Debugging to understand why 'trace function changed' appears.

2 files changed, 10 insertions, 2 deletions

diff --git a/coverage/collector.py b/coverage/collector.py
index 1b807b27..bf465939 100644
--- a/coverage/collector.py
+++ b/coverage/collector.py
@@ -2,6 +2,8 @@
 
 import os, sys, threading
 
+from coverage.misc import short_stack
+
 try:
     # Use the C extension code when we can, for speed.
     from coverage.tracer import CTracer         # pylint: disable=F0401,E0611
@@ -124,8 +126,8 @@ class PyTracer(object):
         """Stop this Tracer."""
         if hasattr(sys, "gettrace") and self.warn:
             if sys.gettrace() != self._trace:
-                msg = "Trace function changed, measurement is likely wrong: %r"
-                self.warn(msg % sys.gettrace())
+                msg = "Trace function changed, measurement is likely wrong: %r\n%s"
+                self.warn(msg % (sys.gettrace(), short_stack()))
         sys.settrace(None)
 
     def get_stats(self):
diff --git a/coverage/misc.py b/coverage/misc.py
index 3ed854a7..b4d1c833 100644
--- a/coverage/misc.py
+++ b/coverage/misc.py
@@ -54,6 +54,12 @@ def format_lines(statements, lines):
     return ret
 
 
+def short_stack():
+    """Return a string summarizing the call stack."""
+    stack = inspect.stack()[:0:-1]
+    return "\n".join("%30s : %s @%d" % (t[3],t[1],t[2]) for t in stack)
+
+
 def expensive(fn):
     """A decorator to cache the result of an expensive operation.