def eval_sum(f, limits):
    from sympy.concrete.delta import deltasummation, _has_simple_delta
    from sympy.functions import KroneckerDelta

    (i, a, b) = limits
    if f is S.Zero:
        return S.Zero
    if i not in f.free_symbols:
        return f*(b - a + 1)
    if a == b:
        return f.subs(i, a)
    if isinstance(f, Piecewise):
        if not any(i in arg.args[1].free_symbols for arg in f.args):
            # Piecewise conditions do not depend on the dummy summation variable,
            # therefore we can fold:     Sum(Piecewise((e, c), ...), limits)
            #                        --> Piecewise((Sum(e, limits), c), ...)
            newargs = []
            for arg in f.args:
                newexpr = eval_sum(arg.expr, limits)
                if newexpr is None:
                    return None
                newargs.append((newexpr, arg.cond))
            return f.func(*newargs)

    if f.has(KroneckerDelta) and _has_simple_delta(f, limits[0]):
        return deltasummation(f, limits)

    dif = b - a
    definite = dif.is_Integer
    # Doing it directly may be faster if there are very few terms.
    if definite and (dif < 100):
        return eval_sum_direct(f, (i, a, b))
    if isinstance(f, Piecewise):
        return None
    # Try to do it symbolically. Even when the number of terms is known,
    # this can save time when b-a is big.
    # We should try to transform to partial fractions
    value = eval_sum_symbolic(f.expand(), (i, a, b))
    if value is not None:
        return value
    # Do it directly
    if definite:
        return eval_sum_direct(f, (i, a, b))
dataset/ETHPy150Open sympy/sympy/sympy/concrete/summations.py/eval_sum/original