Existence of pyproject.toml does not imply PEP 517

As the PEP itself says, if pyproject.toml exists but doesn't have the
build-system table or the build-backend key in it, then tools should
fall back to legacy behaviour of running python setup.py.

See https://github.com/mgedmin/check-manifest/issues/110#issuecomment-571175201

See also #105.

.gitignore

@@ -9,3 +9,4 @@ tmp/
 .coverage
 build/
 tags
+coverage.xml

Makefile

@@ -16,6 +16,11 @@ check:
 coverage:
 	tox -e coverage
 
+.PHONY: diff-cover
+diff-cover: coverage
+	coverage xml
+	diff-cover coverage.xml
+
 .PHONY: distcheck
 distcheck: distcheck-self  # also release.mk will add other checks
 

check_manifest.py

@@ -844,13 +844,29 @@ def extract_version_from_filename(filename):
     return filename.partition('-')[2]
 
 
+def should_use_pep_517():
+    """Check if the project uses PEP-517 builds."""
+    # https://www.python.org/dev/peps/pep-0517/#build-system-table says
+    # "If the pyproject.toml file is absent, or the build-backend key is
+    # missing, the source tree is not using this specification, and tools
+    # should revert to the legacy behaviour of running setup.py".
+    if not os.path.exists('pyproject.toml'):
+        return False
+    config = toml.load("pyproject.toml")
+    if "build-system" not in config:
+        return False
+    if "build-backend" not in config["build-system"]:
+        return False
+    return True
+
+
 def build_sdist(tempdir, python=sys.executable):
     """Build a source distribution in a temporary directory.
 
     Should be run with the current working directory inside the Python package
     you want to build.
     """
-    if os.path.exists('pyproject.toml'):
+    if should_use_pep_517():
         # I could do this in-process with
         #   import pep517.envbuild
         #   pep517.envbuild.build_sdist('.', tempdir)

tests.py

@@ -595,6 +595,51 @@ class Tests(unittest.TestCase):
             "%s, line 2: continuation line immediately precedes end-of-file" % filename,
         ])
 
+    def test_should_use_pep517_no_pyproject_toml(self):
+        from check_manifest import should_use_pep_517, cd
+        src_dir = self.make_temp_dir()
+        with cd(src_dir):
+            self.assertFalse(should_use_pep_517())
+
+    def test_should_use_pep517_no_build_system(self):
+        from check_manifest import should_use_pep_517, cd
+        src_dir = self.make_temp_dir()
+        filename = os.path.join(src_dir, 'pyproject.toml')
+        self.create_file(filename, textwrap.dedent('''
+            [tool.check-manifest]
+        '''))
+        with cd(src_dir):
+            self.assertFalse(should_use_pep_517())
+
+    def test_should_use_pep517_no_build_backend(self):
+        from check_manifest import should_use_pep_517, cd
+        src_dir = self.make_temp_dir()
+        filename = os.path.join(src_dir, 'pyproject.toml')
+        self.create_file(filename, textwrap.dedent('''
+            [build-system]
+            requires = [
+                "setuptools >= 40.6.0",
+                "wheel",
+            ]
+        '''))
+        with cd(src_dir):
+            self.assertFalse(should_use_pep_517())
+
+    def test_should_use_pep517_yes_please(self):
+        from check_manifest import should_use_pep_517, cd
+        src_dir = self.make_temp_dir()
+        filename = os.path.join(src_dir, 'pyproject.toml')
+        self.create_file(filename, textwrap.dedent('''
+            [build-system]
+            requires = [
+                "setuptools >= 40.6.0",
+                "wheel",
+            ]
+            build-backend = "setuptools.build_meta"
+        '''))
+        with cd(src_dir):
+            self.assertTrue(should_use_pep_517())
+
     def test_build_sdist(self):
         from check_manifest import build_sdist, cd, get_one_file_in
         src_dir = self.make_temp_dir()