Adjust expectation that 'extra' is not in the marker evaluation if no…

… extras demanded the requirement.

pkg_resources/__init__.py

@@ -35,6 +35,7 @@
 import email.parser
 import tempfile
 import textwrap
+import itertools
 from pkgutil import get_importer
 
 try:
@@ -986,16 +987,17 @@ def markers_pass(self, req):
         Return False if the req has a marker and fails
         evaluation. Otherwise, return True.
         """
-        if not req.marker:
-            return True
-
-        result = []
-        if req in self:
-            for extra in self[req] or ['']:
-                result.append(req.marker.evaluate({'extra': extra}))
-        else:
-            result.append(req.marker.evaluate())
-        return any(result)
+        extra_evals = (
+            req.marker.evaluate({'extra': extra})
+            for extra in self.get(req, ())
+        )
+        # set up a late-evaluated simple marker evaluation.
+        simple_eval = (
+            req.marker.evaluate()
+            for _ in (None,)
+        )
+        evals = itertools.chain(extra_evals, simple_eval)
+        return not req.marker or any(evals)
 
 
 class Environment(object):

pkg_resources/tests/test_resources.py

@@ -201,11 +201,13 @@ def test_environment_marker_evaluation_called(self):
         req_extras = pkg_resources._ReqExtras({req: parent_req.extras})
         assert req_extras.markers_pass(req)
 
-        # this is a little awkward; I would want this to fail
+        # extra should not be present in the marker namespace if
+        # no markers were supplied
         parent_req, = parse_requirements("foo")
-        req, = parse_requirements("bar;python_version>='2' and extra==''")
+        req, = parse_requirements("bar;extra==''")
         req_extras = pkg_resources._ReqExtras({req: parent_req.extras})
-        assert req_extras.markers_pass(req)
+        with pytest.raises(packaging.markers.UndefinedEnvironmentName):
+            req_extras.markers_pass(req)
 
     def test_marker_evaluation_with_extras(self):
         """Extras are also evaluated as markers at resolution time."""