Permit --no-use-pep517 with editable mode in more cases.

news/6434.feature

@@ -0,0 +1,7 @@
+Allow ``--no-use-pep517`` to be used as a work-around when installing a
+project in editable mode, even when `PEP 517
+<https://www.python.org/dev/peps/pep-0517/>`_ mandates
+``pyproject.toml``-style processing (i.e. when the project has a
+``pyproject.toml`` file as well as a ``"build-backend"`` key for the
+``"build_system"`` value). Since this option conflicts with the PEP 517 spec,
+this mode of operation is officially unsupported.

src/pip/_internal/pyproject.py

@@ -1,6 +1,7 @@
 from __future__ import absolute_import
 
 import io
+import logging
 import os
 import sys
 
@@ -15,6 +16,9 @@
     Pep517Data = Tuple[str, List[str]]
 
 
+logger = logging.getLogger(__name__)
+
+
 def _is_list_of_str(obj):
     # type: (Any) -> bool
     return (
@@ -133,7 +137,11 @@ def resolve_pyproject_toml(
     # opposed to False can occur when the value is provided via an
     # environment variable or config file option (due to the quirk of
     # strtobool() returning an integer in pip's configuration code).
-    if has_pyproject and not has_setup:
+    if editable and use_pep517:
+        raise make_editable_error(
+            req_name, 'PEP 517 processing was explicitly requested'
+        )
+    elif has_pyproject and not has_setup:
         if use_pep517 is not None and not use_pep517:
             raise InstallationError(
                 "Disabling PEP 517 processing is invalid: "
@@ -145,26 +153,45 @@ def resolve_pyproject_toml(
             )
         use_pep517 = True
     elif build_system and "build-backend" in build_system:
-        if use_pep517 is not None and not use_pep517:
+        if editable:
+            if use_pep517 is None:
+                message = (
+                    'Error installing {!r}: editable mode is not supported '
+                    'for pyproject.toml-style projects. '
+                    'This project is pyproject.toml-style because it has a '
+                    'pyproject.toml file and a "build-backend" key for the '
+                    '"build_system" value, but editable mode is undefined '
+                    'for pyproject.toml-style projects. '
+                    'Since the project has a setup.py, you may pass '
+                    '--no-use-pep517 to opt out of pyproject.toml-style '
+                    'processing. However, this is an unsupported combination. '
+                    'See PEP 517 for details on pyproject.toml-style projects.'
+                ).format(req_name)
+                raise InstallationError(message)
+
+            # The case of `editable and use_pep517` being true was already
+            # handled above.
+            assert not use_pep517
+            message = (
+                'Installing {!r} in editable mode, which is not supported '
+                'for pyproject.toml-style projects: '
+                'this project is pyproject.toml-style because it has a '
+                'pyproject.toml file and a "build-backend" key for the '
+                '"build_system" value, but editable mode is undefined '
+                'for pyproject.toml-style projects. '
+                'See PEP 517 for details on pyproject.toml-style projects.'
+            ).format(req_name)
+            logger.warning(message)
+        elif use_pep517 is not None and not use_pep517:
             raise InstallationError(
                 "Disabling PEP 517 processing is invalid: "
                 "project specifies a build backend of {} "
                 "in pyproject.toml".format(
                     build_system["build-backend"]
                 )
             )
-        if editable:
-            reason = (
-                'it has a pyproject.toml file with a "build-backend" key '
-                'in the "build_system" value'
-            )
-            raise make_editable_error(req_name, reason)
-        use_pep517 = True
-    elif use_pep517:
-        if editable:
-            raise make_editable_error(
-                req_name, 'PEP 517 processing was explicitly requested'
-            )
+        else:
+            use_pep517 = True
 
     # If we haven't worked out whether to use PEP 517 yet, and the user
     # hasn't explicitly stated a preference, we do so if the project has

tests/unit/test_pep517.py

@@ -84,9 +84,6 @@ def test_resolve_pyproject_toml__editable_without_use_pep517_false():
     'has_pyproject, has_setup, use_pep517, build_system, expected_err', [
         # Test pyproject.toml with no setup.py.
         (True, False, None, None, 'has a pyproject.toml file and no setup.py'),
-        # Test "build-backend" present.
-        (True, True, None, {'build-backend': 'foo'},
-         'has a pyproject.toml file with a "build-backend" key'),
         # Test explicitly requesting PEP 517 processing.
         (True, True, True, None,
          'PEP 517 processing was explicitly requested'),
@@ -115,6 +112,55 @@ def test_resolve_pyproject_toml__editable_and_pep_517_required(
         )
 
 
+def test_resolve_pyproject_toml__editable_build_backend_use_pep517_none():
+    """
+    Test editable=True with "build-backend" and use_pep517=None.
+    """
+    expected_start = (
+        "Error installing 'my-package': editable mode is not supported "
+    )
+    expected_substr = 'you may pass --no-use-pep517 to opt out'
+
+    with assert_error_startswith(
+        InstallationError, expected_start, expected_substr=expected_substr,
+    ):
+        resolve_pyproject_toml(
+            build_system={'requires': ['my-package'], 'build-backend': 'foo'},
+            has_pyproject=True,
+            has_setup=True,
+            use_pep517=None,
+            editable=True,
+            req_name='my-package',
+        )
+
+
+def test_resolve_pyproject_toml__editable_build_backend_use_pep517_false(
+    caplog
+):
+    """
+    Test editable=True with "build-backend" and use_pep517=False.
+    """
+    resolve_pyproject_toml(
+        build_system={'requires': ['my-package'], 'build-backend': 'foo'},
+        has_pyproject=True,
+        has_setup=True,
+        use_pep517=False,
+        editable=True,
+        req_name='my-package',
+    )
+
+    records = caplog.records
+    assert len(records) == 1
+    record = records[0]
+    assert record.levelname == 'WARNING'
+    expected_start = (
+        "Installing 'my-package' in editable mode, which is not supported "
+    )
+    assert record.message.startswith(expected_start), (
+        'full message: {}'.format(record.message)
+    )
+
+
 @pytest.mark.parametrize(
     'has_pyproject, has_setup, use_pep517, editable, build_system, '
     'expected_err', [