Evaluate markers under environment with no 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
 ~~~~~~~~~~~~~~~~~

packaging/markers.py

@@ -274,6 +274,7 @@ 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"),