Evaluate markers under environment with empty "extra"

CHANGELOG.rst

@@ -4,7 +4,9 @@ Changelog
 *unreleased*
 ~~~~~~~~~~~~
 
-No unreleased changes.
+* ``Marker.evaluate`` will now assume evaluation environment with empty ``extra``.
+  Evaluating markers like ``"extra == 'xyz'"`` without passing any extra in the
+  ``environment`` will no longer raise an exception.
 
 21.3 - 2021-11-17
 ~~~~~~~~~~~~~~~~~

docs/markers.rst

@@ -34,17 +34,6 @@ Usage
     >>> # Markers can be also used with extras, to pull in dependencies if
     >>> # a certain extra is being installed
     >>> extra = Marker('extra == "bar"')
-    >>> # Evaluating an extra marker with no environment is an error
-    >>> try:
-    ...     extra.evaluate()
-    ... except UndefinedEnvironmentName:
-    ...     pass
-    >>> extra_environment = {'extra': ''}
-    >>> extra.evaluate(environment=extra_environment)
-    False
-    >>> extra_environment['extra'] = 'bar'
-    >>> extra.evaluate(environment=extra_environment)
-    True
     >>> # You can do simple comparisons between marker objects:
     >>> Marker("python_version > '3.6'") == Marker("python_version > '3.6'")
     True

packaging/markers.py

@@ -215,24 +215,6 @@ def _eval_op(lhs: str, op: Op, rhs: str) -> bool:
     return oper(lhs, rhs)
 
 
-class Undefined:
-    pass
-
-
-_undefined = Undefined()
-
-
-def _get_env(environment: Dict[str, str], name: str) -> str:
-    value: Union[str, Undefined] = environment.get(name, _undefined)
-
-    if isinstance(value, Undefined):
-        raise UndefinedEnvironmentName(
-            f"{name!r} does not exist in evaluation environment."
-        )
-
-    return value
-
-
 def _normalize(*values: str, key: str) -> Tuple[str, ...]:
     # PEP 685 – Comparison of extra names for optional distribution dependencies
     # https://peps.python.org/pep-0685/
@@ -258,12 +240,12 @@ def _evaluate_markers(markers: List[Any], environment: Dict[str, str]) -> bool:
 
             if isinstance(lhs, Variable):
                 environment_key = lhs.value
-                lhs_value = _get_env(environment, environment_key)
+                lhs_value = environment[environment_key]
                 rhs_value = rhs.value
             else:
                 lhs_value = lhs.value
                 environment_key = rhs.value
-                rhs_value = _get_env(environment, environment_key)
+                rhs_value = environment[environment_key]
 
             lhs_value, rhs_value = _normalize(lhs_value, rhs_value, key=environment_key)
             groups[-1].append(_eval_op(lhs_value, op, rhs_value))
@@ -287,6 +269,7 @@ def default_environment() -> Dict[str, str]:
     iver = format_full_version(sys.implementation.version)
     implementation_name = sys.implementation.name
     return {
+        "extra": "",
         "implementation_name": implementation_name,
         "implementation_version": iver,
         "os_name": os.name,

tests/test_markers.py

@@ -15,7 +15,6 @@
     Marker,
     Node,
     UndefinedComparison,
-    UndefinedEnvironmentName,
     default_environment,
     format_full_version,
 )
@@ -110,6 +109,7 @@ def test_matches_expected(self):
             )
 
         assert environment == {
+            "extra": "",
             "implementation_name": sys.implementation.name,
             "implementation_version": iver,
             "os_name": os.name,
@@ -253,11 +253,8 @@ def test_compare_markers_to_other_objects(self):
         # Markers should not be comparable to other kinds of objects.
         assert Marker("os_name == 'nt'") != "os_name == 'nt'"
 
-    def test_extra_with_no_extra_in_environment(self):
-        # We can't evaluate an extra if no extra is passed into the environment
-        m = Marker("extra == 'security'")
-        with pytest.raises(UndefinedEnvironmentName):
-            m.evaluate()
+    def test_environment_assumes_empty_extra(self):
+        assert Marker('extra == "im_valid"').evaluate() is False
 
     @pytest.mark.parametrize(
         ("marker_string", "environment", "expected"),