[Types-sig] A type checking system in Python

Paul Prescod paulp@ActiveState.com
Wed, 21 Mar 2001 11:13:00 -0800

"hzhu@users.sourceforge.net" wrote:
> ...
> One solution promoted by some other languages is to define some abstract
> classes for type checking purpose, and require them to be inherited by any
> class that behaves like them.  This is like what interfaces do in Java.
> This solution is not satisfactory because
> - It breaks polymorphism.  One has to harness the property at the time the
>   classes are defined, instead of when they are used.  For example, if an
>   application requires a property of "ReadableFile" which is satisfied by
>   sys.stdin, it is difficult to retrofit the definition of sys.stdin to be
>   an instance of such.

It is too strong to say it breaks polymorphism. It can in some cases
*hamper* polymporhism. But Java is a fundamentally polymorphic language,
as are all other object oriented languages I have used. Polymorphism is
the central point of OO. Even languages that do not support either
encapsulation or inheritance support polymorphism.

> - It mixes two incompatible inheritances.  For example, suppose that class A
>   has method A.m() but its subclass B overrides it with B.m().  The property
>   A satisfies is not necessarily satisfied by B.

You cite Java interfaces above and yet Java interfaces are *specifically
designed* to separate out the two different kinds of inheritance. That's
why there are interfaces which are distict from classes!

> From the above it is clear that there is a useful conceptual hierarchy that
> is independent of the class hierarchy.  For lack of a better name let us
> just call them Types.  

We've been calling them interfaces and/or protocols. Python already has
a meaning for "type" which is somewhat confusing in this context.

> - classes embody implementations
> - Types embody requirements

This is exactly the distinction between Java classes and interfaces. I
have a prototype implementation that embodies this distinction here:


We've been discussing these issues in the type-sig. You are invited to
join us there. 
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