[Python-3000] i guess i was misunderstood

[Robin Bryce]

[Talin]
> b=B() # error, A.__init__ expects an instanc of A
>
>why is this kind of type-checking enforced by the language?

[greg]
>Since type-class unification, there is no clear boundary <snip> If you were
allowed to pass any object to any method, there would be a danger of
crashing the interpreter.

I've always expected `AttributeError` if I call `A.somemethod(otherself)`
AND otherself does not conform to the expectations of `A.somemethod`. I
still find `error, A.somemethod expects an instance of A` *very* surprising.

`TypeError` is something I would expect if I broke the typing rules of the
underlying implementation. By 'underlying implementation' I mean the
interpreter AND any extension library that uses the interpreters foreign
function provisions to provide extension types. IIUC greg is saying this is
exactly what Talin's example does.

In the python tutorial 'self' is introduced like this: "the special thing
about methods is that the object is passed as the first argument of the
function". And ever since reading that I've always expected to be able to
treat 'self' as just another parameter. Talins' first example illustrates
that 'self' can not be treated this way.

If there was a way to allow `A.somemethod(otherself)` where
`isinstance(otherself, A) is False`, as a language feature, then I would
find my self reaching for things like PyProtocols less often. I'm not
knocking PyProtocols, I just wish there was a middle ground available that
didn't nobble future adoption (no pun intended) of PyProtocols et al.

Does resolving the can of worms that greg alludes to belong in the 'very
difficult' or 'insufficient motivation & use' camp  ?

I don't care too much about making life harder for extention authors.
Software never so much resembeled the cavalry to me as when I first
encountered Pyrex.


Robin

http://docs.python.org/tut/node11.html#SECTION0011340000000000000000

On 12/05/06, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
>
> tomer filiba wrote:
>
> > b=B() # error, A.__init__ expects an instanc of A
> >
> > why is this kind of type-checking enforced by the language?
>
> Since type-class unification, there is no clear boundary
> between built-in and user-defined classes. Any class can
> potentially add C-level data to its instances and have
> methods implemented in C that operate on it. If you were
> allowed to pass any object to any method, there would
> be a danger of crashing the interpreter.
>
> > what this code does is basically changing the __mro__ of the object. had
> > __mro__ been mutable, it would all be simpler.
>
> Being able to change the mro after class creation would
> lead to similar problems.
>
> --
> Greg
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