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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