diff --git a/Lib/idlelib/PyShell.py b/Lib/idlelib/PyShell.py
--- a/Lib/idlelib/PyShell.py
+++ b/Lib/idlelib/PyShell.py
@@ -61,7 +61,6 @@
                                               lineno, line=line))
         except OSError:
             pass  ## file (probably __stderr__) is invalid, warning dropped.
-    warnings.showwarning = idle_showwarning
     def idle_formatwarning(message, category, filename, lineno, line=None):
         """Format warnings the IDLE way"""
         s = "\nWarning (from warnings module):\n"
@@ -73,7 +72,27 @@
             s += "    %s\n" % line
         s += "%s: %s\n>>> " % (category.__name__, message)
         return s
-    warnings.formatwarning = idle_formatwarning
+
+    _warnings_showwarning = None
+    _warnings_formatwarning = None
+
+    def captureWarnings(capture):
+        global _warnings_showwarning, _warnings_formatwarning
+        if capture:
+            if _warnings_showwarning is None:
+                _warnings_showwarning = warnings.showwarning
+                warnings.showwarning = idle_showwarning
+            if _warnings_formatwarning is None:
+                _warnings_formatwarning = warnings.formatwarning
+                warnings.formatwarning = idle_formatwarning
+        else:
+            if _warnings_showwarning is not None:
+                warnings.showwarning = _warnings_showwarning
+                _warnings_showwarning = None
+            if _warnings_formatwarning is not None:
+                warnings.formatwarning = _warnings_formatwarning
+                _warnings_formatwarning = None
+
 
 def extended_linecache_checkcache(filename=None,
                                   orig_checkcache=linecache.checkcache):
@@ -1425,6 +1444,8 @@
 def main():
     global flist, root, use_subprocess
 
+    captureWarnings(True)
+
     use_subprocess = True
     enable_shell = False
     enable_edit = False
diff --git a/Lib/idlelib/run.py b/Lib/idlelib/run.py
--- a/Lib/idlelib/run.py
+++ b/Lib/idlelib/run.py
@@ -40,7 +40,19 @@
             s += "    %s\n" % line
         s += "%s: %s\n" % (category.__name__, message)
         return s
-    warnings.formatwarning = idle_formatwarning_subproc
+
+    _warnings_formatwarning = None
+
+    def captureWarnings(capture):
+        global _warnings_formatwarning
+        if capture:
+            if _warnings_formatwarning is None:
+                _warnings_formatwarning = warnings.formatwarning
+                warnings.formatwarning = idle_formatwarning_subproc
+        else:
+            if _warnings_formatwarning is not None:
+                warnings.formatwarning = _warnings_formatwarning
+                _warnings_formatwarning = None
 
 
 tcl = tkinter.Tcl()
@@ -92,6 +104,7 @@
               file=sys.__stderr__)
         return
     sys.argv[:] = [""]
+    captureWarnings(True)
     sockthread = threading.Thread(target=manage_socket,
                                   name='SockThread',
                                   args=((LOCALHOST, port),))