Apps registry public API methods lack input validation

[Nicholas Salzman]

The Apps class in django.apps.registry accepts arbitrary argument types on its public API methods. Passing a non-string or empty string produces confusing errors deep in the call stack rather than clear messages at the API boundary.

For example, apps.get_app_config(42) raises KeyError: 42 from the dict lookup inside the method. With validation, it would raise TypeError: app_label must be a string, not int. at the entry point.

Affected methods: get_app_config(), get_model(), register_model(), is_installed(), lazy_model_operation().

I can submit a PR that already passed your tests, other than not having created a ticket first! :)

Nicholas

[Simon Charette]

The Django public API has hundreds of methods that can crash if you provide it the inadequate type, we can't gate them all with isinstance check.

Unless you can provide a real world example where this is an issue (e.g. a documented use case that isn't explicit about a proper app label needing to be provided) I don't see the value of peppering this particular API with instance type checks as there are projects that provide typing stubs for Django already and ​they have pretty good coverage for this API.