code: def get_function(self, name: str) -> AbiFunction or None:
          
          for func in self.functions:
              if func['name'] == name:
                  return AbiFunction(func['name'], func['parameters'], func.get('returntype', ''))
          return None
docs: This interface is used to get an AbiFunction object from AbiInfo object by given function name.
  
          :param name: the function name in abi file
          :return: if succeed, an AbiFunction will constructed based on given function name