior to 2.4.1 with a MinGW inferior to 3.0.0 (with binutils-2.13.90-20030111-1).

These compilers require some special libraries. This task is more complex than for Borland's C++, because there is no program to convert the library. First you have to create a list of symbols which the Python DLL exports. (You can find a good program for this task at http://www.emmestech.com/software/pexports-0.43/download_pexports.html).

pexports python25.dll > python25.def

The location of an installed :file:`python25.dll` will depend on the installation options and the version and language of Windows. In a "just for me" installation, it will appear in the root of the installation directory. In a shared installation, it will be located in the system directory.

Then you can create from these information an import library for gcc.

/cygwin/bin/dlltool --dllname python25.dll --def python25.def --output-lib libpython25.a

The resulting library has to be placed in the same directory as :file:`python25.lib`. (Should be the :file:`libs` directory under your Python installation directory.)

If your extension uses other libraries (zlib,...) you might have to convert them too. The converted files have to reside in the same directories as the normal libraries do.

Footnotes

[1]This also means you could replace all existing COFF-libraries with OMF-libraries of the same name.
[2]Check http://sources.redhat.com/cygwin/ and http://www.mingw.org/ for more information
[3]Then you have no POSIX emulation available, but you also don't need :file:`cygwin1.dll`.