If the concern is that a developer implementing this mixin in one of their views wouldn't see the exception because it gets triggered when a 404 happens, then there are a few options.  Calling upon the template name get method in __init__ can work; however, doing so would require that the developer make the Custom404Mixin class be first in a view that inherits multiple classes because the built-in CBV don't participate in co-operative multiple inheritance.  It also feels a bit non-DRY to call a method early just to attempt to trigger the error, when it would have to be called again later.  Given that, I'm a bit hesitant to use __init__, so I'd suggest either not doing it and adding some developer doc that highlights this case, or we forgo a bit of code beauty and call the get method in the dispatch sequence before the try block.