[Python-ideas] @Override decorator

[Nick Coghlan]

On 19 July 2016 at 14:04, Guido van Rossum <guido at python.org> wrote:
> The same argument would apply against abstractmethod... I have definitely
> wondered when looking over some code whether it was defining or overriding a
> method. But requiring people to always decorate overrides seems hard, unless
> we made it a property that can be enforced by a metaclass, maybe?

This is trickier in Python than it is in Java, since we support
multiple inheritance, and we don't require that subclasses be drop-in
replacements for their parent class.

The key difference I see between @abstractmethod and this proposal is
that @abstractmethod is a service that base class authors provide to
subclass authors to say "Hey, when subclassing this, you *need* to
override these, or your subclass won't work properly". In effect, it's
an implicit test case for all future subclass definitions.

By contrast, this proposal would go in the other direction: it would
be an assertion by the author of a *subclass* that that particular
method isn't new, it's inherited from a parent class, and they're
replacing the existing implementation with a new one. It would only
add new information for readers in cases where the method wasn't
already calling super() (since the latter already implies you expect
there to be at least one further implementation along the MRO to
call).

In my view, that puts it more in the category of type annotations and
the "missing attribute" detection in tools like pylint than it does
abstractmethod: rather than checking it at runtime, you'd want static
checkers like mypy to flag it as a structural error if a method
declared as replacing a base class method wasn't actually doing so.

So a "typing.override" to complement "typing.overload" would make
sense to me, but a checked-at-runtime decorator wouldn't (doing it
that way would also let you put the override annotation in stub files
instead of directly in the code).

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia