[Python-3000] iostack and Oh Oh
talin at acm.org
Thu Dec 7 19:12:44 CET 2006
Phillip J. Eby wrote:
> At 09:59 AM 12/5/2006 -0600, Guido van Rossum wrote:
>> My point is that an interface can *document* (at least in English) a
>> "contract" about the invariants between operations. While I'm not into
>> enforcing or verifying such contracts, I'm very interested in
>> documenting them. For example, something that has "mapping" behavior
>> has a very different relationship between x[y] and "y in x" than
>> something that has "sequence" behavior.
> I assumed this didn't need answering. If you're using the interface solely
> for documentation, then a namespace-oriented interface suffices to provide it.
I'm guessing that Guido's use of the word 'document' means something
more than just conveying information to a human reader.
From what I can tell, the argument boils down to this: You are saying
that a class is merely the sum of its attributes and methods, and Guido
is saying that it's not.
For example: You cannot deduce by examining the methods of a list that
the keys are required to be consecutive integers starting from 0. Lists
have __getitem__, but so do maps; They support len(), but so do maps;
and so on.
Thus, lists embody a particular behavior contract (called 'concept' in
C++ template land). (The word 'interface' and 'ability' has been used in
this context, but that's somewhat misleading.  ) The concept is
supported by the methods of the class, but is only incompletely
derivable from them.
I think what Guido is looking for is a way to signal that a class obeys
a given behavioral contract, and a way to inspect the object, both at
runtime *and* to a human reader, and discover what contracts are
supported by the class.
On the other side of the argument is the fear that such explicitness
will lead to Java-style type bureaucracy, and lead to the death of
implicit, a la 'duck', typing.
The response to that, I think, is that we're not looking to replace duck
typing but only to supplement it - in other words, this extra
information would only be used to convey that which is not derivable by
inspection of types and methods.
 The reason that I think 'ability' is misleading is because what
we're really talking about is a *disability* - a limitation on what
inputs a class can accept. Also, one tends to think of the abilities of
a class as its methods, whereas what we're talking about here is a way
to describe the relationships *between* the methods.
 The reason I think 'interface' are misleading is due to its Java
connotations, in which an interface is a description of a bunch of
methods as well as a signal of an abstract type, but in this case I
think what they want is *only* the contract signal and not the method
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