text: #!/usr/bin/python2.4
  #
  # Copyright 2008 Google Inc.
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #      http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  
  import inspect
  import six
  
  
  class StubOutForTesting:
      """Sample Usage:
         You want os.path.exists() to always return true during testing.
  
         stubs = StubOutForTesting()
         stubs.Set(os.path, 'exists', lambda x: 1)
           ...
         stubs.UnsetAll()
  
         The above changes os.path.exists into a lambda that returns 1.  Once
         the ... part of the code finishes, the UnsetAll() looks up the old value
         of os.path.exists and restores it.
  
      """
  
      def __init__(self):
          self.cache = []
          self.stubs = []
  
      def __del__(self):
          self.SmartUnsetAll()
          self.UnsetAll()
  
      def SmartSet(self, obj, attr_name, new_attr):
          """Replace obj.attr_name with new_attr. This method is smart and works
             at the module, class, and instance level while preserving proper
             inheritance. It will not stub out C types however unless that has
             been explicitly allowed by the type.
  
             This method supports the case where attr_name is a staticmethod or a
             classmethod of obj.
  
             Notes:
            - If obj is an instance, then it is its class that will actually be
              stubbed. Note that the method Set() does not do that: if obj is
              an instance, it (and not its class) will be stubbed.
            - The stubbing is using the builtin getattr and setattr. So, the
              __get__ and __set__ will be called when stubbing (TODO: A better
              idea would probably be to manipulate obj.__dict__ instead of
              getattr() and setattr()).
  
             Raises AttributeError if the attribute cannot be found.
          """
          if (inspect.ismodule(obj) or
                  (not inspect.isclass(obj) and attr_name in obj.__dict__)):
              orig_obj = obj
              orig_attr = getattr(obj, attr_name)
  
          else:
              if not inspect.isclass(obj):
                  mro = list(inspect.getmro(obj.__class__))
              else:
                  mro = list(inspect.getmro(obj))
  
              mro.reverse()
  
              orig_attr = None
  
              for cls in mro:
                  try:
                      orig_obj = cls
                      orig_attr = getattr(obj, attr_name)
                  except AttributeError:
                      continue
  
          if orig_attr is None:
              raise AttributeError("Attribute not found.")
  
          # Calling getattr() on a staticmethod transforms it to a 'normal'
          # function. We need to ensure that we put it back as a staticmethod.
          old_attribute = obj.__dict__.get(attr_name)
          if old_attribute is not None and isinstance(
                  old_attribute,
                  staticmethod):
              orig_attr = staticmethod(orig_attr)
  
          self.stubs.append((orig_obj, attr_name, orig_attr))
          setattr(orig_obj, attr_name, new_attr)
  
      def SmartUnsetAll(self):
          """Reverses all the SmartSet() calls, restoring things to their
          original definition.  Its okay to call SmartUnsetAll() repeatedly, as
          later calls have no effect if no SmartSet() calls have been made.
  
          """
          self.stubs.reverse()
  
          for args in self.stubs:
              setattr(*args)
  
          self.stubs = []
  
      def Set(self, parent, child_name, new_child):
          """Replace child_name's old definition with new_child, in the context
          of the given parent.  The parent could be a module when the child is a
          function at module scope.  Or the parent could be a class when a class'
          method is being replaced.  The named child is set to new_child, while
          the prior definition is saved away for later, when UnsetAll() is
          called.
  
          This method supports the case where child_name is a staticmethod or a
          classmethod of parent.
          """
          old_child = getattr(parent, child_name)
  
          old_attribute = parent.__dict__.get(child_name)
          if old_attribute is not None:
              if isinstance(old_attribute, staticmethod):
                  old_child = staticmethod(old_child)
              elif isinstance(old_attribute, classmethod):
                  if six.PY2:
                      old_child = classmethod(old_child.im_func)
                  else:
                      old_child = classmethod(old_child.__func__)
  
          self.cache.append((parent, old_child, child_name))
          setattr(parent, child_name, new_child)
  
      def UnsetAll(self):
          """Reverses all the Set() calls, restoring things to their original
          definition.  Its okay to call UnsetAll() repeatedly, as later calls
          have no effect if no Set() calls have been made.
  
          """
          # Undo calls to Set() in reverse order, in case Set() was called on the
          # same arguments repeatedly (want the original call to be last one
          # undone)
          self.cache.reverse()
  
          for (parent, old_child, child_name) in self.cache:
              setattr(parent, child_name, old_child)
          self.cache = []
  