自定义基于类的上下文管理器中“with”块中的返回语句行为

Question

更新到 Python，我正在尝试创建我自己的上下文管理器来调用某个 REST API 的登录/注销方法。这样我就可以做其他 API 的事情，登录/注销将为我处理。 但是，我对下面代码中with语句中return语句的行为感到困惑。 我希望它返回到with块之外的代码执行，但它反而退出整个__enter__ function？ 这是为什么？

# test.py

import requests

class LoginLogoutContextManager:
    def __enter__(self):

        print("**entered __enter__")
        print("Logging in...")

        # --- Login --- #

        # Let's pretend this GET request
        # is the Login Code

        self.url = "http://www.google.com"
        self.session = requests.Session()

        with self.session.get(self.url) as self.response:
            print("Start Login attempt (inside WITH but before RETURN)")
            return len(self.response.content)
            print("Login complete (inside WITH but after RETURN)")

        # !!! Login Code finished !!! #

        # OK login code is now done and we're out of the WITH statement

        print("I would expect the above WITH block to Return to here")
        print("!!exiting __enter__")

    def __exit__(self, exc_type, exc_value, exc_tb):

        print("**entered __exit__")
        print("Logging OUT...")

        # Logout code goes here

        print("!!exiting __exit__")


with LoginLogoutContextManager() as manager:
    print("Cool now the Login/Logout API calls are handled for me")

    # Does other API things here

    print(f"Login attempt response length: {manager}")

# python3 test.py

**entered __enter__
Logging in...
Start Login attempt (inside WITH but before RETURN)
Cool now the Login/Logout API calls are handled for me
Login attempt response length: 14126
**entered __exit__
Logging OUT...
!!exiting __exit__

Answer 1

我希望它返回到with块之外的代码执行

你可能会这样想，但事实并非如此。

但它反而退出了整个__enter__ function？ 这是为什么？

语言参考如此指定：

https://docs.python.org/3/reference/simple_stmts.html#the-return-statement

return离开当前 function 调用，表达式列表（或None ）作为返回值。