diff -r 948d8ea8e7a6 Lib/warnings.py
--- a/Lib/warnings.py	Mon Aug 17 13:04:41 2015 -0400
+++ b/Lib/warnings.py	Mon Aug 17 18:03:18 2015 -0700
@@ -160,6 +160,12 @@
     return cat
 
 
+def _internal_frame(frame):
+    """Signal whether the frame is an internal CPython implementation detail."""
+    filename = frame.f_code.co_filename
+    return 'importlib' in filename and '_bootstrap' in filename
+
+
 # Code typically replaced by _warnings
 def warn(message, category=None, stacklevel=1):
     """Issue a warning, or maybe ignore it or raise an exception."""
@@ -174,13 +180,25 @@
                         "not '{:s}'".format(type(category).__name__))
     # Get context information
     try:
-        caller = sys._getframe(stacklevel)
+        if stacklevel < 0:
+            frame = sys._getframe(stacklevel)
+        else:
+            # Basically a funky do...while loop.
+            frame = sys._getframe()
+            if not _internal_frame(frame):
+                stacklevel -= 1
+            while stacklevel >= 0:
+                frame = frame.f_back
+                if frame is None:
+                    raise ValueError
+                elif not _internal_frame(frame):
+                    stacklevel -= 1
     except ValueError:
         globals = sys.__dict__
         lineno = 1
     else:
-        globals = caller.f_globals
-        lineno = caller.f_lineno
+        globals = frame.f_globals
+        lineno = frame.f_lineno
     if '__name__' in globals:
         module = globals['__name__']
     else:
@@ -370,11 +388,10 @@
 _warnings_defaults = False
 try:
     from _warnings import (filters, _defaultaction, _onceregistry,
-                           warn, warn_explicit, _filters_mutated)
+                           warn_explicit, _filters_mutated)
     defaultaction = _defaultaction
     onceregistry = _onceregistry
     _warnings_defaults = True
-
 except ImportError:
     filters = []
     defaultaction = "default"