parameters: (self, mode, callback)
return_statement: return cbname
docstring: Define a trace callback for the variable.
  
          MODE is one of "r", "w", "u" for read, write, undefine.
          CALLBACK must be a function which is called when
          the variable is read, written or undefined.
  
          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_variable(self, mode, callback):
          """Define a trace callback for the variable.
  
          MODE is one of "r", "w", "u" for read, write, undefine.
          CALLBACK must be a function which is called when
          the variable is read, written or undefined.
  
          Return the name of the callback.
          """
          cbname = self._master._register(callback)
          self._tk.call("trace", "variable", self._name, mode, cbname)
          return cbname
function_tokens: def trace_variable ( self , mode , callback ) : cbname = self . _master . _register ( callback ) self . _tk . call ( "trace" , "variable" , self . _name , mode , cbname ) return cbname