[BUG] Children prop is not always validated

[jdamiba]

moved from https://github.com/plotly/dash-core/issues/214

@alexcjohnson

I would expect the Dash app that follows to throw an error when the dcc.Input component tries to set children={"a dict": "is not allowed for children-input"}.

However, the error is only thrown when the html.Div component attempts to do the same.

import dash
import dash_html_components as html
import dash_core_components as dcc


app = dash.Dash(__name__)

app.layout = html.Div([
    dcc.Input(children={"a dict": "is not allowed for children-input"}),
    html.Div(children={"a dict": "is not allowed for children-div"})

])

if __name__ == '__main__':
    app.run_server(debug=True)

Error:

An object was provided as `children` instead of a component, string, or number (or list of those). Check the children property that looks something like:
{
  "a dict": "is not allowed for children-div"
}

[alstn2468]

I think it is a little more right raise unexpected keyword argument: children error .
The reason seems to be that the __init__ function was called with children missing from args in the generated file.

• dash/development/base_component.py (134-141)

if not k_in_propnames and not k_in_wildcards:
    allowed_args = ", ".join(
        sorted(self._prop_names)
    )  # pylint: disable=no-member
    raise TypeError(
        f"{error_string_prefix} received an unexpected keyword argument: `{k}`"
        f"\nAllowed arguments: {allowed_args}"
   )

• Input.py

args = {k: _locals[k] for k in _explicit_args if k != "children"}
for k in []:
    if k not in args:
        raise TypeError("Required argument `" + k + "` was not specified.")
super(Input, self).__init__(**args)

• Link.py

args = {k: _locals[k] for k in _explicit_args if k != "children"}
for k in ["href"]:
    if k not in args:
        raise TypeError("Required argument `" + k + "` was not specified.")
super(Link, self).__init__(children=children, **args)

Is there a reason for separately pass children?
If put children in args together, the validation will be the same.