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

Co-Authored-By: Tzu-ping Chung <uranusjr@gmail.com>
Co-Authored-By: Pradyun Gedam <3275593+pradyunsg@users.noreply.github.com>

news/9547.feature.rst

@@ -0,0 +1 @@
+Add support for editable installs for project with only setup.cfg files.

src/pip/_internal/req/constructors.py

@@ -77,16 +77,19 @@ 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')):
+        setup_py = os.path.join(url_no_extras, 'setup.py')
+        setup_cfg = os.path.join(url_no_extras, 'setup.cfg')
+        if not os.path.exists(setup_py) and not os.path.exists(setup_cfg):
             msg = (
-                'File "setup.py" not found. Directory cannot be installed '
-                'in editable mode: {}'.format(os.path.abspath(url_no_extras))
+                '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.)'
                 )
             raise InstallationError(msg)
 

src/pip/_internal/utils/setuptools_build.py

@@ -8,9 +8,11 @@
 # 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__);"
-    "code=f.read().replace('\\r\\n', '\\n');"
+    "import io, os, sys, setuptools, tokenize; sys.argv[0] = {0!r}; __file__={0!r};"
+    "f = getattr(tokenize, 'open', open)(__file__) "
+    "if os.path.exists(__file__) "
+    "else io.StringIO('from setuptools import setup; setup()');"
+    "code = f.read().replace('\\r\\n', '\\n');"
     "f.close();"
     "exec(compile(code, __file__, 'exec'))"
 )

tests/functional/test_install.py

@@ -641,9 +641,9 @@ def test_editable_install__local_dir_no_setup_py(
 
     msg = result.stderr
     if deprecated_python:
-        assert 'File "setup.py" not found. ' in msg
+        assert 'File "setup.py" or "setup.cfg" not found. ' in msg
     else:
-        assert msg.startswith('ERROR: File "setup.py" not found. ')
+        assert msg.startswith('ERROR: File "setup.py" or "setup.cfg" not found. ')
     assert 'pyproject.toml' not in msg
 
 
@@ -663,9 +663,9 @@ def test_editable_install__local_dir_no_setup_py_with_pyproject(
 
     msg = result.stderr
     if deprecated_python:
-        assert 'File "setup.py" not found. ' in msg
+        assert 'File "setup.py" or "setup.cfg" not found. ' in msg
     else:
-        assert msg.startswith('ERROR: File "setup.py" not found. ')
+        assert msg.startswith('ERROR: File "setup.py" or "setup.cfg" not found. ')
     assert 'A "pyproject.toml" file was found' in msg
 
 
@@ -1034,15 +1034,13 @@ def test_install_package_with_prefix(script, data):
     result.did_create(install_path)
 
 
-def test_install_editable_with_prefix(script):
+def _test_install_editable_with_prefix(script, files):
     # make a dummy project
     pkga_path = script.scratch_path / 'pkga'
     pkga_path.mkdir()
-    pkga_path.joinpath("setup.py").write_text(textwrap.dedent("""
-        from setuptools import setup
-        setup(name='pkga',
-              version='0.1')
-    """))
+
+    for fn, contents in files.items():
+        pkga_path.joinpath(fn).write_text(textwrap.dedent(contents))
 
     if hasattr(sys, "pypy_version_info"):
         site_packages = os.path.join(
@@ -1087,6 +1085,28 @@ def test_install_editable_with_target(script):
     result.did_create(script.scratch / 'target' / 'watching_testrunner.py')
 
 
+def test_install_editable_with_prefix_setup_py(script):
+    setup_py = """
+from setuptools import setup
+setup(name='pkga', version='0.1')
+"""
+    _test_install_editable_with_prefix(script, {"setup.py": setup_py})
+
+
+def test_install_editable_with_prefix_setup_cfg(script):
+    setup_cfg = """[metadata]
+name = pkga
+version = 0.1
+"""
+    pyproject_toml = """[build-system]
+requires = ["setuptools", "wheel"]
+build-backend = "setuptools.build_meta"
+"""
+    _test_install_editable_with_prefix(
+        script, {"setup.cfg": setup_cfg, "pyproject.toml": pyproject_toml}
+    )
+
+
 def test_install_package_conflict_prefix_and_user(script, data):
     """
     Test installing a package using pip install --prefix --user errors out