Fixed --editable install for setuptools projects without setup.py.

news/9547.bugfix.rst

@@ -0,0 +1 @@
+When --editable install is used for setup.cfg-only setuptools-based projects it doesn't work.

src/pip/_internal/req/constructors.py

@@ -82,16 +82,17 @@ def parse_editable(editable_req):
     url_no_extras, extras = _strip_extras(url)
 
     if os.path.isdir(url_no_extras):
-        if not os.path.exists(os.path.join(url_no_extras, 'setup.py')):
+        if not os.path.exists(os.path.join(url_no_extras, 'setup.py')) and not os.path.exists(os.path.join(url_no_extras, 'setup.cfg')):
             msg = (
-                'File "setup.py" not found. Directory cannot be installed '
+                'File "setup.py" or "setup.cfg" not found. Directory cannot be installed '
                 'in editable mode: {}'.format(os.path.abspath(url_no_extras))
             )
             pyproject_path = make_pyproject_path(url_no_extras)
             if os.path.isfile(pyproject_path):
                 msg += (
                     '\n(A "pyproject.toml" file was found, but editable '
-                    'mode currently requires a setup.py based build.)'
+                    'mode currently requires a setuptools-based build'
+                    ' ("setup.py" or "setup.cfg" or both).)'
                 )
             raise InstallationError(msg)
 

src/pip/_internal/utils/setuptools_build.py

@@ -12,8 +12,8 @@
 # invoking via the shim.  This avoids e.g. the following manifest_maker
 # warning: "warning: manifest_maker: standard file '-c' not found".
 _SETUPTOOLS_SHIM = (
-    "import sys, setuptools, tokenize; sys.argv[0] = {0!r}; __file__={0!r};"
-    "f=getattr(tokenize, 'open', open)(__file__);"
+    "import sys, setuptools.build_meta, tokenize; sys.argv[0] = {0!r}; __file__={0!r};"
+    "f = setuptools.build_meta._open_setup_script(__file__) if hasattr(setuptools.build_meta, '_open_setup_script') else getattr(tokenize, 'open', open)(__file__);"
     "code=f.read().replace('\\r\\n', '\\n');"
     "f.close();"
     "exec(compile(code, __file__, 'exec'))"