Reject projects that have neither a pyproject.toml nor a setup.py

news/10531.bugfix.rst

@@ -0,0 +1,2 @@
+Always refuse installing or building projects that have no ``pyproject.toml`` nor
+``setup.py``.

src/pip/_internal/pyproject.py

@@ -48,6 +48,12 @@ def load_pyproject_toml(
     has_pyproject = os.path.isfile(pyproject_toml)
     has_setup = os.path.isfile(setup_py)
 
+    if not has_pyproject and not has_setup:
+        raise InstallationError(
+            f"{req_name} does not appear to be a Python project: "
+            f"neither 'setup.py' nor 'pyproject.toml' found."
+        )
+
     if has_pyproject:
         with open(pyproject_toml, encoding="utf-8") as f:
             pp_toml = tomli.load(f)

src/pip/_internal/req/constructors.py

@@ -235,6 +235,8 @@ def _get_url_from_path(path: str, name: str) -> Optional[str]:
     if _looks_like_path(name) and os.path.isdir(path):
         if is_installable_dir(path):
             return path_to_url(path)
+        # TODO: The is_installable_dir test here might not be necessary
+        #       now that it is done in load_pyproject_toml too.
         raise InstallationError(
             f"Directory {name!r} is not installable. Neither 'setup.py' "
             "nor 'pyproject.toml' found."

tests/data/src/pep517_setup_cfg_only/setup.cfg

@@ -0,0 +1,3 @@
+[metadata]
+name = "dummy"
+version = "0.1"

tests/functional/test_install.py

@@ -653,7 +653,6 @@ def test_install_from_local_directory_with_no_setup_py(script, data):
     """
     result = script.pip("install", data.root, expect_error=True)
     assert not result.files_created
-    assert "is not installable." in result.stderr
     assert "Neither 'setup.py' nor 'pyproject.toml' found." in result.stderr
 
 
@@ -663,11 +662,10 @@ def test_editable_install__local_dir_no_setup_py(script, data):
     """
     result = script.pip("install", "-e", data.root, expect_error=True)
     assert not result.files_created
-
-    msg = result.stderr
-    assert msg.startswith("ERROR: File 'setup.py' or 'setup.cfg' not found ")
-    assert "cannot be installed in editable mode" in msg
-    assert "pyproject.toml" not in msg
+    assert (
+        "does not appear to be a Python project: "
+        "neither 'setup.py' nor 'pyproject.toml' found" in result.stderr
+    )
 
 
 def test_editable_install__local_dir_no_setup_py_with_pyproject(script):

tests/unit/test_pep517.py

@@ -27,6 +27,22 @@ def test_use_pep517(shared_data: TestData, source: str, expected: bool) -> None:
     assert req.use_pep517 is expected
 
 
+def test_use_pep517_rejects_setup_cfg_only(shared_data: TestData) -> None:
+    """
+    Test that projects with setup.cfg but no pyproject.toml are rejected.
+    """
+    src = shared_data.src.joinpath("pep517_setup_cfg_only")
+    req = InstallRequirement(None, None)
+    req.source_dir = src  # make req believe it has been unpacked
+    with pytest.raises(InstallationError) as e:
+        req.load_pyproject_toml()
+    err_msg = e.value.args[0]
+    assert (
+        "does not appear to be a Python project: "
+        "neither 'setup.py' nor 'pyproject.toml' found" in err_msg
+    )
+
+
 @pytest.mark.parametrize(
     ("source", "msg"),
     [