Fix branch coverage for yield statements. #308 #324

Turns out the "call" and "return" trace events are really "start frame" and
"end frame".  They happen not only when functions are entered and left, but
when generators yield and resume.  We aren't interested in arcs into and out
of yield statements, so the trace functions look more closely to see what's
really happening, and record an arc in human-friendly terms.

Thanks for Mickie Betz for pushing on this bug, although her code is no
longer here. :(

2 files changed, 91 insertions, 2 deletions

diff --git a/tests/test_arcs.py b/tests/test_arcs.py
index d3717a88..1098dbdf 100644
--- a/tests/test_arcs.py
+++ b/tests/test_arcs.py
@@ -558,6 +558,93 @@ class ExceptionArcTest(CoverageTest):
             arcz_missing="67 7B", arcz_unpredicted="68")
 
 
+class YieldTest(CoverageTest):
+    """Arc tests for generators."""
+
+    def test_yield_in_loop(self):
+        self.check_coverage("""\
+            def gen(inp):
+                for n in inp:
+                    yield n
+
+            list(gen([1,2,3]))
+            """,
+            arcz=".1 .2 23 2. 32 15 5.",
+            arcz_missing="",
+            arcz_unpredicted="")
+
+    def test_padded_yield_in_loop(self):
+        self.check_coverage("""\
+            def gen(inp):
+                i = 2
+                for n in inp:
+                    i = 4
+                    yield n
+                    i = 6
+                i = 7
+
+            list(gen([1,2,3]))
+            """,
+            arcz=".1 19 9.  .2 23 34 45 56 63 37 7.",
+            arcz_missing="",
+            arcz_unpredicted="")
+
+    def test_bug_308(self):
+        self.check_coverage("""\
+            def run():
+                for i in range(10):
+                    yield lambda: i
+
+            for f in run():
+                print(f())
+            """,
+            arcz=".1 15 56 65 5.  .2 23 32 2.  .3 3-3",
+            arcz_missing="",
+            arcz_unpredicted="")
+
+        self.check_coverage("""\
+            def run():
+                yield lambda: 100
+                for i in range(10):
+                    yield lambda: i
+
+            for f in run():
+                print(f())
+            """,
+            arcz=".1 16 67 76 6.  .2 23 34 43 3.   2-2  .4 4-4",
+            arcz_missing="",
+            arcz_unpredicted="")
+
+        self.check_coverage("""\
+            def run():
+                yield lambda: 100  # no branch miss
+
+            for f in run():
+                print(f())
+            """,
+            arcz=".1 14 45 54 4.  .2 2.  2-2",
+            arcz_missing="",
+            arcz_unpredicted="")
+
+    def test_bug_324(self):
+        # This code is tricky: the list() call pulls all the values from gen(),
+        # but each of them is a generator itself that is never iterated.  As a
+        # result, the generator expression on line 3 is never entered or run.
+        self.check_coverage("""\
+            def gen(inp):
+                for n in inp:
+                    yield (i * 2 for i in range(n))
+
+            list(gen([1,2,3]))
+            """,
+            arcz=
+                ".1 15 5. "     # The module level
+                ".2 23 32 2. "  # The gen() function
+                ".3 3-3",       # The generator expression
+            arcz_missing=".3 3-3",
+            arcz_unpredicted="")
+
+
 class MiscArcTest(CoverageTest):
     """Miscellaneous arc-measuring tests."""
 
diff --git a/tests/test_parser.py b/tests/test_parser.py
index 04c345ec..81916a98 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -35,7 +35,6 @@ class PythonParserTest(CoverageTest):
             })
 
     def test_generator_exit_counts(self):
-        # Generators' yield lines should only have one exit count.
         # https://bitbucket.org/ned/coveragepy/issue/324/yield-in-loop-confuses-branch-coverage
         parser = self.parse_source("""\
             def gen(input):
@@ -45,7 +44,10 @@ class PythonParserTest(CoverageTest):
             list(gen([1,2,3]))
             """)
         self.assertEqual(parser.exit_counts(), {
-           1:1, 2:2, 3:1, 5:1
+            1:1,    # def -> list
+            2:2,    # for -> yield; for -> exit
+            3:2,    # yield -> for;  genexp exit
+            5:1,    # list -> exit
         })
 
     def test_try_except(self):