Add error message specifying that we can't parse pyproject.toml (#734)

.pre-commit-config.yaml

@@ -14,6 +14,7 @@ repos:
       - id: check-case-conflict
       - id: check-json
       - id: check-toml
+        exclude: tests/fixtures/project_duplicate_dependency/pyproject.toml
       - id: check-yaml
       - id: pretty-format-json
         args:

src/poetry/core/pyproject/toml.py

@@ -28,8 +28,21 @@ def data(self) -> dict[str, Any]:
             if not self.path.exists():
                 self._data = {}
             else:
-                with self.path.open("rb") as f:
-                    self._data = tomllib.load(f)
+                try:
+                    with self.path.open("rb") as f:
+                        self._data = tomllib.load(f)
+                except tomllib.TOMLDecodeError as e:
+                    from poetry.core.pyproject.exceptions import PyProjectException
+
+                    msg = (
+                        f"{self._path.as_posix()} is not a valid TOML file.\n"
+                        f"{e.__class__.__name__}: {e}"
+                    )
+
+                    if str(e).startswith("Cannot overwrite a value"):
+                        msg += "\nThis is often caused by a duplicate entry."
+
+                    raise PyProjectException(msg) from e
 
         return self._data
 

tests/fixtures/project_duplicate_dependency/pyproject.toml

@@ -0,0 +1,7 @@
+[project]
+name = "minimal"
+version = "1"
+
+[tool.poetry.dependencies]
+python = "^3.8"
+python = "^3.8"

tests/pyproject/test_pyproject_toml.py

@@ -72,3 +72,21 @@ def test_pyproject_toml_non_existent(pyproject_toml: Path) -> None:
     assert pyproject.data == {}
     assert build_system.requires == ["poetry-core"]
     assert build_system.build_backend == "poetry.core.masonry.api"
+
+
+def test_unparseable_pyproject_toml() -> None:
+    pyproject_toml = (
+        Path(__file__).parent.parent
+        / "fixtures"
+        / "project_duplicate_dependency"
+        / "pyproject.toml"
+    )
+
+    with pytest.raises(PyProjectException) as excval:
+        _ = PyProjectTOML(pyproject_toml).build_system
+
+    assert (
+        f"{pyproject_toml.as_posix()} is not a valid TOML file.\n"
+        "TOMLDecodeError: Cannot overwrite a value (at line 7, column 16)\n"
+        "This is often caused by a duplicate entry"
+    ) in str(excval.value)