_sympy_ conversion for BuiltinFunction and CallableSymbolicExpression objects

[mkoeppe]

Currently we have:

sage: from sympy import sympify
sage: sympify(sin(x))
sin(x)
sage: 
sage: sympify(sin)
SymPyDeprecationWarning: 

String fallback in sympify has been deprecated since SymPy 1.6. Use
sympify(str(obj)) or sympy.core.sympify.converter or obj._sympy_
instead. 

We should fix this by providing functions with a _sympy_ method that implements the same conversion as SympyConverter.composition.

Also, converting CallableSymbolicExpression to sympy forgets about the formal arguments:

sage: from sympy import sympify
sage: f(x, y) = x^2 + y^2; f
(x, y) |--> x^2 + y^2
sage: sympify(f)
x**2 + y**2

We change it to return a SymPy Lambda instead.

CC: @egourgoulhon @nbruin @kcrisman @EmmanuelCharpentier @mwageringel

Component: symbolics

Author: Matthias Koeppe

Branch/Commit: d9cff76

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/32130

[mkoeppe]

Branch: u/mkoeppe/_sympy__conversion_for_builtinfunction_and_callablesymbolicexpression_objects

[mkoeppe]

New commits:

d9cff76

Function._sympy_, SympyConverter.__call__: New

[mkoeppe]

Author: Matthias Koeppe

[mkoeppe]

Commit: d9cff76

[tscrim]

comment:7

LGTM.

[tscrim]

Reviewer: Travis Scrimshaw

[mkoeppe]

comment:8

Thanks!

[vbraun]

Changed branch from u/mkoeppe/_sympy__conversion_for_builtinfunction_and_callablesymbolicexpression_objects to d9cff76