Fix readline backend check and stdin isatty condition in cmd and pdb modules

Author: zelosleone

Lib/cmd.py

@@ -111,7 +111,7 @@ def cmdloop(self, intro=None):
                 import readline
                 self.old_completer = readline.get_completer()
                 readline.set_completer(self.complete)
-                if readline.backend == "editline":
+                if getattr(readline, 'backend', None) == "editline":
                     if self.completekey == 'tab':
                         # libedit uses "^I" instead of "tab"
                         command_string = "bind ^I rl_complete"

Lib/pdb.py

@@ -364,7 +364,7 @@ def __init__(self, completekey='tab', stdin=None, stdout=None, skip=None,
         bdb.Bdb.__init__(self, skip=skip, backend=backend if backend else get_default_backend())
         cmd.Cmd.__init__(self, completekey, stdin, stdout)
         sys.audit("pdb.Pdb")
-        if stdin:
+        if stdin and not getattr(stdin, 'isatty', lambda: False)():
             self.use_rawinput = False
         self.prompt = '(Pdb) '
         self.aliases = {}