Add detailed error message for unsupported dependency case (#184)

Author: gbmarc1

src/poetry_plugin_export/walker.py

@@ -242,10 +242,27 @@ def get_locked_package(
 
     # If we have an overlapping candidate, we must use it.
     if overlapping_candidates:
-        compatible_candidates = [
+        filtered_compatible_candidates = [
             package
             for package in compatible_candidates
             if package in overlapping_candidates
         ]
 
+        if not filtered_compatible_candidates:
+            # TODO: Support this case:
+            # https://github.com/python-poetry/poetry-plugin-export/issues/183
+            raise DependencyWalkerError(
+                f"The `{dependency.name}` package has the following compatible"
+                f" candidates `{compatible_candidates}`;  but, the exporter dependency"
+                f" walker previously elected `{overlapping_candidates.pop()}` which is"
+                f" not compatible with the dependency `{dependency}`. Please contribute"
+                " to `poetry-plugin-export` to solve this problem."
+            )
+
+        compatible_candidates = filtered_compatible_candidates
+
     return next(iter(compatible_candidates), None)
+
+
+class DependencyWalkerError(Exception):
+    pass

tests/test_walker.py

@@ -0,0 +1,27 @@
+from __future__ import annotations
+
+import pytest
+
+from packaging.utils import NormalizedName
+from poetry.core.packages.dependency import Dependency
+from poetry.core.packages.package import Package
+
+from poetry_plugin_export.walker import DependencyWalkerError
+from poetry_plugin_export.walker import walk_dependencies
+
+
+def test_walk_dependencies_multiple_versions_when_latest_is_not_compatible() -> None:
+    # TODO: Support this case:
+    # https://github.com/python-poetry/poetry-plugin-export/issues/183
+    with pytest.raises(DependencyWalkerError):
+        walk_dependencies(
+            dependencies=[
+                Dependency("grpcio", ">=1.42.0"),
+                Dependency("grpcio", ">=1.42.0,<=1.49.1"),
+                Dependency("grpcio", ">=1.47.0,<2.0dev"),
+            ],
+            packages_by_name={
+                "grpcio": [Package("grpcio", "1.51.3"), Package("grpcio", "1.49.1")]
+            },
+            root_package_name=NormalizedName("package-name"),
+        )