build-system: Add pyproject-build-system.

This is an experimental build system based on python-build-system
that implements PEP 517-compliant builds.

* doc/guix.texi (Build Systems): Add pyproject-build-system section.
* doc/contributing.texi (Python Modules): Mention pyproject.toml and the
PYTHON-TOOLCHAIN package, as well as differences to python-build-system.
* guix/build-system/pyproject.scm,
guix/build/pyproject-build-system.scm,
gnu/packages/aux-files/python/sanity-check-next.py,
gnu/packages/python-commencement.scm: New files.
* Makefile.am (MODULES): Register the new build systems.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add python-commencement.scm.
* gnu/packages/python.scm (python-sans-pip, python-sans-pip-wrapper): New
variables.

Co-authored-by: Marius Bakke <marius@gnu.org>

7 files changed, 732 insertions, 9 deletions

diff --git a/Makefile.am b/Makefile.am
index 22dcc43f99..6ccb790c11 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -166,6 +166,7 @@ MODULES =					\
   guix/build-system/maven.scm			\
   guix/build-system/node.scm			\
   guix/build-system/perl.scm			\
+  guix/build-system/pyproject.scm		\
   guix/build-system/python.scm			\
   guix/build-system/renpy.scm			\
   guix/build-system/ocaml.scm			\
@@ -222,6 +223,7 @@ MODULES =					\
   guix/build/minetest-build-system.scm		\
   guix/build/node-build-system.scm		\
   guix/build/perl-build-system.scm		\
+  guix/build/pyproject-build-system.scm		\
   guix/build/python-build-system.scm		\
   guix/build/ocaml-build-system.scm		\
   guix/build/qt-build-system.scm		\
diff --git a/doc/contributing.texi b/doc/contributing.texi
index 4b1eed1cb1..c3221d23e4 100644
--- a/doc/contributing.texi
+++ b/doc/contributing.texi
@@ -786,12 +786,29 @@ for instance, the module python-dateutil is packaged under the names
 starts with @code{py} (e.g.@: @code{pytz}), we keep it and prefix it as
 described above.
 
+@quotation Note
+Currently there are two different build systems for Python packages in Guix:
+@var{python-build-system} and @var{pyproject-build-system}.  For the
+longest time, Python packages were built from an informally specified
+@file{setup.py} file.  That worked amazingly well, considering Python's
+success, but was difficult to build tooling around.  As a result, a host
+of alternative build systems emerged and the community eventually settled on a
+@url{https://peps.python.org/pep-0517/, formal standard} for specifying build
+requirements.  @var{pyproject-build-system} is Guix's implementation of this
+standard.  It is considered ``experimental'' in that it does not yet support
+all the various PEP-517 @emph{build backends}, but you are encouraged to try
+it for new Python packages and report any problems.  It will eventually be
+deprecated and merged into @var{python-build-system}.
+@end quotation
+
 @subsubsection Specifying Dependencies
 @cindex inputs, for Python packages
 
 Dependency information for Python packages is usually available in the
 package source tree, with varying degrees of accuracy: in the
-@file{setup.py} file, in @file{requirements.txt}, or in @file{tox.ini}.
+@file{pyproject.toml} file, the @file{setup.py} file, in
+@file{requirements.txt}, or in @file{tox.ini} (the latter mostly for
+test dependencies).
 
 Your mission, when writing a recipe for a Python package, is to map
 these dependencies to the appropriate type of ``input'' (@pxref{package
@@ -802,10 +819,12 @@ following check list to determine which dependency goes where.
 @itemize
 
 @item
-We currently package Python 2 with @code{setuptools} and @code{pip}
-installed like Python 3.4 has per default.  Thus you don't need to
-specify either of these as an input.  @command{guix lint} will warn you
-if you do.
+We currently package Python with @code{setuptools} and @code{pip}
+installed per default.  This is about to change, and users are encouraged
+to use @code{python-toolchain} if they want a build environment for Python.
+
+@command{guix lint} will warn if @code{setuptools} or @code{pip} are
+added as native-inputs because they are generally not necessary.
 
 @item
 Python dependencies required at run time go into
@@ -814,9 +833,10 @@ Python dependencies required at run time go into
 @file{requirements.txt} file.
 
 @item
-Python packages required only at build time---e.g., those listed with
-the @code{setup_requires} keyword in @file{setup.py}---or only for
-testing---e.g., those in @code{tests_require}---go into
+Python packages required only at build time---e.g., those listed under
+@code{build-system.requires} in @file{pyproject.toml} or with the
+@code{setup_requires} keyword in @file{setup.py}---or dependencies only
+for testing---e.g., those in @code{tests_require} or @file{tox.ini}---go into
 @code{native-inputs}.  The rationale is that (1) they do not need to be
 propagated because they are not needed at run time, and (2) in a
 cross-compilation context, it's the ``native'' input that we'd want.
diff --git a/doc/guix.texi b/doc/guix.texi
index 2f7ab61aec..3bfb89bc33 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -9313,7 +9313,36 @@ instead of the default @code{"out"} output. This is useful for packages that
 include a Python package as only a part of the software, and thus want to
 combine the phases of @code{python-build-system} with another build system.
 Python bindings are a common usecase.
+@end defvr
+
+@defvr {Scheme Variable} pyproject-build-system
+This is a variable exported by @code{guix build-system pyproject}.  It
+is based on @var{python-build-system}, and adds support for
+@file{pyproject.toml} and @url{https://peps.python.org/pep-0517/, PEP 517}.
+It also supports a variety of build backends and test frameworks.
+
+The API is slightly different from @var{python-build-system}:
+@itemize
+@item
+@code{#:use-setuptools?} and @code{#:test-target} is removed.
+@item
+@code{#:build-backend} is added.  It defaults to @code{#false} and will try
+to guess the appropriate backend based on @file{pyproject.toml}.
+@item
+@code{#:test-backend} is added.  It defaults to @code{#false} and will guess
+an appropriate test backend based on what is available in package inputs.
+@item
+@code{#:test-flags} is added.  The default is @code{#false}, and varies based
+on the detected @code{#:test-backend}.
+@end itemize
+
+It is considered ``experimental'' in that the implementation details are
+not set in stone yet, however users are encouraged to try it for new
+Python projects (even those using @file{setup.py}).  The API is subject to
+change, but any breaking changes in the Guix channel will be dealt with.
 
+Eventually this build system will be deprecated and merged back into
+@var{python-build-system}, probably some time in 2024.
 @end defvr
 
 @defvr {Scheme Variable} perl-build-system
diff --git a/gnu/packages/aux-files/python/sanity-check-next.py b/gnu/packages/aux-files/python/sanity-check-next.py
new file mode 100644
index 0000000000..a206b51d0a
--- /dev/null
+++ b/gnu/packages/aux-files/python/sanity-check-next.py
@@ -0,0 +1,99 @@
+# -*- coding: utf-8 -*-
+# GNU Guix --- Functional package management for GNU
+# Copyright © 2021, 2022 Lars-Dominik Braun <lars@6xq.net>
+#
+# This file is part of GNU Guix.
+#
+# GNU Guix is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or (at
+# your option) any later version.
+#
+# GNU Guix is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+# This version adds a small change to accommodate missing python-setuptools.
+# TODO: Merge with sanity-check.py in the next core-updates cycle.
+
+from __future__ import print_function  # Python 2 support.
+import importlib
+import sys
+import traceback
+try:
+    import pkg_resources
+except ImportError:
+    print('Warning: Skipping, because python-setuptools are not available.')
+    sys.exit(0)
+
+try:
+    from importlib.machinery import PathFinder
+except ImportError:
+    PathFinder = None
+
+ret = 0
+
+# Only check site-packages installed by this package, but not dependencies
+# (which pkg_resources.working_set would include). Path supplied via argv.
+ws = pkg_resources.find_distributions(sys.argv[1])
+
+for dist in ws:
+    print('validating', repr(dist.project_name), dist.location)
+    try:
+        print('...checking requirements: ', end='')
+        req = str(dist.as_requirement())
+        # dist.activate() is not enough to actually check requirements, we
+        # have to .require() it.
+        pkg_resources.require(req)
+        print('OK')
+    except Exception as e:
+        print('ERROR:', req, repr(e))
+        ret = 1
+        continue
+
+    # Try to load top level modules. This should not have any side-effects.
+    try:
+        metalines = dist.get_metadata_lines('top_level.txt')
+    except (KeyError, EnvironmentError):
+        # distutils (i.e. #:use-setuptools? #f) will not install any metadata.
+        # This file is also missing for packages built using a PEP 517 builder
+        # such as poetry.
+        print('WARNING: cannot determine top-level modules')
+        continue
+    for name in metalines:
+        # Only available on Python 3.
+        if PathFinder and PathFinder.find_spec(name) is None:
+            # Ignore unavailable modules, often C modules, which were not
+            # installed at the top-level. Cannot use ModuleNotFoundError,
+            # because it is raised by failed imports too.
+            continue
+        try:
+            print('...trying to load module', name, end=': ')
+            importlib.import_module(name)
+            print('OK')
+        except Exception:
+            print('ERROR:')
+            traceback.print_exc(file=sys.stdout)
+            ret = 1
+
+    # Try to load entry points of console scripts too, making sure they
+    # work. They should be removed if they don't. Other groups may not be
+    # safe, as they can depend on optional packages.
+    for group, v in dist.get_entry_map().items():
+        if group not in {'console_scripts', 'gui_scripts'}:
+            continue
+        for name, ep in v.items():
+            try:
+                print('...trying to load endpoint', group, name, end=': ')
+                ep.load()
+                print('OK')
+            except Exception:
+                print('ERROR:')
+                traceback.print_exc(file=sys.stdout)
+                ret = 1
+
+sys.exit(ret)
diff --git a/gnu/packages/python.scm b/gnu/packages/python.scm
index 00c3eb7774..0c164d7991 100644
--- a/gnu/packages/python.scm
+++ b/gnu/packages/python.scm
@@ -26,7 +26,7 @@
 ;;; Copyright © 2016, 2017 Nikita <nikita@n0.is>
 ;;; Copyright © 2016 Dylan Jeffers <sapientech@sapientech@openmailbox.org>
 ;;; Copyright © 2016 David Craven <david@craven.ch>
-;;; Copyright © 2016, 2017, 2018, 2019, 2020, 2021 Marius Bakke <marius@gnu.org>
+;;; Copyright © 2016-2022 Marius Bakke <marius@gnu.org>
 ;;; Copyright © 2016, 2017 Stefan Reichör <stefan@xsteve.at>
 ;;; Copyright © 2016, 2017 Alex Vong <alexvong1995@gmail.com>
 ;;; Copyright © 2016, 2017, 2018 Arun Isaac <arunisaac@systemreboot.net>
@@ -60,6 +60,7 @@
 ;;; Copyright © 2020, 2021 Greg Hogan <code@greghogan.com>
 ;;; Copyright © 2022 Philip McGrath <philip@philipmcgrath.com>
 ;;; Copyright © 2022 jgart <jgart@dismail.de>
+;;; Copyright © 2021 Lars-Dominik Braun <lars@6xq.net>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -87,6 +88,7 @@
   #:use-module (gnu packages hurd)
   #:use-module (gnu packages libffi)
   #:use-module (gnu packages pkg-config)
+  #:use-module (gnu packages python-build)
   #:use-module (gnu packages readline)
   #:use-module (gnu packages sqlite)
   #:use-module (gnu packages tcl)
@@ -674,6 +676,37 @@ and the unversioned commands available.")))
 (define-public python-wrapper (wrap-python3 python))
 (define-public python-minimal-wrapper (wrap-python3 python-minimal))
 
+;; The Python used in pyproject-build-system.
+(define-public python-sans-pip
+  (hidden-package
+   (package/inherit python
+     (arguments
+      (substitute-keyword-arguments (package-arguments python)
+        ((#:configure-flags flags #~())
+         #~(append '("--with-ensurepip=no")
+                   (delete "--with-ensurepip=install" #$flags))))))))
+
+(define-public python-sans-pip-wrapper
+  (wrap-python3 python-sans-pip))
+
+(define-public python-toolchain
+  (let ((base (package/inherit python-sans-pip-wrapper)))
+    (package
+      (inherit base)
+      (properties '())
+      (name "python-toolchain")
+      (propagated-inputs
+       (modify-inputs (package-propagated-inputs base)
+         (append python-pip
+                 python-pypa-build
+                 python-setuptools
+                 python-wheel)))
+      (synopsis "Python toolchain")
+      (description
+       "Python toolchain including Python itself, setuptools and pip.
+Use this package if you need a minimal Python toolchain instead of just
+the interpreter."))))
+
 (define-public micropython
   (package
     (name "micropython")
diff --git a/guix/build-system/pyproject.scm b/guix/build-system/pyproject.scm
new file mode 100644
index 0000000000..1e365d4f21
--- /dev/null
+++ b/guix/build-system/pyproject.scm
@@ -0,0 +1,147 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2021 Lars-Dominik Braun <lars@6xq.net>
+;;; Copyright © 2022 Marius Bakke <marius@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (guix build-system pyproject)
+  #:use-module ((gnu packages) #:select (search-auxiliary-file))
+  #:use-module (guix gexp)
+  #:use-module (guix store)
+  #:use-module (guix utils)
+  #:use-module (guix memoization)
+  #:use-module (guix gexp)
+  #:use-module (guix monads)
+  #:use-module (guix packages)
+  #:use-module (guix derivations)
+  #:use-module (guix search-paths)
+  #:use-module (guix build-system)
+  #:use-module (guix build-system gnu)
+  #:use-module (guix build-system python)
+  #:use-module (ice-9 match)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-26)
+  #:export (%pyproject-build-system-modules
+            default-python
+            pyproject-build
+            pyproject-build-system))
+
+;; Commentary:
+;;
+;; Standard build procedure for Python packages using 'pyproject.toml'.
+;; This is implemented as an extension of 'python-build-system'.
+;;
+;; Code:
+
+(define %pyproject-build-system-modules
+  ;; Build-side modules imported by default.
+  `((guix build pyproject-build-system)
+    (guix build json)
+    ,@%python-build-system-modules))
+
+(define (default-python)
+  "Return the default Python package."
+  ;; Lazily resolve the binding to avoid a circular dependency.
+  (let ((python (resolve-interface '(gnu packages python))))
+    (module-ref python 'python-toolchain)))
+
+(define sanity-check.py
+  ;; TODO: Merge with sanity-check.py in the next rebuild cycle.
+  (search-auxiliary-file "python/sanity-check-next.py"))
+
+(define* (lower name
+                #:key source inputs native-inputs outputs system target
+                (python (default-python))
+                #:allow-other-keys
+                #:rest arguments)
+  "Return a bag for NAME."
+  (define private-keywords
+    '(#:target #:python #:inputs #:native-inputs))
+
+  (and (not target)                               ;XXX: no cross-compilation
+       (bag
+         (name name)
+         (system system)
+         (host-inputs `(,@(if source
+                              `(("source" ,source))
+                              '())
+                        ,@inputs
+
+                        ;; Keep the standard inputs of 'gnu-build-system'.
+                        ,@(standard-packages)))
+         (build-inputs `(("python" ,python)
+                         ("sanity-check.py" ,(local-file sanity-check.py))
+                         ,@native-inputs))
+         (outputs (append outputs '(wheel)))
+         (build pyproject-build)
+         (arguments (strip-keyword-arguments private-keywords arguments)))))
+
+(define* (pyproject-build name inputs
+                          #:key source
+                          (tests? #t)
+                          (configure-flags ''())
+                          (build-backend #f)
+                          (test-backend #f)
+                          (test-flags #f)
+                          (phases '%standard-phases)
+                          (outputs '("out" "wheel"))
+                          (search-paths '())
+                          (system (%current-system))
+                          (guile #f)
+                          (imported-modules %pyproject-build-system-modules)
+                          (modules '((guix build pyproject-build-system)
+                                     (guix build utils))))
+  "Build SOURCE using PYTHON, and with INPUTS."
+  (define build
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@(sexp->gexp modules))
+
+          #$(with-build-variables inputs outputs
+              #~(pyproject-build
+                 #:name #$name
+                 #:source #+source
+                 #:configure-flags #$configure-flags
+                 #:system #$system
+                 #:build-backend #$build-backend
+                 #:test-backend #$test-backend
+                 #:test-flags #$test-flags
+                 #:tests? #$tests?
+                 #:phases #$(if (pair? phases)
+                                (sexp->gexp phases)
+                                phases)
+                 #:outputs %outputs
+                 #:search-paths '#$(sexp->gexp
+                                    (map search-path-specification->sexp
+                                         search-paths))
+                 #:inputs %build-inputs)))))
+
+
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name build
+                      #:system system
+                      #:graft? #f                 ;consistent with 'gnu-build'
+                      #:target #f
+                      #:guile-for-build guile)))
+
+(define pyproject-build-system
+  (build-system
+    (name 'pyproject)
+    (description "The PEP517-compliant Python build system")
+    (lower lower)))
+
+;;; pyproject.scm ends here
diff --git a/guix/build/pyproject-build-system.scm b/guix/build/pyproject-build-system.scm
new file mode 100644
index 0000000000..d247fd77a8
--- /dev/null
+++ b/guix/build/pyproject-build-system.scm
@@ -0,0 +1,393 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2021 Lars-Dominik Braun <lars@6xq.net>
+;;; Copyright © 2022 Marius Bakke <marius@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (guix build pyproject-build-system)
+  #:use-module ((guix build python-build-system) #:prefix python:)
+  #:use-module (guix build utils)
+  #:use-module (guix build json)
+  #:use-module (ice-9 match)
+  #:use-module (ice-9 ftw)
+  #:use-module (ice-9 format)
+  #:use-module (ice-9 rdelim)
+  #:use-module (ice-9 regex)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-34)
+  #:use-module (srfi srfi-35)
+  #:export (%standard-phases
+            add-installed-pythonpath
+            site-packages
+            python-version
+            pyproject-build))
+
+;;; Commentary:
+;;;
+;;; PEP 517-compatible build system for Python packages.
+;;;
+;;; PEP 517 mandates the use of a TOML file called pyproject.toml at the
+;;; project root, describing build and runtime dependencies, as well as the
+;;; build system, which can be different from setuptools. This module uses
+;;; that file to extract the build system used and call its wheel-building
+;;; entry point build_wheel (see 'build). setuptools’ wheel builder is
+;;; used as a fallback if either no pyproject.toml exists or it does not
+;;; declare a build-system. It supports config_settings through the
+;;; standard #:configure-flags argument.
+;;;
+;;; This wheel, which is just a ZIP file with a file structure defined
+;;; by PEP 427 (https://www.python.org/dev/peps/pep-0427/), is then unpacked
+;;; and its contents are moved to the appropriate locations in 'install.
+;;;
+;;; Then entry points, as defined by the PyPa Entry Point Specification
+;;; (https://packaging.python.org/specifications/entry-points/) are read
+;;; from a file called entry_points.txt in the package’s site-packages
+;;; subdirectory and scripts are written to bin/. These are not part of a
+;;; wheel and expected to be created by the installing utility.
+;;; TODO: Add support for PEP-621 entry points.
+;;;
+;;; Caveats:
+;;; - There is no support for in-tree build backends.
+;;;
+;;; Code:
+;;;
+
+;; Re-export these variables from python-build-system as many packages
+;; rely on these.
+(define python-version python:python-version)
+(define site-packages python:site-packages)
+(define add-installed-pythonpath python:add-installed-pythonpath)
+
+;; Base error type.
+(define-condition-type &python-build-error &error python-build-error?)
+
+;; Raised when 'check cannot find a valid test system in the inputs.
+(define-condition-type &test-system-not-found &python-build-error
+                       test-system-not-found?)
+
+;; Raised when multiple wheels are created by 'build.
+(define-condition-type &cannot-extract-multiple-wheels &python-build-error
+                       cannot-extract-multiple-wheels?)
+
+;; Raised, when no wheel has been built by the build system.
+(define-condition-type &no-wheels-built &python-build-error no-wheels-built?)
+
+(define* (build #:key outputs build-backend configure-flags #:allow-other-keys)
+  "Build a given Python package."
+
+  (define (pyproject.toml->build-backend file)
+    "Look up the build backend in a pyproject.toml file."
+    (call-with-input-file file
+      (lambda (in)
+        (let loop
+          ((line (read-line in
+                            'concat)))
+          (if (eof-object? line) #f
+              (let ((m (string-match "build-backend = [\"'](.+)[\"']" line)))
+                (if m
+                    (match:substring m 1)
+                    (loop (read-line in
+                                     'concat)))))))))
+
+  (let* ((wheel-output (assoc-ref outputs "wheel"))
+         (wheel-dir (if wheel-output wheel-output "dist"))
+         ;; There is no easy way to get data from Guile into Python via
+         ;; s-expressions, but we have JSON serialization already, which Python
+         ;; also supports out-of-the-box.
+         (config-settings (call-with-output-string (cut write-json
+                                                        configure-flags <>)))
+         ;; python-setuptools’ default backend supports setup.py *and*
+         ;; pyproject.toml. Allow overriding this automatic detection via
+         ;; build-backend.
+         (auto-build-backend (if (file-exists? "pyproject.toml")
+                                 (pyproject.toml->build-backend
+                                  "pyproject.toml") #f))
+         ;; Use build system detection here and not in importer, because a) we
+         ;; have alot of legacy packages and b) the importer cannot update arbitrary
+         ;; fields in case a package switches its build system.
+         (use-build-backend (or build-backend auto-build-backend
+                                "setuptools.build_meta")))
+    (format #t
+     "Using '~a' to build wheels, auto-detected '~a', override '~a'.~%"
+     use-build-backend auto-build-backend build-backend)
+    (mkdir-p wheel-dir)
+    ;; Call the PEP 517 build function, which drops a .whl into wheel-dir.
+    (invoke "python"
+     "-c"
+     "import sys, importlib, json\nconfig_settings = json.loads (sys.argv[3])\nbuilder = importlib.import_module(sys.argv[1])\nbuilder.build_wheel(sys.argv[2], config_settings=config_settings)"
+     use-build-backend
+     wheel-dir
+     config-settings)))
+
+(define* (check #:key inputs
+                outputs
+                tests?
+                test-backend
+                test-flags
+                #:allow-other-keys)
+  "Run the test suite of a given Python package."
+  (if tests?
+      ;; Unfortunately with PEP 517 there is no common method to specify test
+      ;; systems.  Guess test system based on inputs instead.
+      (let* ((pytest (which "pytest"))
+             (nosetests (which "nosetests"))
+             (nose2 (which "nose2"))
+             (have-setup-py (file-exists? "setup.py"))
+             (use-test-backend
+              (or test-backend
+                  ;; Prefer pytest
+                  (if pytest 'pytest #f)
+                  (if nosetests 'nose #f)
+                  (if nose2 'nose2 #f)
+                  ;; But fall back to setup.py, which should work for most
+                  ;; packages. XXX: would be nice not to depend on setup.py here?
+                  ;; fails more often than not to find any tests at all.  Maybe
+                  ;; we can run `python -m unittest`?
+                  (if have-setup-py 'setup.py #f))))
+        (format #t "Using ~a~%" use-test-backend)
+        (match use-test-backend
+          ('pytest
+           (apply invoke (cons pytest (or test-flags '("-vv")))))
+          ('nose
+           (apply invoke (cons nosetests (or test-flags '("-v")))))
+          ('nose2
+           (apply invoke (cons nose2 (or test-flags '("-v" "--pretty-assert")))))
+          ('setup.py
+           (apply invoke (append '("python" "setup.py")
+                                 (or test-flags '("test" "-v")))))
+          ;; The developer should explicitly disable tests in this case.
+          (else (raise (condition (&test-system-not-found))))))
+      (format #t "test suite not run~%")))
+
+(define* (install #:key inputs outputs #:allow-other-keys)
+  "Install a wheel file according to PEP 427"
+  ;; See https://www.python.org/dev/peps/pep-0427/#installing-a-wheel-distribution-1-0-py32-none-any-whl
+  (let ((site-dir (site-packages inputs outputs))
+        (python (assoc-ref inputs "python"))
+        (out (assoc-ref outputs "out")))
+    (define (extract file)
+      "Extract wheel (ZIP file) into site-packages directory"
+      ;; Use Python’s zipfile to avoid extra dependency
+      (invoke "python"
+              "-m"
+              "zipfile"
+              "-e"
+              file
+              site-dir))
+
+    (define python-hashbang
+      (string-append "#!" python "/bin/python"))
+
+    (define* (merge-directories source destination
+                                #:optional (post-move #f))
+      "Move all files in SOURCE into DESTINATION, merging the two directories."
+      (format #t "Merging directory ~a into ~a~%" source destination)
+      (for-each (lambda (file)
+                  (format #t
+                          "~a/~a -> ~a/~a~%"
+                          source
+                          file
+                          destination
+                          file)
+                  (mkdir-p destination)
+                  (rename-file (string-append source "/" file)
+                               (string-append destination "/" file))
+                  (when post-move
+                    (post-move file)))
+                (scandir source
+                         (negate (cut member <>
+                                      '("." "..")))))
+      (rmdir source))
+
+    (define (expand-data-directory directory)
+      "Move files from all .data subdirectories to their respective\ndestinations."
+      ;; Python’s distutils.command.install defines this mapping from source to
+      ;; destination mapping.
+      (let ((source (string-append directory "/scripts"))
+            (destination (string-append out "/bin")))
+        (when (file-exists? source)
+          (merge-directories source destination
+                             (lambda (f)
+                               (let ((dest-path (string-append destination "/"
+                                                               f)))
+                                 (chmod dest-path #o755)
+                                 (substitute* dest-path
+                                   (("#!python")
+                                    python-hashbang)))))))
+      ;; Data can be contained in arbitrary directory structures. Most
+      ;; commonly it is used for share/.
+      (let ((source (string-append directory "/data"))
+            (destination out))
+        (when (file-exists? source)
+          (merge-directories source destination)))
+      (let* ((distribution (car (string-split (basename directory) #\-)))
+             (source (string-append directory "/headers"))
+             (destination (string-append out "/include/python"
+                                         (python-version python) "/"
+                                         distribution)))
+        (when (file-exists? source)
+          (merge-directories source destination))))
+
+    (define (list-directories base predicate)
+      ;; Cannot use find-files here, because it’s recursive.
+      (scandir base
+               (lambda (name)
+                 (let ((stat (lstat (string-append base "/" name))))
+                   (and (not (member name
+                                     '("." "..")))
+                        (eq? (stat:type stat)
+                             'directory)
+                        (predicate name stat))))))
+
+    (let* ((wheel-output (assoc-ref outputs "wheel"))
+           (wheel-dir (if wheel-output wheel-output "dist"))
+           (wheels (map (cut string-append wheel-dir "/" <>)
+                        (scandir wheel-dir
+                                 (cut string-suffix? ".whl" <>)))))
+      (cond
+        ((> (length wheels) 1)
+          ;This code does not support multiple wheels
+         ;; yet, because their outputs would have to be
+         ;; merged properly.
+         (raise (condition (&cannot-extract-multiple-wheels))))
+        ((= (length wheels) 0)
+         (raise (condition (&no-wheels-built)))))
+      (for-each extract wheels))
+    (let ((datadirs (map (cut string-append site-dir "/" <>)
+                         (list-directories site-dir
+                                           (file-name-predicate "\\.data$")))))
+      (for-each (lambda (directory)
+                  (expand-data-directory directory)
+                  (rmdir directory)) datadirs))))
+
+(define* (compile-bytecode #:key inputs outputs #:allow-other-keys)
+  "Compile installed byte-code in site-packages."
+  (let* ((site-dir (site-packages inputs outputs))
+         (python (assoc-ref inputs "python"))
+         (major-minor (map string->number
+                           (take (string-split (python-version python) #\.) 2)))
+         (<3.7? (match major-minor
+                  ((major minor)
+                   (or (< major 3)
+                       (and (= major 3)
+                            (< minor 7)))))))
+    (if <3.7?
+        ;; These versions don’t have the hash invalidation modes and do
+        ;; not produce reproducible bytecode files.
+        (format #t "Skipping bytecode compilation for Python version ~a < 3.7~%"
+                (python-version python))
+        (invoke "python" "-m" "compileall"
+                "--invalidation-mode=unchecked-hash" site-dir))))
+
+(define* (create-entrypoints #:key inputs outputs (configure-flags '()) #:allow-other-keys)
+  "Implement Entry Points Specification
+(https://packaging.python.org/specifications/entry-points/) by PyPa,
+which creates runnable scripts in bin/ from entry point specification
+file entry_points.txt.  This is necessary, because wheels do not contain
+these binaries and installers are expected to create them."
+
+  (define (entry-points.txt->entry-points file)
+    "Specialized parser for Python configfile-like files, in particular
+entry_points.txt.  Returns a list of console_script and gui_scripts
+entry points."
+    (call-with-input-file file
+      (lambda (in)
+        (let loop ((line (read-line in))
+                   (inside #f)
+                   (result '()))
+          (if (eof-object? line)
+              result
+              (let* ((group-match (string-match "^\\[(.+)\\]$" line))
+                     (group-name (if group-match
+                                     (match:substring group-match 1)
+                                     #f))
+                     (next-inside (if (not group-name)
+                                      inside
+                                      (or (string=? group-name
+                                                    "console_scripts")
+                                          (string=? group-name "gui_scripts"))))
+                     (item-match (string-match
+                                  "^([^ =]+)\\s*=\\s*([^:]+):(.+)$" line)))
+                (if (and inside item-match)
+                    (loop (read-line in)
+                          next-inside
+                          (cons (list (match:substring item-match 1)
+                                      (match:substring item-match 2)
+                                      (match:substring item-match 3))
+                                result))
+                    (loop (read-line in) next-inside result))))))))
+
+  (define (create-script path name module function)
+    "Create a Python script from an entry point’s NAME, MODULE and
+  FUNCTION and return write it to PATH/NAME."
+    (let ((interpreter (which "python"))
+          (file-path (string-append path "/" name)))
+      (format #t "Creating entry point for '~a.~a' at '~a'.~%"
+              module function file-path)
+      (call-with-output-file file-path
+        (lambda (port)
+          ;; Technically the script could also include search-paths,
+          ;; but having a generic 'wrap phases also handles manually
+          ;; written entry point scripts.
+          (format port "#!~a
+# Auto-generated entry point script.
+import sys
+import ~a as mod
+sys.exit (mod.~a ())~%" interpreter module function)))
+        (chmod file-path #o755)))
+
+  (let* ((site-dir (site-packages inputs outputs))
+         (out (assoc-ref outputs "out"))
+         (bin-dir (string-append out "/bin"))
+         (entry-point-files (find-files site-dir "^entry_points.txt$")))
+    (mkdir-p bin-dir)
+    (for-each (lambda (f)
+                (for-each (lambda (ep)
+                            (apply create-script
+                                   (cons bin-dir ep)))
+                          (entry-points.txt->entry-points f)))
+              entry-point-files)))
+
+(define* (set-SOURCE-DATE-EPOCH* #:rest _)
+  "Set the 'SOURCE_DATE_EPOCH' environment variable.  This is used by tools
+that incorporate timestamps as a way to tell them to use a fixed timestamp.
+See https://reproducible-builds.org/specs/source-date-epoch/."
+  ;; Use a post-1980 timestamp because the Zip format used in wheels do
+  ;; not support timestamps before 1980.
+  (setenv "SOURCE_DATE_EPOCH" "315619200"))
+
+(define %standard-phases
+  ;; The build phase only builds C extensions and copies the Python sources,
+  ;; while the install phase copies then byte-compiles the sources to the
+  ;; prefix directory.  The check phase is moved after the installation phase
+  ;; to ease testing the built package.
+  (modify-phases python:%standard-phases
+    (replace 'set-SOURCE-DATE-EPOCH set-SOURCE-DATE-EPOCH*)
+    (replace 'build build)
+    (replace 'install install)
+    (delete 'check)
+    ;; Must be before tests, so they can use installed packages’ entry points.
+    (add-before 'wrap 'create-entrypoints create-entrypoints)
+    (add-after 'wrap 'check check)
+    (add-before 'check 'compile-bytecode compile-bytecode)))
+
+(define* (pyproject-build #:key inputs (phases %standard-phases)
+                          #:allow-other-keys #:rest args)
+  "Build the given Python package, applying all of PHASES in order."
+  (apply python:python-build #:inputs inputs #:phases phases args))
+
+;;; pyproject-build-system.scm ends here