fix(init): validate version constraint in interactive prompt

Resolves #8797

Author: SarthakB11

src/poetry/console/commands/init.py

@@ -382,7 +382,7 @@ def _determine_requirements(
                         "(or leave blank to use the latest version):"
                     )
                     question.set_max_attempts(3)
-                    question.set_validator(lambda x: (x or "").strip() or None)
+                    question.set_validator(self._validate_version_constraint)
 
                     package_constraint = self.ask(question)
 
@@ -521,6 +521,21 @@ def _validate_package(package: str | None) -> str | None:
 
         return package
 
+    @staticmethod
+    def _validate_version_constraint(constraint: str | None) -> str | None:
+        from poetry.core.constraints.version import parse_constraint
+
+        constraint = (constraint or "").strip() or None
+        if constraint is None:
+            return None
+
+        try:
+            parse_constraint(constraint)
+        except ValueError as e:
+            raise ValueError(f"Invalid version constraint: {constraint}") from e
+
+        return constraint
+
     def _get_pool(self) -> RepositoryPool:
         from poetry.config.config import Config
         from poetry.repositories import RepositoryPool

tests/console/commands/test_init.py

@@ -1217,3 +1217,34 @@ def test_init_does_not_add_readme_key_when_readme_missing(
     # Assert
     pyproject = (tmp_path / "pyproject.toml").read_text(encoding="utf-8")
     assert "readme =" not in pyproject
+
+
+@pytest.mark.parametrize(
+    ("constraint", "expected"),
+    [
+        ("^1.0", "^1.0"),
+        ("==1.2.3", "==1.2.3"),
+        (">=1,<2", ">=1,<2"),
+        ("", None),
+        (None, None),
+        ("   ", None),
+    ],
+)
+def test_validate_version_constraint_accepts_valid_inputs(
+    constraint: str | None, expected: str | None
+) -> None:
+    assert InitCommand._validate_version_constraint(constraint) == expected
+
+
+@pytest.mark.parametrize(
+    "invalid",
+    [
+        "latest",
+        "isort",
+        "==1.,",
+        "not-a-version",
+    ],
+)
+def test_validate_version_constraint_rejects_invalid_inputs(invalid: str) -> None:
+    with pytest.raises(ValueError, match="Invalid version constraint"):
+        InitCommand._validate_version_constraint(invalid)