In that case, it would be better to model your object after Predicate and
AppliedPredicate, which work the same way (except in Boolean expressions).
Aaron Meurer
> On Feb 5, 2016, at 5:37 PM, Bogdan Opanchuk <manti...@gmail.com> wrote:
>
> Hi Aaron,
>
> Thank you for your reply.
>
>> If you want to give the fields names I guess you should use
>> UndefinedFunction. Note that UndefinedFunction('f') dynamically creates a
>> subclass of AppliedUndef called "f".
>
> The problem is that I need this object to participate in expressions even
> without being applied (that is, both `f` and `f(x)` should be usable in
> expressions), and UndefinedFunction cannot do that. I was thinking about
> subclassing a Symbol and overriding __call__ to return an applied function
> (that's where the error checking will happen, too). Another variant would be
> to give the user a function applied to native dimensions right away, and
> override __call__ in the UndefinedFunction derivative (that would make
> printing more verbose than necessary, but I can live with that). Which way do
> you think fits better in sympy design?
>
>> I think what you are doing is right, although let us know if you run into
>> issues (SymPy has many bugs where things don't work correctly if you
>> subclass things). It's clear to me that we should make this sort of thing
>> easier, though.
>
> The main problem is that I do not know which conventions should my subclassed
> objects conform to (besides the primary invariant). For instance, do I
> understand it correctly that sympy caches objects purely for performance
> reasons, and for two objects to be equal it is enough to have equal
> _hashable_content()? If I want a custom differentiation behavior (e.g.
> `f.diff(x)` equivalent to `f(x).diff(x)`), do I need to override diff(), or
> some underscored method?
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