diff options
| -rw-r--r-- | guix/build/python-build-system.scm | 68 |
1 files changed, 67 insertions, 1 deletions
diff --git a/guix/build/python-build-system.scm b/guix/build/python-build-system.scm index 310ba8aa2e..3f280b0ac0 100644 --- a/guix/build/python-build-system.scm +++ b/guix/build/python-build-system.scm @@ -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" |