Better diagnose when setup.py/cfg cannot be found

news/9944.bugfix.rst

@@ -0,0 +1,2 @@
+Emit clearer error message when a project root does not contain either
+``pyproject.toml``, ``setup.py`` or ``setup.cfg``.

src/pip/_internal/req/req_install.py

@@ -509,13 +509,32 @@ def load_pyproject_toml(self):
  self.unpacked_source_directory, backend, backend_path=backend_path,
  )
 
+ def _check_setup_py_or_cfg_exists(self) -> bool:
+ """Check if the requirement actually has a setuptools build file.
+
+ If setup.py does not exist, we also check setup.cfg in the same
+ directory and allow the directory if that exists.
+ """
+ if os.path.exists(self.setup_py_path):
+ return True
+ stem, ext = os.path.splitext(self.setup_py_path)
+ if ext == ".py" and os.path.exists(f"{stem}.cfg"):
+ return True
+ return False
+
  def _generate_metadata(self):
  # type: () -> str
  """Invokes metadata generator functions, with the required arguments.
  """
  if not self.use_pep517:
  assert self.unpacked_source_directory
 
+ if not self._check_setup_py_or_cfg_exists():
+ raise InstallationError(
+ f'File "setup.py" or "setup.cfg" not found for legacy '
+ f'project {self}.'
+ )
+
  return generate_metadata_legacy(
  build_env=self.build_env,
  setup_py_path=self.setup_py_path,