[BUG] cannot pickle 'SSLContext' with background callback about dash HOT 5 OPEN

Are you using the SQLAlchemy engine defined globally elsewhere?
The multiprocess will try to transfer it when used but that is not valid pickle/multiprocess object. May need to establish the engine inside the callback for that to work or switch to celery.

from dash.

The engine is not a global object but a property of an object created in the callback. Roughly:

class DBHandle:
    def __init__(self, url, schema):
        connect_args = {...}
        self.engine = sqlalchemy.create_engine(
            url,
            echo=True,
            pool_pre_ping=True,
            connect_args=connect_args,
            execution_options={"schema_translate_map": {None: schema}},
        )

@dash.callback(
    [...]
    prevent_initial_call=True,
    background=True,
    manager=BACKGROUND_CALLBACK_MANAGER,
)
def _callback():
    dbh = DBHandle(url, schema)

I'll try with Celery if I can't find a solution but it would be nice and make development easier if it could work with a local cache.

from dash.

Could you try running your app with:

app.run(debug=True, dev_tools_prune_errors=False)

With that option the stacktrace in the screenshot should have more info to help identify which variable is causing the issue.

from dash.

Thanks for the suggestion. After some digging, I realized the SSLContext instance is a nested property of botocore.client.S3. This S3 client is not used in the background callback that is the source of the error, but elsewhere in the app. I guess dash serializes the whole execution environment?

from dash.

I guess dash serializes the whole execution environment?

Yes, we use multiprocess to spawn the background process and that will try to transfer all the global variables.

from dash.