Flask Traceback仅显示外部库，无法定位自有代码错误行原因咨询

运行Flask应用时遇到500错误，对应的Traceback只显示外部库的调用栈信息，没法定位到自有代码里引发错误的具体行；但写小型测试应用复现该错误时，Traceback却能正常显示错误行号，想知道这一现象的原因。

正式应用错误信息 #

* Debugger is active!
 * Debugger PIN: ******
127.0.0.1 - - [29/Dec/2022 12:21:53] "GET /authorization/****/authorize HTTP/1.1" 302 -
127.0.0.1 - - [29/Dec/2022 12:21:53] "GET /authorization/****/test HTTP/1.1" 500 -
Traceback (most recent call last):
  File "******/lib/python3.10/site-packages/flask/app.py", line 2548, in __call__
    return self.wsgi_app(environ, start_response)
  File "******/lib/python3.10/site-packages/flask/app.py", line 2528, in wsgi_app
    response = self.handle_exception(e)
  File "******/lib/python3.10/site-packages/flask/app.py", line 2525, in wsgi_app
    response = self.full_dispatch_request()
  File "******/lib/python3.10/site-packages/flask/app.py", line 1823, in full_dispatch_request
    return self.finalize_request(rv)
  File "******/lib/python3.10/site-packages/flask/app.py", line 1844, in finalize_request
    response = self.process_response(response)
  File "******/lib/python3.10/site-packages/flask/app.py", line 2340, in process_response
    self.session_interface.save_session(self, ctx.session, response)
  File "******/lib/python3.10/site-packages/flask/sessions.py", line 409, in save_session
    val = self.get_signing_serializer(app).dumps(dict(session))  # type: ignore
  File "******/lib/python3.10/site-packages/itsdangerous/serializer.py", line 207, in dumps
    payload = want_bytes(self.dump_payload(obj))
  File "******/lib/python3.10/site-packages/itsdangerous/url_safe.py", line 53, in dump_payload
    json = super().dump_payload(obj)
  File "******/lib/python3.10/site-packages/itsdangerous/serializer.py", line 169, in dump_payload
    return want_bytes(self.serializer.dumps(obj, **self.serializer_kwargs))
  File "******/lib/python3.10/site-packages/flask/json/tag.py", line 308, in dumps
    return dumps(self.tag(value), separators=(",", ":"))
  File "******/lib/python3.10/site-packages/flask/json/__init__.py", line 124, in dumps
    return app.json.dumps(obj, **kwargs)
  File "******/lib/python3.10/site-packages/flask/json/provider.py", line 230, in dumps
    return json.dumps(obj, **kwargs)
  File "/usr/lib/python3.10/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
  File "/usr/lib/python3.10/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python3.10/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "******/lib/python3.10/site-packages/flask/json/provider.py", line 122, in _default
    raise TypeError(f"Object of type {type(o).__name__} is not JSON serializable")

测试应用代码 #

import flask
import json
app = flask.Flask(__name__)
app.secret_key = "####"

@app.route("/")
def index():
    json.dumps(json)  # Line that causes error

if __name__ == "__main__":
    app.run("localhost", 8080, debug=True)

• 错误触发时机差异：
  正式应用的错误是在视图函数执行完毕后，Flask进入process_response阶段保存session时触发的。这个过程属于框架内部的后续处理，和你的业务代码已经不在同一调用栈，所以Traceback只会展示框架库的调用信息，无法关联到你写入session的代码行。
  测试应用的错误是在视图函数内部直接调用序列化函数时即时触发，调用栈自然包含你的业务代码行。

• 错误根源的间接性：
  正式应用的问题本质是你在某个业务逻辑中往session存入了无法JSON序列化的对象，但存入操作本身不会立刻报错，直到Flask尝试序列化session时才暴露问题。此时错误栈只能追踪到框架执行序列化的代码，无法回溯到原始的存入操作位置。
  测试应用的错误是直接在视图中触发序列化失败，错误栈直接指向问题代码。

• 代码复杂度与框架流程的包裹：
  正式应用可能包含更多中间件、扩展或复杂的代码结构，导致错误被框架的异常处理流程多层包裹，进一步掩盖了原始调用栈中业务代码的痕迹。

内容的提问来源于stack exchange，提问作者Raphael