_evalf_ handling of backends

[eviatarbach]

As per Burcin, with the option of the algorithm keyword for _evalf_ in #12289,

ATM, if you implement different backends, you need to write a lot of
boilerplate code in _evalf_() to parse the algorithm argument, do the
right thing if it is not given etc. Can we have a default _evalf_
method, which dispatches to _evalf_<system>_ methods if they exist and
raise error if not? Perhaps we can also figure out how to annotate
these methods to indicate if they support arbitrary precision
evaluation or not.

CC: @burcin @kcrisman

Component: symbolics

Issue created by migration from https://trac.sagemath.org/ticket/15200

[eviatarbach]

Dependencies: #12289

[eviatarbach]

comment:2

I think "the right thing if it is not given" is resort to a method _evalf_default_. In cases where _evalf_ has already been defined, overwriting the _evalf_ which this patch would define, this will not change anything. The only problem is that for most functions the algorithm keyword won't do anything. I can't think of any good solution to this (such as raising a warning or error) that wouldn't involve modifying all the existing symbolic functions.

[eviatarbach]

comment:3

Attachment: trac15200.patch.gz

This patch works for BuiltinFunction. Unfortunately it breaks GinacFunction (since they often don't define an _evalf_ method, they don't overwrite the _evalf_ in the base class) and gives an error as in #14743 on startup:

Exception AttributeError: "'builtin_function_or_method' object has no attribute 'func_code'" in 'sage.symbolic.function.SymbolicFunction._hash_' ignored

[jdemeyer]

comment:7

I think the best solution would be to hardcode a set of common algorithm/parent choices like:

def _evalf_(self, *args, parent=None, algorithm=None):
    if hasattr(self, '_evalf_scipy_') and (some_condition_on_parent_and_algorithm):
        return parent(self._evalf_scipy_(*args))
    if hasattr(self, '_evalf_mpmath_') and (some_condition_on_parent_and_algorithm):
        return parent(self._evalf_mpmath_(*args))
    [...]
    return self._evalf_default_(*args, parent=parent, algorithm=algorithm)

[jdemeyer]

Changed dependencies from #12289 to #17130