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. :(

1 files changed, 3 insertions, 0 deletions

diff --git a/coverage/results.py b/coverage/results.py
index 0b27971..def3a07 100644
--- a/coverage/results.py
+++ b/coverage/results.py
@@ -101,10 +101,13 @@ class Analysis(object):
         # Exclude arcs here which connect a line to itself.  They can occur
         # in executed data in some cases.  This is where they can cause
         # trouble, and here is where it's the least burden to remove them.
+        # Also, generators can somehow cause arcs from "enter" to "exit", so
+        # make sure we have at least one positive value.
         unpredicted = (
             e for e in executed
                 if e not in possible
                     and e[0] != e[1]
+                    and (e[0] > 0 or e[1] > 0)
         )
         return sorted(unpredicted)