fix(markers): only parse versions on certain keys

According to the [environment
markers](https://packaging.python.org/en/latest/specifications/dependency-specifiers/#environment-markers),
most markers are strings, with only a small subset being use to handle
versions.

This commit ensures that only those keys which _are_ versions get
compared as versions, and all other keys are compared as string
literals.

Signed-off-by: JP-Ellis <[email protected]>

src/packaging/markers.py

@@ -29,6 +29,11 @@
 Operator = Callable[[str, Union[str, AbstractSet[str]]], bool]
 EvaluateContext = Literal["metadata", "lock_file", "requirement"]
 MARKERS_ALLOWING_SET = {"extras", "dependency_groups"}
+MARKERS_REQUIRING_VERSION = {
+    "python_version",
+    "python_full_version",
+    "implementation_version",
+}
 
 
 class InvalidMarker(ValueError):
@@ -186,16 +191,17 @@ def _format_marker(
 }
 
 
-def _eval_op(lhs: str, op: Op, rhs: str | AbstractSet[str]) -> bool:
-    if isinstance(rhs, str):
+def _eval_op(lhs: str, op: Op, rhs: str | AbstractSet[str], *, key: str) -> bool:
+    op_str = op.serialize()
+    if key in MARKERS_REQUIRING_VERSION:
         try:
-            spec = Specifier(f"{op.serialize()}{rhs}")
+            spec = Specifier(f"{op_str}{rhs}")
         except InvalidSpecifier:
             pass
         else:
             return spec.contains(lhs, prereleases=True)
 
-    oper: Operator | None = _operators.get(op.serialize())
+    oper: Operator | None = _operators.get(op_str)
     if oper is None:
         raise UndefinedComparison(f"Undefined {op!r} on {lhs!r} and {rhs!r}.")
 
@@ -242,9 +248,10 @@ def _evaluate_markers(
                 lhs_value = lhs.value
                 environment_key = rhs.value
                 rhs_value = environment[environment_key]
+
             assert isinstance(lhs_value, str), "lhs must be a string"
             lhs_value, rhs_value = _normalize(lhs_value, rhs_value, key=environment_key)
-            groups[-1].append(_eval_op(lhs_value, op, rhs_value))
+            groups[-1].append(_eval_op(lhs_value, op, rhs_value, key=environment_key))
         elif marker == "or":
             groups.append([])
         elif marker == "and":

tests/test_markers.py

@@ -443,3 +443,24 @@ def test_extras_and_dependency_groups_disallowed(self, variable: str) -> None:
 
         with pytest.raises(KeyError):
             marker.evaluate(context="requirement")
+
+    @pytest.mark.parametrize(
+        ("marker_string", "environment", "expected"),
+        [
+            ('extra == "v2"', None, False),
+            ('extra == "v2"', {"extra": ""}, False),
+            ('extra == "v2"', {"extra": "v2"}, True),
+            ('extra == "v2"', {"extra": "v2a3"}, False),
+            ('extra == "v2a3"', {"extra": "v2"}, False),
+            ('extra == "v2a3"', {"extra": "v2a3"}, True),
+        ],
+    )
+    def test_version_like_equality(
+        self, marker_string: str, environment: dict[str, str] | None, expected: bool
+    ) -> None:
+        """
+        Test for issue #938: Extras are meant to be literal strings, even if
+        they look like versions, and therefore should not be parsed as version.
+        """
+        marker = Marker(marker_string)
+        assert marker.evaluate(environment) is expected