overriding methods - two questions

[Bruno Desthuilliers]

Donn Ingle a écrit :
> Hi,
> Here's a framework for the questions:
> 
> --- In a module, part of an API ---
> class Basis ( object ):
>  def foo ( self, arg ):
>   pass
> 
> --- In user's own code ---
> class Child ( Basis ):
>  def foo ( self, not, sure ):
>   ...
> 
> 
> 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.

> Question 2:
> 
> Say I am in class Basis, doing a loop and I have a list of Child objects. I
> want to run the foo() method for each one that *has* a foo() method. i.e.
> user has done this:
> 
> class Sam ( Child ):
>  ...
> *Sam does not define foo()
> 
> class Judy ( Child ):
>  def foo ( self, arg ):
>   ...
> * Judy does define foo()
> 
> Instances of Sam and Judy have been put into the list (within the instance)
> of Basis. I want Basis to detect that Judy has foo() and run it.
> 
> I can handle question 2 by using a flag that must be set by the user.
> Something like:
> class Judy ( child ):
>  def __init__( self ):
>   self.pleaseCallFoo = true
> 
> And now, Basis can check for that var and only then call foo(), but this is
> ugly and means more for the user to learn API-wise.

Indeed.

> Any ideas?

Quite a few, but I don't have enough context to tell which one would be 
the best - nor why you want to do such a thing. Anyway, the simplest is 
to just check :

for child in self.childrens:
   if 'foo' in child.__class__.__dict__:
     child.foo()

but this won't call foo for :

class Dude(Judy):
   pass

Don't know if that's what you want. If not (ie, you want to call 
child.foo if foo is not Basis.foo), then:

for child in self.childrens:
   if child.foo.im_func is not self.foo.im_func:
     child.foo()

HTH