Pyproject validation: report an error when a license file does not exist

src/poetry/core/factory.py

@@ -577,6 +577,17 @@ def validate(
                 for e in validate_object(project, "project-schema")
             ]
             result["errors"] += project_validation_errors
+
+        # Ensure license file exists, if project.license.file is set
+        if project is not None and project.get("license") is not None:
+            license_data = project.get("license")
+            if isinstance(license_data, dict) and "file" in license_data:
+                license_path = Path(license_data["file"]).absolute()
+                if not license_path.exists():
+                    result["errors"].append(
+                        f"project.license.file '{license_path}' does not exist"
+                    )
+
         # With PEP 621 [tool.poetry] is not mandatory anymore. We still create and
         # validate it so that default values (e.g. for package-mode) are set.
         tool_poetry = toml_data.setdefault("tool", {}).setdefault("poetry", {})

tests/fixtures/missing_license_file/pyproject.toml

@@ -0,0 +1,5 @@
+[project]
+name = "my-package"
+version = "0.1"
+license = { file = "LICENSE" } # License file is intentionally missing
+keywords = ["special"]         # field that comes after license in core metadata

tests/test_factory.py

@@ -19,6 +19,7 @@
 from poetry.core.pyproject.tables import BuildSystem
 from poetry.core.utils._compat import tomllib
 from poetry.core.version.markers import SingleMarker
+from tests.testutils import temporary_cd
 
 
 if TYPE_CHECKING:
@@ -636,6 +637,31 @@ def test_validate_fails() -> None:
     assert Factory.validate(content) == {"errors": [expected], "warnings": []}
 
 
+def test_validate_missing_license_file() -> None:
+    complete = fixtures_dir / "missing_license_file/pyproject.toml"
+    with complete.open("rb") as f:
+        content = tomllib.load(f)
+
+    with temporary_cd(complete.parent):
+        license_path = Path(content["project"]["license"]["file"]).absolute()
+        expected = f"project.license.file '{license_path}' does not exist"
+
+        assert Factory.validate(content) == {"errors": [expected], "warnings": []}
+
+
+def test_validate_existing_license_file() -> None:
+    # Ensure that no error is reported when a project.license.file is given,
+    # and the file does exist.
+    complete = fixtures_dir / "with_license_type_file/pyproject.toml"
+    with complete.open("rb") as f:
+        content = tomllib.load(f)
+
+    # Still need to cd to avoid spurious pass due to the project's root LICENSE
+    # file.
+    with temporary_cd(complete.parent):
+        assert Factory.validate(content) == {"errors": [], "warnings": []}
+
+
 def test_validate_without_strict_fails_only_non_strict() -> None:
     project_failing_strict_validation = (
         fixtures_dir / "project_failing_strict_validation" / "pyproject.toml"

tests/testutils.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import os
 import shutil
 import subprocess
 import sys
@@ -29,6 +30,23 @@
 }
 
 
+@contextmanager
+def temporary_cd(path: Path) -> Generator[None, None, None]:
+    """
+    Context manager that temporarily CDs into the given directory.
+
+    Once the context exits, the cwd is returned to its previous state.
+
+    :param path: Path to cd into.
+    """
+    old_path = Path.cwd()
+    os.chdir(path)
+    try:
+        yield
+    finally:
+        os.chdir(old_path)
+
+
 @contextmanager
 def temporary_project_directory(
     path: Path, toml_patch: dict[str, Any] | None = None