On Fri, 16 Nov 2007 18:28:59 +0100, Bruno Desthuilliers wrote:
>> Question 1:
>>
>> Given that the user of the API can choose to override foo() or not, how
>> can I control the signature that they use?
>
> While technically possible (using inspect.getargspec), trying to make
> your code idiot-proof is a lost fight and a pure waste of time.
Worse: it's actually counter-productive!
The whole idea of being able to subclass a class means that the user
should be able to override foo() *including* the signature. Why do you
want to stop them? It's their subclass, not yours. You don't know what
arguments it needs.
Let me give a practical example: in mathematics there is a construct
known as a continued fraction. What it is isn't especially important, if
you're curious you can google for it. If you were defining a class for
continued fractions, you might do this:
class ContinuedFraction(object):
def __init__(self, list_of_numerators, list_of_denominators):
pass
# lots of other methods
cf = ContinuedFraction([3, 7, 2, 8, 9, 5], [2, 3, 1, 5, 3, 7])
If I wanted to subclass your ContinuedFraction class to provide regular
continued fractions, I could do this:
class RegularCF(ContinuedFraction):
def __init__(self, *denominators):
numerators = [1]*len(denominators)
super(RegularCF, self).__init__(numerators, denominators)
# all other methods are inherited from super-class without change
cf = RegularCF(4, 9, 1, 2, 6, 3)
But if you did what you're proposing to do, I couldn't do that. I'd need
to do something silly like this:
class RegularCF(ContinuedFraction):
def __init__(self, list_of_numerators, list_of_denominators):
numerators = [1]*len(list_of_denominators)
super(RegularCF, self).__init__(numerators, list_of_denominators)
cf = RegularCF(None, [4, 9, 1, 2, 6, 3])
just so that the signatures matched. What a waste of time.
And worse, what if my subclass needed *more* arguments than your
signature provided? The hoops I would have to jump through would not only
be flaming, they'd be spinning and flying through the air, with rotating
knives and trip-wires.
--
Steven
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