Merge pull request #2306 from takluyver/pep517-setup-requires

Don't install setup_requires when run as a PEP-517 backend.

changelog.d/2306.change.rst

+When running as a PEP 517 backend, setuptools does not try to install
+``setup_requires`` itself. They are reported as build requirements for the
+frontend to install.

setuptools/build_meta.py

@@ -75,6 +75,22 @@ class Distribution(setuptools.dist.Distribution):
             distutils.core.Distribution = orig
 
 
+@contextlib.contextmanager
+def no_install_setup_requires():
+    """Temporarily disable installing setup_requires
+
+    Under PEP 517, the backend reports build dependencies to the frontend,
+    and the frontend is responsible for ensuring they're installed.
+    So setuptools (acting as a backend) should not try to install them.
+    """
+    orig = setuptools._install_setup_requires
+    setuptools._install_setup_requires = lambda attrs: None
+    try:
+        yield
+    finally:
+        setuptools._install_setup_requires = orig
+
+
 def _to_str(s):
     """
     Convert a filename to a string (on Python 2, explicitly
@@ -154,7 +170,8 @@ class _BuildMetaBackend(object):
                                          config_settings=None):
         sys.argv = sys.argv[:1] + ['dist_info', '--egg-base',
                                    _to_str(metadata_directory)]
-        self.run_setup()
+        with no_install_setup_requires():
+            self.run_setup()
 
         dist_info_directory = metadata_directory
         while True:
@@ -194,7 +211,8 @@ class _BuildMetaBackend(object):
             sys.argv = (sys.argv[:1] + setup_command +
                         ['--dist-dir', tmp_dist_dir] +
                         config_settings["--global-option"])
-            self.run_setup()
+            with no_install_setup_requires():
+                self.run_setup()
 
             result_basename = _file_with_extension(
                 tmp_dist_dir, result_extension)

setuptools/tests/test_build_meta.py

@@ -380,7 +380,7 @@ class TestBuildMetaBackend:
                 setup(
                     name="qux",
                     version="0.0.0",
-                    py_modules=["hello.py"],
+                    py_modules=["hello"],
                     setup_requires={setup_literal},
                 )
             """).format(setup_literal=setup_literal),
@@ -407,6 +407,35 @@ class TestBuildMetaBackend:
 
         assert expected == sorted(actual)
 
+    def test_dont_install_setup_requires(self, tmpdir_cwd):
+        files = {
+            'setup.py': DALS("""
+                        from setuptools import setup
+
+                        setup(
+                            name="qux",
+                            version="0.0.0",
+                            py_modules=["hello"],
+                            setup_requires=["does-not-exist >99"],
+                        )
+                    """),
+            'hello.py': DALS("""
+                    def run():
+                        print('hello')
+                    """),
+        }
+
+        build_files(files)
+
+        build_backend = self.get_build_backend()
+
+        dist_dir = os.path.abspath('pip-dist-info')
+        os.makedirs(dist_dir)
+
+        # does-not-exist can't be satisfied, so if it attempts to install
+        # setup_requires, it will fail.
+        build_backend.prepare_metadata_for_build_wheel(dist_dir)
+
     _sys_argv_0_passthrough = {
         'setup.py': DALS("""
             import os