diff --git a/Lib/pdb.py b/Lib/pdb.py
index 3e5e898..683abb9 100755
--- a/Lib/pdb.py
+++ b/Lib/pdb.py
@@ -171,14 +171,18 @@
 
     def user_return(self, frame, return_value):
         """This function is called when a return trap is set here."""
+        if self._wait_for_mainpyfile:
+            return
         frame.f_locals['__return__'] = return_value
         print >>self.stdout, '--Return--'
         self.interaction(frame, None)
 
     def user_exception(self, frame, exc_info):
-        exc_type, exc_value, exc_traceback = exc_info
         """This function is called if an exception occurs,
         but only if we are to stop at or just below this level."""
+        if self._wait_for_mainpyfile:
+            return
+        exc_type, exc_value, exc_traceback = exc_info
         frame.f_locals['__exception__'] = exc_type, exc_value
         if type(exc_type) == type(''):
             exc_type_name = exc_type
@@ -265,8 +269,10 @@
             return self.handle_command_def(line)
 
     def handle_command_def(self,line):
-        """ Handles one command line during command list definition. """
+        """Handles one command line during command list definition."""
         cmd, arg, line = self.parseline(line)
+        if not cmd:
+            return
         if cmd == 'silent':
             self.commands_silent[self.commands_bnum] = True
             return # continue to handle other cmd def in the cmd list
@@ -274,7 +280,7 @@
             self.cmdqueue = []
             return 1 # end of cmd list
         cmdlist = self.commands[self.commands_bnum]
-        if (arg):
+        if arg:
             cmdlist.append(cmd+' '+arg)
         else:
             cmdlist.append(cmd)
@@ -313,9 +319,11 @@
         prompt_back = self.prompt
         self.prompt = '(com) '
         self.commands_defining = True
-        self.cmdloop()
-        self.commands_defining = False
-        self.prompt = prompt_back
+        try:
+            self.cmdloop()
+        finally:
+            self.commands_defining = False
+            self.prompt = prompt_back
 
     def do_break(self, arg, temporary = 0):
         # break [ ([filename:]lineno | function) [, "condition"] ]
@@ -451,7 +459,10 @@
         Return `lineno` if it is, 0 if not (e.g. a docstring, comment, blank
         line or EOF). Warning: testing is not comprehensive.
         """
-        line = linecache.getline(filename, lineno, self.curframe.f_globals)
+        # this method should be callable before starting debugging, so default
+        # to "no globals" if there is no current frame
+        globs = self.curframe.f_globals if hasattr(self, 'curframe') else None
+        line = linecache.getline(filename, lineno, globs)
         if not line:
             print >>self.stdout, 'End of file'
             return 0
@@ -1280,7 +1291,7 @@
     # changed by the user from the command line. There is a "restart" command which
     # allows explicit specification of command line arguments.
     pdb = Pdb()
-    while 1:
+    while True:
         try:
             pdb._runscript(mainpyfile)
             if pdb._user_requested_quit: