Why is there an error in the code?

Question

Python

Hello. In search of speeding up the calculation of the exponential moving average, I decided to write a recursive algorithm for this. I love recursion and try to use it more often.
Here is the actual code:

def get_ema(array, N: int, prev_ema: float = None):
    alpha = 2 / (1 + N)
    if len(array) == 1:
        ema = []
        ema.append(array[0] * alpha + (1 - alpha) * prev_ema)
        return ema

    if not prev_ema:
        for_sma = array[:N]
        sma = sum(for_sma) / len(for_sma)
        return get_ema(array[N:], N, prev_ema=sma).append(sma)
    else:
        new_ema = array[0] * alpha + (1 - alpha) * prev_ema
        return get_ema(array[1:], N, prev_ema=new_ema).append(new_ema)

On startup, an AttributeError: 'NoneType' object has no attribute 'append' error points to the last line. I don't understand why. It seems that the list should return, so append should work. Where did I go wrong?

Answer 1

append()returns None. So any return(except return ema) will return None. Well, at the next iteration, the code will crumble.
You can quickly check by replacing the first if block

tmp = get_ema(array[N:], N, prev_ema=sma)
print(type(tmp))
return tmp.append(sma)

and second

tmp = get_ema(array[1:], N, prev_ema=new_ema)
print(type(tmp))
return tmp.append(new_ema)