Show constraint in error message

news/9300.bugfix.rst

@@ -0,0 +1,2 @@
+New resolver: Show relevant entries from user-supplied constraint files in the
+error message to improve debuggability.

src/pip/_internal/resolution/resolvelib/factory.py

@@ -404,8 +404,24 @@ def _report_requires_python_error(
         )
         return UnsupportedPythonVersion(message)
 
-    def get_installation_error(self, e):
-        # type: (ResolutionImpossible) -> InstallationError
+    def _report_single_requirement_conflict(self, req, parent):
+        # type: (Requirement, Candidate) -> DistributionNotFound
+        if parent is None:
+            req_disp = str(req)
+        else:
+            req_disp = f"{req} (from {parent.name})"
+        logger.critical(
+            "Could not find a version that satisfies the requirement %s",
+            req_disp,
+        )
+        return DistributionNotFound(f"No matching distribution found for {req}")
+
+    def get_installation_error(
+        self,
+        e,  # type: ResolutionImpossible
+        constraints,  # type: Dict[str, Constraint]
+    ):
+        # type: (...) -> InstallationError
 
         assert e.causes, "Installation error reported with no cause"
 
@@ -425,15 +441,8 @@ def get_installation_error(self, e):
         # satisfied. We just report that case.
         if len(e.causes) == 1:
             req, parent = e.causes[0]
-            if parent is None:
-                req_disp = str(req)
-            else:
-                req_disp = f"{req} (from {parent.name})"
-            logger.critical(
-                "Could not find a version that satisfies the requirement %s",
-                req_disp,
-            )
-            return DistributionNotFound(f"No matching distribution found for {req}")
+            if req.name not in constraints:
+                return self._report_single_requirement_conflict(req, parent)
 
         # OK, we now have a list of requirements that can't all be
         # satisfied at once.
@@ -475,13 +484,20 @@ def describe_trigger(parent):
         )
         logger.critical(msg)
         msg = "\nThe conflict is caused by:"
+
+        relevant_constraints = set()
         for req, parent in e.causes:
+            if req.name in constraints:
+                relevant_constraints.add(req.name)
             msg = msg + "\n    "
             if parent:
                 msg = msg + "{} {} depends on ".format(parent.name, parent.version)
             else:
                 msg = msg + "The user requested "
             msg = msg + req.format_for_error()
+        for key in relevant_constraints:
+            spec = constraints[key].specifier
+            msg += f"\n    The user requested (constraint) {key}{spec}"
 
         msg = (
             msg

src/pip/_internal/resolution/resolvelib/resolver.py

@@ -122,7 +122,7 @@ def resolve(self, root_reqs, check_supported_wheels):
             )
 
         except ResolutionImpossible as e:
-            error = self.factory.get_installation_error(e)
+            error = self.factory.get_installation_error(e, constraints)
             six.raise_from(error, e)
 
         req_set = RequirementSet(check_supported_wheels=check_supported_wheels)

tests/functional/test_install_reqs.py

@@ -357,7 +357,7 @@ def test_constraints_local_editable_install_causes_error(
         assert 'Could not satisfy constraints' in result.stderr, str(result)
     else:
         # Because singlemodule only has 0.0.1 available.
-        assert 'No matching distribution found' in result.stderr, str(result)
+        assert 'Cannot install singlemodule 0.0.1' in result.stderr, str(result)
 
 
 @pytest.mark.network
@@ -386,7 +386,7 @@ def test_constraints_local_install_causes_error(
         assert 'Could not satisfy constraints' in result.stderr, str(result)
     else:
         # Because singlemodule only has 0.0.1 available.
-        assert 'No matching distribution found' in result.stderr, str(result)
+        assert 'Cannot install singlemodule 0.0.1' in result.stderr, str(result)
 
 
 def test_constraints_constrain_to_local_editable(

tests/functional/test_new_resolver.py

@@ -687,7 +687,7 @@ def test_new_resolver_constraint_on_dependency(script):
 @pytest.mark.parametrize(
     "constraint_version, expect_error, message",
     [
-        ("1.0", True, "ERROR: No matching distribution found for foo 2.0"),
+        ("1.0", True, "Cannot install foo 2.0"),
         ("2.0", False, "Successfully installed foo-2.0"),
     ],
 )

tests/functional/test_new_resolver_errors.py

@@ -24,3 +24,24 @@ def test_new_resolver_conflict_requirements_file(tmpdir, script):
 
     message = "package versions have conflicting dependencies"
     assert message in result.stderr, str(result)
+
+
+def test_new_resolver_conflict_constraints_file(tmpdir, script):
+    create_basic_wheel_for_package(script, "pkg", "1.0")
+
+    constrats_file = tmpdir.joinpath("constraints.txt")
+    constrats_file.write_text("pkg!=1.0")
+
+    result = script.pip(
+        "install",
+        "--no-cache-dir", "--no-index",
+        "--find-links", script.scratch_path,
+        "-c", constrats_file,
+        "pkg==1.0",
+        expect_error=True,
+    )
+
+    assert "ResolutionImpossible" in result.stderr, str(result)
+
+    message = "The user requested (constraint) pkg!=1.0"
+    assert message in result.stdout, str(result)