reject empty python versions (#761)

src/poetry/core/packages/package.py

@@ -278,6 +278,9 @@ def python_versions(self, value: str) -> None:
         except ParseConstraintError:
             raise ParseConstraintError(f"Invalid python versions '{value}' on {self}")
 
+        if constraint.is_empty():
+            raise ParseConstraintError(f"Python versions '{value}' on {self} is empty")
+
         self._python_versions = value
         self._python_constraint = constraint
         self._python_marker = parse_marker(

src/poetry/core/packages/utils/utils.py

@@ -239,7 +239,9 @@ def create_nested_marker(
 
         marker = f'{name} == "{constraint.text}"'
     else:
-        assert isinstance(constraint, VersionRange)
+        assert isinstance(
+            constraint, VersionRange
+        ), f"Unexpected constraint of type {type(constraint)}"
         min_name = max_name = name
 
         parts = []

tests/packages/test_package.py

@@ -658,3 +658,12 @@ def test_package_invalid_python_versions() -> None:
 
     expected = "Invalid python versions '>=3.6.y' on foo (1.2.3)"
     assert str(exc_info.value) == expected
+
+
+def test_package_empty_python_versions() -> None:
+    package = Package("foo", "1.2.3")
+    with pytest.raises(ParseConstraintError) as exc_info:
+        package.python_versions = "~2.7, >=3.4, <3.8"
+
+    expected = "Python versions '~2.7, >=3.4, <3.8' on foo (1.2.3) is empty"
+    assert str(exc_info.value) == expected