POST /_dash-update-component HTTP/1.1" 500

Dash Python
1

Hi.

I’m very new to Dash development.

I am getting the aforementioned internal server error accompanied by the following in the browser:

Callback error updating ..login-url.pathname...login-alert.children..", html: '<!doctype html>\n<html lang=en>\n  <head>\n    <title>TypeError: Object of type builtin_function_or_method is not JSON serializable

And the following from the console logs:
127.0.0.1 - - [25/Jul/2024 17:32:08] "POST /_dash-update-component HTTP/1.1" 500 -
Traceback (most recent call last):
  File "C:\Python312\Lib\site-packages\flask\app.py", line 1498, in __call__
    return self.wsgi_app(environ, start_response)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python312\Lib\site-packages\flask\app.py", line 1476, in wsgi_app
    response = self.handle_exception(e)
               ^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python312\Lib\site-packages\flask\app.py", line 1473, in wsgi_app
    response = self.full_dispatch_request()
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python312\Lib\site-packages\flask\app.py", line 883, in full_dispatch_request
    return self.finalize_request(rv)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python312\Lib\site-packages\flask\app.py", line 904, in finalize_request
    response = self.process_response(response)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python312\Lib\site-packages\flask\app.py", line 1284, in process_response
    self.session_interface.save_session(self, ctx.session, response)
  File "C:\Python312\Lib\site-packages\flask\sessions.py", line 368, in save_session
    val = self.get_signing_serializer(app).dumps(dict(session))  # type: ignore
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python312\Lib\site-packages\itsdangerous\serializer.py", line 316, in dumps
    payload = want_bytes(self.dump_payload(obj))
                         ^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python312\Lib\site-packages\itsdangerous\url_safe.py", line 56, in dump_payload
    json = super().dump_payload(obj)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python312\Lib\site-packages\itsdangerous\serializer.py", line 278, in dump_payload
    return want_bytes(self.serializer.dumps(obj, **self.serializer_kwargs))
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python312\Lib\site-packages\flask\json\tag.py", line 323, in dumps
    return dumps(self.tag(value), separators=(",", ":"))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python312\Lib\site-packages\flask\json\__init__.py", line 41, in dumps
    return current_app.json.dumps(obj, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python312\Lib\site-packages\flask\json\provider.py", line 179, in dumps
    return json.dumps(obj, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python312\Lib\json\__init__.py", line 238, in dumps
    **kw).encode(obj)
          ^^^^^^^^^^^
  File "C:\Python312\Lib\json\encoder.py", line 200, in encode
    chunks = self.iterencode(o, _one_shot=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python312\Lib\json\encoder.py", line 258, in iterencode
    return _iterencode(o, 0)
           ^^^^^^^^^^^^^^^^^
  File "C:\Python312\Lib\site-packages\flask\json\provider.py", line 121, in _default
    raise TypeError(f"Object of type {type(o).__name__} is not JSON serializable")
TypeError: Object of type builtin_function_or_method is not JSON serializable

Here is the callback code that I believe to be causing the trouble. 
@dashApp.callback(
    [
        Output(component_id='login-url', component_property='pathname'),
        Output(component_id='login-alert', component_property='children')
    ],
    [
        Input(component_id='login-button', component_property='n_clicks'),
        Input(component_id='login-password', component_property='n_submit')
    ],
    [
        State('login-username', 'value'),
        State('login-password', 'value')
    ],
    prevent_initial_call=True
)
def login_success(n_clicks, n_submit, username, password):

    if (n_submit and n_submit > 0) or (n_clicks and n_clicks > 0):

        user = User.find_by_username(username)
        if user and user.verify_password(password) == True:
            login_user(user)
            logger.info("Login success %s" % user)
            return ('/home', success_alert)
        else:
            logger.info("Login error %s" % username)
            return ('/logout', failure_alert)
    else:
        return no_update, ''

Can you please review the above and let me know if you spot an obvious issue with the code?
Its a multipage app that upon login, I want to transition to home page.

Thanks,
Ger.

2

Hello @shadowofthewind,

Welcome to the community!

You could wrap your callback in a try - except clause. And print your traceback error for it, it seems like you are encountering a server error.

In the future, on_error will be part of the dash app process with init in a future release.

3

Hi @jinnyzor - adding the try - except logic within the callback handler does not seem to raise anything additional. Could that mean the issue is not in the callback but somewhere else.
Are there any obvious reasons why I might get the POST /_dash-update-component HTTP/1.1" 500 - does it mean it cannot locate the component or handler for the component?

Thanks,
Ger.

4

I can’t see anything where it would be an issue, are you able to share more of your code?

5

Hi @jinnyzor - I’ll try and share some of the code shortly.

1 Like