sympy.Sum error: IndexError: only integers, slices (`:`), ellipsis (`...`), numpy.newaxis (`None`) and integer or boolean arrays are valid indices

I am programming the following function (in python):

def draw_fourier(a, n):
    if len(a) < 2*n + 1:
        return NotImplemented
    else:
        return (1/n) * sy.Sum(a[y] * np.exp(1j * y * t), (y, -n, n))

(sy. signifies sympy and np. signifies numpy)
And I get the following error:

line 91, in draw_fourier
    return (1/n) * sy.Sum(a[y] * np.exp(1j * y * t), (y, -n, n))
IndexError: only integers, slices (`:`), ellipsis (`...`), numpy.newaxis (`None`) and integer or boolean arrays are valid indices

I previously also got similar errors with sympy.Sum - I have yet to actually successfully use it, so I am curious as to what I am doing wrong, or understanding wrong about the function. I would like it to evaluate the sum for $y$ from $-n$ to $n$. a is a list and n is an (positive) integer. The reason I am using sympy is that 't' is a symbol, which I would like to assign a value to later.

Many thanks,
Hugo

When you use sy.Sum(expression, (dummy_variable, start, end)) your dummy_variable (in this case y) is a symbol. The error is that y cannot be an index of the list a because it is not a valid type for an index, as it is a symbol.

A simple way to write that symbolic summation where the coefficient in a changes in every term could be:

summation=0 #some initialization
for i in range(-n,n):
    summation+=a[i]*sy.exp(1j*i*t)
summation=(1/n)*summation #final expression