Move default_backend to module level and provide utils

Author: gaogaotiantian

Lib/pdb.py

@@ -301,22 +301,38 @@ def write(self, data):
 line_prefix = '\n-> '   # Probably a better default
 
 
+# The default backend to use for Pdb instances if not specified
+# Should be either 'settrace' or 'monitoring'
+_default_backend = 'settrace'
+
+
+def set_default_backend(backend):
+    """Set the default backend to use for Pdb instances."""
+    global _default_backend
+    if backend not in ('settrace', 'monitoring'):
+        raise ValueError("Invalid backend: %s" % backend)
+    _default_backend = backend
+
+
+def get_default_backend():
+    """Get the default backend to use for Pdb instances."""
+    return _default_backend
+
 
 class Pdb(bdb.Bdb, cmd.Cmd):
     _previous_sigint_handler = None
 
     # Limit the maximum depth of chained exceptions, we should be handling cycles,
     # but in case there are recursions, we stop at 999.
     MAX_CHAINED_EXCEPTION_DEPTH = 999
-    DEFAULT_BACKEND = 'settrace'
 
     _file_mtime_table = {}
 
     _last_pdb_instance = None
 
     def __init__(self, completekey='tab', stdin=None, stdout=None, skip=None,
                  nosigint=False, readrc=True, mode=None, backend=None):
-        bdb.Bdb.__init__(self, skip=skip, backend=backend if backend else self.DEFAULT_BACKEND)
+        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 stdout:

Lib/test/test_pdb.py

@@ -4449,7 +4449,7 @@ def load_tests(loader, tests, pattern):
     def setUpPdbBackend(backend):
         def setUp(test):
             import pdb
-            pdb.Pdb.DEFAULT_BACKEND = backend
+            pdb.set_default_backend(backend)
         return setUp
     tests.addTest(doctest.DocTestSuite(test_pdb, setUp=setUpPdbBackend('monitoring')))
     tests.addTest(doctest.DocTestSuite(test_pdb, setUp=setUpPdbBackend('settrace')))