Feature request: delayed validation

[0zeroth]

Validation is awesome, but if all the components are not defined upfront it must be disabled (eg when creating multi-page / multi-file apps).

It is possible to monkey-patch delayed validation. An example is below, but since the introduction of multi-output it is probably not correct. I find this extremely useful, as I always get the @app.callback strings wrong, and even though immediate validation is best, this is much better than nothing.

Perhaps this could be added as a feature! (I of course leave the decision to accept/reject this proposal in your capable hands!)

import dash

_old_validate = dash.Dash._validate_callback
_list = []


def new_validate(self, output, inputs, state):
    _list.append(tuple((output, inputs, state)))

dash.Dash._validate_callback = new_validate


def validate_callbacks(app):
    from dash._utils import create_callback_id as _create_callback_id
    old_map = app.callback_map
    app.callback_map = {} # Monkey patch to trick Dash from raising DuplicateCallbackOutput
    for output, inputs, state in _list:
        _old_validate(app, output, inputs, state)
        callback_id = _create_callback_id
    app.callback_map = old_map

app = dash.Dash(__name__)

# After defining all the callbacks ...

validate_callbacks(app)
app.run_server(**CONFIG)

[ned2]

I very much agree, and in fact, have just submitted a proposed PR for doing this: #644.

I ended up taking the same basic approach as yours, but with some extra logic for detecting duplicated Outputs in callbacks (including handling multi-output callbacks). Feedback welcome!

[alexcjohnson]

Combining this with #519