6 files changed, 68 insertions, 15 deletions
diff --git a/coverage/html.py b/coverage/html.py
index 9471db93..d4fb7516 100644
--- a/coverage/html.py
+++ b/coverage/html.py
@@ -190,6 +190,7 @@ class HtmlReporter(Reporter):
 
         if self.has_arcs:
             missing_branch_arcs = analysis.missing_branch_arcs()
+            arcs_executed = analysis.arcs_executed()
 
         # These classes determine which lines are highlighted by default.
         c_run = "run hide_run"
@@ -219,7 +220,7 @@ class HtmlReporter(Reporter):
                         shorts.append("exit")
                     else:
                         shorts.append(b)
-                    longs.append(fr.missing_arc_description(lineno, b))
+                    longs.append(fr.missing_arc_description(lineno, b, arcs_executed))
                 # 202F is NARROW NO-BREAK SPACE.
                 # 219B is RIGHTWARDS ARROW WITH STROKE.
                 short_fmt = "%s&#x202F;&#x219B;&#x202F;%s"
diff --git a/coverage/parser.py b/coverage/parser.py
index f4dd02d4..8fb5d89b 100644
--- a/coverage/parser.py
+++ b/coverage/parser.py
@@ -21,8 +21,12 @@ from coverage.phystokens import compile_unicode, generate_tokens, neuter_encodin
 
 
 class PythonParser(object):
-    """Parse code to find executable lines, excluded lines, etc."""
+    """Parse code to find executable lines, excluded lines, etc.
 
+    This information is all based on static analysis: no code execution is
+    involved.
+
+    """
     @contract(text='unicode|None')
     def __init__(self, text=None, filename=None, exclude=None):
         """
@@ -304,11 +308,23 @@ class PythonParser(object):
 
         return exit_counts
 
-    def missing_arc_description(self, start, end):
+    def missing_arc_description(self, start, end, executed_arcs=None):
         """Provide an English sentence describing a missing arc."""
         if self._missing_arc_fragments is None:
             self._analyze_ast()
 
+        actual_start = start
+
+        if (
+            executed_arcs and
+            end < 0 and end == -start and
+            (end, start) not in executed_arcs and
+            (end, start) in self._missing_arc_fragments
+        ):
+            # It's a one-line callable, and we never even started it,
+            # and we have a message about not starting it.
+            start, end = end, start
+
         fragment_pairs = self._missing_arc_fragments.get((start, end), [(None, None)])
 
         msgs = []
@@ -325,9 +341,9 @@ class PythonParser(object):
                     emsg = "didn't jump to line {lineno}"
             emsg = emsg.format(lineno=end)
 
-            msg = "line {start} {emsg}".format(start=start, emsg=emsg)
+            msg = "line {start} {emsg}".format(start=actual_start, emsg=emsg)
             if smsg is not None:
-                msg += ", because {smsg}".format(smsg=smsg.format(lineno=start))
+                msg += ", because {smsg}".format(smsg=smsg.format(lineno=actual_start))
 
             msgs.append(msg)
 
@@ -939,10 +955,10 @@ class AstArcAnalyzer(object):
         """A function to make methods for online callable _code_object__ methods."""
         def _code_object__oneline_callable(self, node):
             start = self.line_for_node(node)
-            self.add_arc(-start, start)
+            self.add_arc(-start, start, None, "didn't run the {0} on line {1}".format(noun, start))
             self.add_arc(
                 start, -start, None,
-                "didn't run the {0} on line {1}".format(noun, start),
+                "didn't finish the {0} on line {1}".format(noun, start),
             )
         return _code_object__oneline_callable
 
diff --git a/coverage/plugin.py b/coverage/plugin.py
index 97d9c16e..a7b95466 100644
--- a/coverage/plugin.py
+++ b/coverage/plugin.py
@@ -329,9 +329,15 @@ class FileReporter(object):
         """
         return {}
 
-    def missing_arc_description(self, start, end):
+    def missing_arc_description(self, start, end, executed_arcs=None):     # pylint: disable=unused-argument
         """Provide an English sentence describing a missing arc.
 
+        The `start` and `end` arguments are the line numbers of the missing
+        arc. Negative numbers indicate entering or exiting code objects.
+
+        The `executed_arcs` argument is a set of line number pairs, the arcs
+        that were executed in this file.
+
         By default, this simply returns the string "Line {start} didn't jump
         to {end}".
 
diff --git a/coverage/python.py b/coverage/python.py
index 0d2fb3b4..0cd2181c 100644
--- a/coverage/python.py
+++ b/coverage/python.py
@@ -165,8 +165,8 @@ class PythonFileReporter(FileReporter):
     def exit_counts(self):
         return self.parser.exit_counts()
 
-    def missing_arc_description(self, start, end):
-        return self.parser.missing_arc_description(start, end)
+    def missing_arc_description(self, start, end, executed_arcs=None):
+        return self.parser.missing_arc_description(start, end, executed_arcs)
 
     @contract(returns='unicode')
     def source(self):
diff --git a/tests/test_arcs.py b/tests/test_arcs.py
index 826256b0..16efbcdf 100644
--- a/tests/test_arcs.py
+++ b/tests/test_arcs.py
@@ -1105,6 +1105,36 @@ class MiscArcTest(CoverageTest):
             arcz_missing="27",
         )
 
+    def test_partial_generators(self):
+        # https://bitbucket.org/ned/coveragepy/issues/475/generator-expression-is-marked-as-not
+        # Line 2 is executed completely.
+        # Line 3 is started but not finished, because zip ends when #2 ends.
+        # Line 4 is never started.
+        cov = self.check_coverage("""\
+            def f(a, b):
+                c = (i for i in a)          # 2
+                d = (j for j in b)          # 3
+                e = (k for k in b)          # 4
+                return dict(zip(c, d))
+
+            f(['a', 'b'], [1, 2])
+            """,
+            arcz=".1 17 7.  .2 23 34 45 5.  -22 2-2  -33 3-3  -44 4-4",
+            arcz_missing="3-3 -44 4-4",
+        )
+        # ugh, unexposed methods??
+        filename = self.last_module_name + ".py"
+        fr = cov._get_file_reporter(filename)
+        arcs_executed = cov._analyze(filename).arcs_executed()
+        self.assertEqual(
+            fr.missing_arc_description(3, -3, arcs_executed),
+            "line 3 didn't finish the generator expression on line 3"
+        )
+        self.assertEqual(
+            fr.missing_arc_description(4, -4, arcs_executed),
+            "line 4 didn't run the generator expression on line 4"
+        )
+
 
 class DecoratorArcTest(CoverageTest):
     """Tests of arcs with decorators."""
diff --git a/tests/test_parser.py b/tests/test_parser.py
index 17f81ad8..ed18ccaa 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -256,19 +256,19 @@ class ParserMissingArcDescriptionTest(CoverageTest):
             """)
         self.assertEqual(
             parser.missing_arc_description(2, -2),
-            "line 2 didn't run the lambda on line 2"
+            "line 2 didn't finish the lambda on line 2"
         )
         self.assertEqual(
             parser.missing_arc_description(3, -3),
-            "line 3 didn't run the generator expression on line 3"
+            "line 3 didn't finish the generator expression on line 3"
         )
         self.assertEqual(
             parser.missing_arc_description(4, -4),
-            "line 4 didn't run the dictionary comprehension on line 4"
+            "line 4 didn't finish the dictionary comprehension on line 4"
         )
         self.assertEqual(
             parser.missing_arc_description(5, -5),
-            "line 5 didn't run the set comprehension on line 5"
+            "line 5 didn't finish the set comprehension on line 5"
         )
 
     def test_missing_arc_descriptions_for_exceptions(self):
@@ -340,7 +340,7 @@ class ParserMissingArcDescriptionTest(CoverageTest):
             """)
         self.assertEqual(
             parser.missing_arc_description(2, -3),
-            "line 3 didn't run the lambda on line 3",
+            "line 3 didn't finish the lambda on line 3",
         )