On Sat, 21 Aug 2010 08:08:56 am Alan Gauld wrote:
> Every time you change the interface of inherited methods you
> create for yourself extra work in converting types to match the
> superclass. But that is often easier than rewriting the methods
> from scratch.
Every time you change the interface of inherited methods, you probably
shouldn't.
Firstly, it probably breaks the Liskov Substitution Principle. The LSP
says, essentially, if you subclass A to make B, you should be able to
use a B anywhere you can use an A. (After all, instances of B are also
instances of A.) Here's an example of what not to do:
class Vehicle:
def start(self, key):
ignition.insert(key)
ignition.turn() # raises exception if out of fuel
def go(self, key):
self.start(key)
self.handbrake = 'off'
self.gear = 'drive'
self.accelerate()
class Truck(Vehicle):
# add other truck-like methods
class KeylessTruck(Truck):
# Some military vehicles are designed to not require keys.
# When on a battlefield, there's nothing worse than being
# unable to find the car keys!
def go(self):
self.start()
self.handbrake = 'off'
self.gear = 'drive'
self.accelerate()
def start(self):
ignition.turn() # Actually a push button, but nevermind.
Can you see the problem? If the caller is expecting a Truck, and pass a
key to the truck.go() method, they will get an exception if you give
them a KeylessTruck instead of a Truck. This is a Bad Thing.
Secondly, changing method interfaces is not compatible with multiple
inheritance and super(). You can probably get away with it if you stick
to single inheritance, but it's still a bad thing to do.
--
Steven D'Aprano
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