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

Chris Rebert pyideas at rebertia.com
Fri Jan 13 01:22:26 EST 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 'odd_ge_3'
> It would look better if you used standard syntax and do the inclusion check
> in a __contains__ method.
>>>> 3 in odd_ge_3
> 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

>> 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.


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