[Python-ideas] Symbolic expressions (or: partials and closures from the inside out) (original) (raw)

Chris Rebert pyideas at rebertia.com
Fri Jan 13 07:22:26 CET 2012

On Thu, Jan 12, 2012 at 3:03 PM, Terry Reedy <tjreedy at udel.edu> wrote:

On 1/12/2012 3:45 PM, Nathan Rice wrote:

print isinstance(3, const) True A Contraints instance defines a set. 'const' is the set 'oddge3' It would look better if you used standard syntax and do the inclusion check in a contains method. 3 in oddge3 True

But what are types but abstract sets of values? Phrasing it as a typecheck is perfectly sensible from a type-theoretic standpoint. Also, the problem of representing isinstance(X.attr, someclass) [for non-Constraint someclass, e.g. str] in a Constraint would still remain, since there's no "lcontains" (thus precluding X.attr in str).

>> _so your validations are checked using _instancecheck_._ >> _But it is a fake check in that 3 is not really an instance of the class,_ > _which has no instances._

The same can be true for abstract base classes, which have been sufficiently accepted to warrant adding instancecheck() in the first place and also to be added to the std lib (witness the abc and collections modules). It may seem unfamiliar, but then again it was only made possible starting with v2.6, which is pretty recent.

Cheers, Chris

http://rebertia.com

More information about the Python-ideas mailing list