Operands and Operator of symbolic expressions

[SimonKing]

I think that for all symbolic expression s should hold

sage: s == s.operator()(*s.operands())

That currently does not work, for two reasons.

1. There may be no operator at all:

   sage: print x.operator()
   None
   sage: x.operands()
   []
   

I believe there should instead be an identity operator.

2. The list of operands may be longer than what the operator accepts:

   sage: s = 0.001*x^2+0.01*x+0.1*sin(1.01*x)+1
   sage: s.operands()
   [0.00100000000000000*x^2, 0.0100000000000000*x,
   0.100000000000000*sin(1.01000000000000*x), 1]
   sage: s.operator()(*s.operands())
   ---------------------------------------------------------------------------
   TypeError                                 Traceback (most recent call
   last)
   
   /home/king/<ipython console> in <module>()
   
   TypeError: op_add expected 2 arguments, got 4 
   

Compare this thread at ​sage-devel.

[eviatarbach]

Note to self: after this is fixed (or at least just the arithmetic operators), simplify_hypergeometric should be added to simplify_full. Now it cannot be added because it will sometimes raise errors due to this bug.

[eviatarbach]

Burcin, is there a problem with setting the sum operator to Expression.add, for example? I don't understand why we have to define new functions representing the arithmetic operators.

[rws]

This will not work with some symbolic functions because they may have been called with the hold=True keyword, and calling them again doesn't know about it. See also #17849. The solution of that ticket in line with this one would be to make keywords into operands (strings?, a dedicated class OperatorKeyword?).