TypeError raised for evalf containing summations

[sylee957]

from sympy import *


j = Symbol('j')
expr = 4*Sum(-1, (j, I, 5)) + 3*I*Sum(-1, (j, I, 5))
expr.evalf()

raises TypeError: Invalid comparison of non-real I

[oscarbenjamin]

What do you think Sum(-1, (j, I, 5)) is supposed to represent? Probably it should give an immediate error.

[oscarbenjamin]

There are old open issues about disallowing non-integer limits for summations.

[anutosh491]

Yeah passing non-integer limits should raise an error which is not the case as of now . Something like sympy.Sum(-1, (j, I, 5)).doit() would return -6 + I .
Being relatively new to contributing to sympy ,I would be needing some guidance to approach this . If this issue is more towards the beginner/intermediate side I would try to take it on .

[faze-geek]

There are old open issues about disallowing non-integer limits for summations.
@oscarbenjamin

Is this a needs decision issue now , we can't disallow non-integer limits since many summations can be and should be evaluated by SymPy (eg -Sum(cos(n), (n, x, x + 1/2)).doit() ). Should we raise the TypeError simply when one limit is complex and the other is real !?

[oscarbenjamin]

It's difficult to know what to do in terms of backwards compatibility but I think ideally the limits would be required to be integers (unless infinite).