parameters: (self, mode, callback)
return_statement: return cbname
docstring: Define a trace callback for the variable.
  
          Mode is one of "read", "write", "unset", or a list or tuple of
          such strings.
          Callback must be a function which is called when the variable is
          read, written or unset.
  
          Return the name of the callback.
docstring_summary: Define a trace callback for the variable.
docstring_tokens: Define a trace callback for the variable .
function: def trace_add(self, mode, callback):
          """Define a trace callback for the variable.
  
          Mode is one of "read", "write", "unset", or a list or tuple of
          such strings.
          Callback must be a function which is called when the variable is
          read, written or unset.
  
          Return the name of the callback.
          """
          cbname = self._register(callback)
          self._tk.call('trace', 'add', 'variable',
                        self._name, mode, (cbname,))
          return cbname
function_tokens: def trace_add ( self , mode , callback ) : cbname = self . _register ( callback ) self . _tk . call ( 'trace' , 'add' , 'variable' , self . _name , mode , ( cbname , ) ) return cbname