Home Original page

Real Problems with Python

Michael Hudson mwh21 at cam.ac.uk
Sun Feb 13 18:03:08 EST 2000 
neelk at brick.cswv.com (Neel Krishnaswami) writes:

> > > 3. Multiple inheritance is unsound
> > >
> > >    By 'unsound' I mean that a method call to a method inherited by a
> > >    subclass of two other classes can fail, even if that method call
> > >    would work on an instance of the base class.
> > 
> > Unsure what this is about; am pretty sure nobody has complained about it
> > before.  Certainly "depth first, left to right" is an unprincipled approach,
> > but in practice it's more than predictable enough to use.
> 
> Here's some code:
> 
> class Foo:
>    x = 99
>    def frob(self):
>        print "%d bottles of beer" % self.x
> 
> class Bar:
>    x = "bar"
>    def wibble(self):
>        print "Belly up to the " + self.x
> 
> Note that all the operations on Foo and Bar work safely for direct
> instances of Foo and Bar.  But if you define a subclass like so:
>  
> class Baz(Bar, Foo):
>    pass
> 
> you can get type errors even though both of the superclasses have
> type-safe operations:
> 
> >>> q = Baz()
> >>> q.frob()
> Traceback (innermost last):
>   [...]
> TypeError: illegal argument type for built-in operation
> 
> This is only an annoyance in day-to-day work, but it becomes a big
> problem if you are serious about building automatic code analysis
> tools for Python (eg for CP4E). This is because the soundness theorems
> needed to write things like soft typing tools no longer hold. :(

How is this different from (excuse the dodgy syntax, I haven't done
much Eiffel):

class FOO
   feature
      x: INTEGER

   feature frob: STRING is
      do
          Result = interpolate("%d bottles of beer",x.to_string)
      end
end

class BAR
   feature
      x: STRING

   feature wibble: STRING is
      do
          Result = concatenate("Belly up to the ",x)
      end
end

class BAZ
    inherit FOO,BAR
end

? I mean, that won't compile, presumably, but in the case of a code
analysis tool there comes a point when it can just throw it's hands up
in the air and say "you're not playing by the rules, I give
up". Repeasted feature inheritance is not a solved problem in any
language I'm aware of.

And in this case, the code has a bug, so code analysis catching it is
surely a good thing?

not-at-understanding-yet-ly y'rs
Michael
More information about the Python-list mailing list