Improve and migrate editable errors to diagnostic format

news/10421.feature.rst

@@ -0,0 +1,2 @@
+Present clearer errors when an invalid editable requirement is given or
+when a project's build backend does not support editable installs (PEP 660).

src/pip/_internal/exceptions.py

@@ -9,6 +9,7 @@
 import contextlib
 import locale
 import logging
+import os
 import pathlib
 import re
 import sys
@@ -775,3 +776,42 @@ def __init__(self, *, distribution: "BaseDistribution") -> None:
             ),
             hint_stmt=None,
         )
+
+
+class InvalidEditableRequirement(DiagnosticPipError):
+    reference = "invalid-editable-requirement"
+
+    def __init__(self, *, requirement: str, vcs_schemes: List[str]) -> None:
+        if os.path.sep in requirement and "://" not in requirement:
+            hint = (
+                "It appears to be a path, but does not exist. Was the right path given?"
+            )
+        else:
+            hint = (
+                "It should either be a path to a local project or a VCS URL "
+                f"(beginning with {', '.join(vcs_schemes)})."
+            )
+
+        super().__init__(
+            message=Text(f"{requirement} is not a valid editable requirement"),
+            context=(
+                "There would be no source tree that can be edited after installation."
+            ),
+            hint_stmt=hint,
+        )
+
+
+class EditableUnsupportedByBackend(DiagnosticPipError):
+    reference = "editable-mode-unsupported-by-backend"
+
+    def __init__(self, *, requirement: "InstallRequirement", backend: str) -> None:
+        super().__init__(
+            message=f"Cannot install {requirement} in editable mode",
+            context=(
+                f"The project's build backend ([magenta]{backend}[/]) does "
+                "not support editable installs.\n"
+                "Falling back to a setuptools legacy editable install is "
+                "unsupported as neither a 'setup.py' nor a 'setup.cfg' was found."
+            ),
+            hint_stmt=Text("Consider using a build backend that supports PEP 660."),
+        )

src/pip/_internal/req/constructors.py

@@ -19,7 +19,7 @@
 from pip._vendor.packaging.requirements import InvalidRequirement, Requirement
 from pip._vendor.packaging.specifiers import Specifier
 
-from pip._internal.exceptions import InstallationError
+from pip._internal.exceptions import InstallationError, InvalidEditableRequirement
 from pip._internal.models.index import PyPI, TestPyPI
 from pip._internal.models.link import Link
 from pip._internal.models.wheel import Wheel
@@ -123,11 +123,8 @@ def parse_editable(editable_req: str) -> Tuple[Optional[str], str, Set[str]]:
     link = Link(url)
 
     if not link.is_vcs:
-        backends = ", ".join(vcs.all_schemes)
-        raise InstallationError(
-            f"{editable_req} is not a valid editable requirement. "
-            f"It should either be a path to a local project or a VCS URL "
-            f"(beginning with {backends})."
+        raise InvalidEditableRequirement(
+            requirement=editable_req, vcs_schemes=vcs.all_schemes
         )
 
     package_name = link.egg_fragment

src/pip/_internal/req/req_install.py

@@ -18,7 +18,11 @@
 from pip._vendor.pyproject_hooks import BuildBackendHookCaller
 
 from pip._internal.build_env import BuildEnvironment, NoOpBuildEnvironment
-from pip._internal.exceptions import InstallationError, PreviousBuildDirError
+from pip._internal.exceptions import (
+    EditableUnsupportedByBackend,
+    InstallationError,
+    PreviousBuildDirError,
+)
 from pip._internal.locations import get_scheme
 from pip._internal.metadata import (
     BaseDistribution,
@@ -541,12 +545,9 @@ def isolated_editable_sanity_check(self) -> None:
             and not os.path.isfile(self.setup_py_path)
             and not os.path.isfile(self.setup_cfg_path)
         ):
-            raise InstallationError(
-                f"Project {self} has a 'pyproject.toml' and its build "
-                f"backend is missing the 'build_editable' hook. Since it does not "
-                f"have a 'setup.py' nor a 'setup.cfg', "
-                f"it cannot be installed in editable mode. "
-                f"Consider using a build backend that supports PEP 660."
+            assert self.pep517_backend is not None, "backend should be loaded!"
+            raise EditableUnsupportedByBackend(
+                requirement=self, backend=self.pep517_backend.build_backend
             )
 
     def prepare_metadata(self) -> None:

tests/functional/test_install.py

@@ -879,9 +879,8 @@ def test_editable_install_legacy__local_dir_no_setup_py_with_pyproject(
     assert not result.files_created
 
     msg = result.stderr
-    assert "has a 'pyproject.toml'" in msg
-    assert "does not have a 'setup.py' nor a 'setup.cfg'" in msg
-    assert "cannot be installed in editable mode" in msg
+    assert "editable-mode-unsupported-by-backend" in msg
+    assert "setuptools.build_meta" in msg
 
 
 def test_editable_install__local_dir_setup_requires_with_pyproject(

tests/functional/test_pep660.py

@@ -62,11 +62,17 @@ def build_editable(wheel_directory, config_settings=None, metadata_directory=Non
 
 
 def _make_project(
-    tmpdir: Path, backend_code: str, with_setup_py: bool, with_pyproject: bool = True
+    tmpdir: Path,
+    backend_code: str,
+    *,
+    with_setup_py: bool,
+    with_setup_cfg: bool = True,
+    with_pyproject: bool = True,
 ) -> Path:
     project_dir = tmpdir / "project"
     project_dir.mkdir()
-    project_dir.joinpath("setup.cfg").write_text(SETUP_CFG)
+    if with_setup_cfg:
+        project_dir.joinpath("setup.cfg").write_text(SETUP_CFG)
     if with_setup_py:
         project_dir.joinpath("setup.py").write_text(SETUP_PY)
     if backend_code:
@@ -259,3 +265,19 @@ def test_download_editable_pep660_basic(
     _assert_hook_not_called(project_dir, "prepare_metadata_for_build_editable")
     _assert_hook_called(project_dir, "prepare_metadata_for_build_wheel")
     assert len(os.listdir(str(download_dir))) == 1, "a zip should have been created"
+
+
+def test_install_editable_unsupported_by_backend(
+    tmpdir: Path, script: PipTestEnvironment
+) -> None:
+    """
+    Check that pip errors out when installing a project whose backend does not
+    support PEP 660 and falling back to a legacy editable install is impossible
+    (no 'setup.py' or 'setup.py.cfg').
+    """
+    project_dir = _make_project(
+        tmpdir, BACKEND_WITHOUT_PEP660, with_setup_py=False, with_setup_cfg=False
+    )
+    result = script.pip("install", "--editable", project_dir, expect_error=True)
+    assert "editable-mode-unsupported-by-backend" in result.stderr
+    assert "Consider using a build backend that supports PEP 660" in result.stderr