[Cython] CEP 507/513

[Martin C. Martin]

Dag Sverre Seljebotn wrote:
> Martin C. Martin wrote:
>> I too would be worried about changing the semantics of cdef T a = x. 
>> But what about:
>>
>> cdef T a = x
>> assert a.__class__ == T
>>
>> This makes it valid Python, which pins down the type of "a" exactly.  
>> In fact, at this point you don't even need the "cdef T".
>>
>> It's slightly ugly, but with luck, that will discourage people from 
>> premature optimization.
> It's still dangerous:
> 
> a.myfunc = myoverride
> assert a.__class__ == T
> a.myfunc()

True.  It would be good to have a way to turn off the per-instance 
metaclass for instances of a given type, i.e. not allow functions in the 
object's __dict__.  It wouldn't be that hard, in the constructor you 
could replace the object's __dict__ with a subclass of __dict__ that 
overrides the setter.

But then you have the same problem with the class, namely

a = T()
T.myfunc = myoverride
a.myfunc() # Calls myoverride.

Not sure how you'd disable it in the class.

> (On the subject of optimizations, one could also augment the CEP by 
> having the notation "exactly(MyClass)" mean "early-bind all calls and I 
> don't care about the consequences". Using "exactly(list)" would then 
> mean that builtins wouldn't need to be a special-case either. But I'm 
> fine with "list" being a special-case here.)

Well, special cases make things very confusing for the programmer, so 
I'm against them when possible.

Best,
Martikn