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guix: python-build-system: Add background about Python installation methods.

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Hartmut Goebel 2016-11-15 16:57:21 +01:00
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guix/build/python-build-system.scm
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@ -36,7 +36,70 @@
;;
;; Builder-side code of the standard Python package build procedure.
;;
;; Code:
;;
;; Backgound about the Python installation methods
;;
;; In Python there are different ways to install packages: distutils,
;; setuptools, easy_install and pip. All of these are sharing the file
;; setup.py, introduced with distutils in Python 2.0. The setup.py file can be
;; considered as a kind of Makefile accepting targets (or commands) like
;; "build" and "install". As of autumn 2016 the recommended way to install
;; Python packages is using pip.
;;
;; For both distutils and setuptools, running "python setup.py install" is the
;; way to install Python packages. With distutils the "install" command
;; basically copies all packages into <prefix>/lib/pythonX.Y/site-packages.
;;
;; Some time later "setuptools" was established to enhance distutils. To use
;; setuptools, the developer imports setuptools in setup.py. When importing
;; setuptools, the original "install" command gets overwritten by setuptools'
;; "install" command.
;;
;; The command-line tools easy_install and pip are both capable of finding and
;; downloading the package source from PyPI (the Python Package Index). Both
;; of them import setuptools and execute the "setup.py" file under their
;; control. Thus the "setup.py" behaves as if the developer had imported
;; setuptools within setup.py - even is still using only distutils.
;;
;; Setuptools' "install" command (to be more precise: the "easy_install"
;; command which is called by "install") will put the path of the currently
;; installed version of each package and it's dependencies (as declared in
;; setup.py) into an "easy-install.pth" file. In Guix each packages gets its
;; own "site-packages" directory and thus an "easy-install.pth" of its own.
;; To avoid conflicts, the python build system renames the file to
;; <packagename>.pth in the phase rename-pth-file. To ensure that Python will
;; process the .pth file, easy_install also creates a basic "site.py" in each
;; "site-packages" directory. The file is the same for all packages, thus
;; there is no need to rename it. For more information about .pth files and
;; the site module, please refere to
;; https://docs.python.org/3/library/site.html.
;;
;; The .pth files contain the file-system paths (pointing to the store) of all
;; dependencies. So the dependency is hidden in the .pth file but is not
;; visible in the file-system. Now if packages A and B both required packages
;; P, but in different versions, Guix will not detect this when installing
;; both A and B to a profile. (For details and example see
;; https://lists.gnu.org/archive/html/guix-devel/2016-10/msg01233.html.)
;;
;; Pip behaves a bit different then easy_install: it always executes
;; "setup.py" with the option "--single-version-externally-managed" set. This
;; makes setuptools' "install" command run the original "install" command
;; instead of the "easy_install" command, so no .pth file (and no site.py)
;; will be created. The "site-packages" directory only contains the package
;; and the related .egg-info directory.
;;
;; This is exactly what we need for Guix and this is what we mimic in the
;; install phase below.
;;
;; As a draw back, the magic of the .pth file of linking to the other required
;; packages is gone and these packages have now to be declared as
;; "propagated-inputs".
;;
;; Note: Importing setuptools also adds two sub-commands: "install_egg_info"
;; and "install_scripts". These sub-commands are executed even if
;; "--single-version-externally-managed" is set, thus the .egg-info directory
;; and the scripts defined in entry-points will always be created.
(define setuptools-shim
;; Run setup.py with "setuptools" being imported, which will patch
@ -149,6 +212,9 @@ when running checks after installing the package."
(define* (rename-pth-file #:key name inputs outputs #:allow-other-keys)
"Rename easy-install.pth to NAME.pth to avoid conflicts between packages
installed with setuptools."
;; Even if the "easy-install.pth" is not longer created, we kept this phase.
;; There still may be packages creating an "easy-install.pth" manually for
;; some good reason.
(let* ((out (assoc-ref outputs "out"))
(python (assoc-ref inputs "python"))
(site-packages (string-append out "/lib/python"