metaclass

05/26/2025

• metaclass
  • A metaclass is a class that creates other classes
  • Just as a class defines how an instance behaves, a metaclass defines how a class behaves
  • The most common use case is to modify class creation and deletion behavior
  • Metaclass's __new__ is called during class definition, not during instance creation
  • Think of metaclass as a decorator for classes - it wraps and modifies classes just like decorators wrap and modify functions


• Example
  • The BaseAgentMeta metaclass wraps the call method to measure API call execution time
  • In this example, BaseAgentMeta.__new__ modifies the class namespace before the class is created

  
      import time
      import copy
  
      from abc import ABC, abstractmethod, ABCMeta
  
      from openai import OpenAI
  
  
      class BaseAgentMeta(ABCMeta):
          @staticmethod
          def runtime_calculator(func):
              def wrapper(self, *args, **kwargs):
                  start_time = time.time()
                  result = func(self, *args, **kwargs)
                  end_time = time.time()
                  print(f"{func.__qualname__} Time taken: {end_time - start_time} seconds")
                  return result
  
              return wrapper
  
          def __new__(metaclass, name, bases, namespace):
              namespace["call"] = metaclass.runtime_calculator(namespace["call"])
  
              return super().__new__(metaclass, name, bases, namespace)
  
  
      class BaseAgent(ABC, metaclass=BaseAgentMeta):
          _system_prompt = None
          _output_default = None
          _output_format = None
          _call_payload = None
  
          def __init__(self, client: OpenAI):
              self.client = client
              self.current_payload = None
  
          @property
          def system_prompt(self):
              if self._system_prompt is None:
                  raise NotImplementedError
  
              return self._system_prompt
  
          @property
          def output_default(self):
              if self._output_default is None:
                  raise NotImplementedError
  
              return self._output_default
  
          @property
          def output_format(self):
              if self._output_format is None:
                  raise NotImplementedError
  
              return self._output_format
  
          @property
          def call_payload(self):
              if self._call_payload is None:
                  raise NotImplementedError
  
              return copy.deepcopy(self._call_payload)
  
          @abstractmethod
          def call(self):
              """Interface for calling the agent."""
              raise NotImplementedError
  
          def create_payload(self, prompt_user: str):
              # Get copied payload
              call_payload = self.call_payload
              call_payload["input"] = [
                  {"role": "system", "content": self.system_prompt},
                  {"role": "user", "content": prompt_user},
              ]
  
              # Store the last payload
              self.current_payload = call_payload
  
              return call_payload
  
  
      # *******************Agent.call Time taken: 1.6251204013824463 seconds
      # *******************Agent.call Time taken: 1.1683950424194336 seconds
      # *******************Agent.call Time taken: 6.725451707839966 seconds
      # ********************Agent.call Time taken: 1.3004052639007568 seconds
      # ************Agent.call Time taken: 0.9431052207946777 seconds
      # ******Agent.call Time taken: 2.2339024543762207 seconds
  

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