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If a apphook needs namespaces it, it should be able define the default instance namespace (see django url namespacing) on the Apphook class.
Since the namespace field is unique, form validation will complain if the namespace is already taken. Otherwise the default can silently be used.
In advanced settings:
Django docs that I think @stefanfoulis meant to reference: https://docs.djangoproject.com/en/dev/topics/http/urls/#reversing-namespaced-urls
“The default application instance is the instance that has an instance namespace matching the application namespace...”
This is implemented, but slightly differently. It doesn't not affect the presentation of the form, but if the user neglects to complete the 'Application instance name[space]:' field, and the app requires one, and the default is not already in use, then it will just automatically apply the default.
The next time the user inspects the Advanced Settings form, they will see it was set.