testing.fix_mock_tests

Goal

• When the user passes a test file, apply the following transformations to remove mocking from unit testing code without changing behavior
• Mocking often signals that your design could be more modular
  • Avoiding mocking in Python unit tests means designing code so dependencies are easy to control naturally, rather than being intercepted artificially

Workflow

• Avoid mocking using the strategies below

Prefer Pure Functions

• Functions that depend only on inputs and return outputs are trivial to test
  • Example:

    def add(a: int, b: int) -> int:
        return a + b
    

• No mocking needed because there are no external dependencies

Design for Dependency Injection (DI)

• Instead of hard coding dependencies inside functions or classes, pass them in
• Bad (hardcodes the dependency, forcing a mock to test it)

  from typing import Any
  import requests
  
  def get_user(user_id: int) -> Any:
    return requests.get(f"https://api.com/users/{user_id}").json()
  

• Good (injects the dependency, so a fake can be substituted in tests)

  from typing import Any, Protocol
  
  class HttpClient(Protocol):
      def get(self, url: str) -> Any: ...
  
  def get_user(user_id: int, http_client: HttpClient) -> Any:
    return http_client.get(f"https://api.com/users/{user_id}").json()
  

• Bad (creates its own dependency inside __init__)

  class Database: pass
  
  class UserService:
    def __init__(self) -> None:
      self.db: Database = Database()
  

• Good (accepts the dependency, so a fake can be passed in)

  class Database: pass
  
  class UserService:
    def __init__(self, db: Database) -> None:
      self.db: Database = db
  

Create Lightweight Fakes Instead of Mocks

• A fake is a real implementation with simplified behavior
  • More maintainable and less brittle than mocks
  • Example:

    from typing import Any
    
    class FakeDatabase:
        def __init__(self) -> None:
            self.data: dict[str, Any] = {}
    
        def save(self, key: str, value: Any) -> None:
            self.data[key] = value
    
        def get(self, key: str) -> Any:
            return self.data.get(key)