Currently when user sets a completer policy override and then changes evaluation mode to one which does not support given override, they get a warning each time they use a completer.

%config IPCompleter.evaluation = 'limited'
%config IPCompleter.policy_overrides = {'allowed_getattr_external': []}
%config IPCompleter.evaluation = 'unsafe'
# see warnings from now on Tab

Instead, the warning should be displayed only once, ideally when changing the evaluation policy.