parameters: (self, fixers, traversal)
docstring: Traverse an AST, applying a set of fixers to each node.
  
          This is a helper method for refactor_tree().
  
          Args:
              fixers: a list of fixer instances.
              traversal: a generator that yields AST nodes.
  
          Returns:
              None
docstring_summary: Traverse an AST, applying a set of fixers to each node.
docstring_tokens: Traverse an AST applying a set of fixers to each node .
function: def traverse_by(self, fixers, traversal):
          """Traverse an AST, applying a set of fixers to each node.
  
          This is a helper method for refactor_tree().
  
          Args:
              fixers: a list of fixer instances.
              traversal: a generator that yields AST nodes.
  
          Returns:
              None
          """
          if not fixers:
              return
          for node in traversal:
              for fixer in fixers[node.type]:
                  results = fixer.match(node)
                  if results:
                      new = fixer.transform(node, results)
                      if new is not None:
                          node.replace(new)
                          node = new
function_tokens: def traverse_by ( self , fixers , traversal ) : if not fixers : return for node in traversal : for fixer in fixers [ node . type ] : results = fixer . match ( node ) if results : new = fixer . transform ( node , results ) if new is not None : node . replace ( new ) node = new