2014-03-15 21:44 GMT+00:00 Nikolaus Rath nikol...@rath.org:
Guido van Rossum gu...@python.org writes:
This downside of using subclassing as an API should be well known by now
and widely warned against.
It wasn't known to me until now. Are these downsides described in some
more detail somewhere?
The short version is: inheritance breaks encapsulation.
As a trivial and stupid example, let's say you need a list object which
counts the number of items inserted/removed (it's completely stupid, but
that's not the point :-):
So you might do something like:
class CountingList(list):
[...]
def append(self, e):
self.inserted += 1
return super().append(e)
def extend(self, l):
self.inserted += len(l)
return super().extend(l)
Looks fine, it would probably work.
Now, it's actually very fragile: imagine what would happen if list.extend()
was internally implemented by calling list.append() for each element: you'd
end up counting each element twice (since the subclass append() method
would be called).
And that's the problem: by deriving from a class, you become dependent of
its implementation, even though you're using its public API. Which means
that it could work with e.g. CPython but not Pypy, or break with a new
version of Python.
Another related problem is, as Guido explained, that if you add a new
method in the subclass, and the parent class gains a method with the same
name in a new version, you're in trouble.
That's why advising inheritance as a silver bullet for code reuses is IMO
one of the biggest mistakes in OOP, simply because although attractive,
inheritance breaks encapsulation.
As a rule of thumb, you should only use inheritance within a
module/package, or in other words only if you're in control of the
implementation.
The alternative is to use composition
For more details, I highly encourage anyone interested in looking at the
book Effective Java by Joshua Bloch (the example above is inspired by his
book). Although Java-centric, it's packed with many advises, patterns and
anti-patterns that are relevant to OOP and just programming in general
(it's in my top-5 books).
cf
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