before_request、after_request、teardown_request

  钩子定义(简写):
      1 | @app.before_request   def b1 / b2         # b2 在 /short 返回响应,在 /before-boom 抛异常
      2 | @app.after_request    def a1 / a2         # 参数是响应,必须返回响应
      3 | @app.teardown_request def t1 / t2         # 参数是异常或 None
  正常 GET /           → 200  before_1 → before_2 → view → after_2 → after_1 → teardown_2(None) → teardown_1(None)
  abort(403)         → 403  before_1 → before_2 → view → after_2 → after_1 → teardown_2(None) → teardown_1(None)
  视图里 ValueError     → 500  before_1 → before_2 → view → after_2 → after_1 → teardown_2(ValueError) → teardown_1(ValueError)
  before 里直接返回响应     → 200  before_1 → before_2 → after_2 → after_1 → teardown_2(None) → teardown_1(None)
  before 里抛异常        → 500  before_1 → before_2 → after_2 → after_1 → teardown_2(RuntimeError) → teardown_1(RuntimeError)
  路径不存在(404)         → 404  before_1 → before_2 → after_2 → after_1 → teardown_2(None) → teardown_1(None)
  静态文件 /static/x     → 404  before_1 → before_2 → after_2 → after_1 → teardown_2(None) → teardown_1(None)

  最后执行的 after_request 返回 None → test_client 抛出 TypeError: 'NoneType' object is not callable
  (after_request_funcs 按注册顺序存放,执行时倒序,所以 insert(0) 的函数最后执行(这里让它返回 None))

  before_0 → before_1 → before_2 → after_2 → after_1 → after_0 → teardown_2 → teardown_1 → teardown_0

  GET /in-bp   → 200  app.before_request → bp.before_app_request → bp.before_request → view(bp) → bp.after_request → app.after_request
  GET /in-app  → 200  app.before_request → bp.before_app_request → view(app) → app.after_request
  GET /nope    → 404  app.before_request → bp.before_app_request → app.after_request

  test_client 向上抛出了: ValueError boom
  before_1 → before_2 → view → teardown_2(ValueError) → teardown_1(ValueError)