While looking at #15786 with an eye to understanding the architecture of
sage, I noticed the following comment before the __call__ method of
Function_ceil and Function_floor.
#FIXME: this should be moved to _eval_
After some rummaging about I see that the problem is that the function
looks something like
def __call__(self, x):
try a bunch of stuff
else:
return BuiltinFunction.__call__(self, SR(x_original))
the intention being, if we cannot find a numerical result, just leave it as
a symbolic result. This works when we overload the __call__ interface
because the BuiltinFunction.__call__ will not reach our code again.
Unfortunately, if we move all of the code from Function_ceil.__call__ to
Function_ceil._eval_ then the call to BuiltinFunction.__call__ will then
call Function_ceil._eval_ and we are on the way to infinite recursion. We
cannot call BuiltinFunction._eval_, because BuiltinFunction does not
implement _eval_.
Is there some way to avoid this recursion? What is the problem with
overloading __call__?
On a not entirely unrelated note, is the expected interface for classes
that inherit from BuiltinFunction documented somewhere. The ones I know
about are listed in the code for BuiltinFunction._register_function, but
don't seem to be documented. For example, I believe that _evalf is
supposed to be the numerical evaluation function, but do not have a clear
understanding of what types it is expected to return, what exceptions it is
expected to raise, etc.
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