debug: better tracing of pytracer changed

coverage/pytracer.py

@@ -68,7 +68,7 @@ def __init__(self):
         atexit.register(setattr, self, 'in_atexit', True)
 
     def __repr__(self):
-        return "<PyTracer at {}: {} lines in {} files>".format(
+        return "<PyTracer at 0x{:x}: {} lines in {} files>".format(
             id(self),
             sum(len(v) for v in self.data.values()),
             len(self.data),
@@ -267,8 +267,10 @@ def stop(self):
             # has changed to None.
             dont_warn = (env.PYPY and env.PYPYVERSION >= (5, 4) and self.in_atexit and tf is None)
             if (not dont_warn) and tf != self._trace:   # pylint: disable=comparison-with-callable
-                msg = f"Trace function changed, measurement is likely wrong: {tf!r}"
-                self.warn(msg, slug="trace-changed")
+                self.warn(
+                    f"Trace function changed, data is likely wrong: {tf!r} != {self._trace!r}",
+                    slug="trace-changed",
+                )
 
     def activity(self):
         """Has there been any activity?"""

tests/test_oddball.py

@@ -4,6 +4,7 @@
 """Oddball cases for testing coverage.py"""
 
 import os.path
+import re
 import sys
 
 from flaky import flaky
@@ -132,7 +133,13 @@ def recur(n):
 
         # Get a warning about the stackoverflow effect on the tracing function.
         if pytrace:                                 # pragma: no metacov
-            assert cov._warnings == ["Trace function changed, measurement is likely wrong: None"]
+            assert len(cov._warnings) == 1
+            assert re.fullmatch(
+                r"Trace function changed, data is likely wrong: None != " +
+                r"<bound method PyTracer._trace of " +
+                "<PyTracer at 0x[0-9a-fA-F]+: 5 lines in 1 files>>",
+                cov._warnings[0],
+            )
         else:
             assert not cov._warnings