Moving average from datastream

class MovingAverage:

    def __init__(self, size: int):
        self.nums = []
        self.size = size
        self.running_sum = 0

    def next(self, val: int) -> float:

        # mistake done here was not defining running_sum globally
        # running_sum = 0 <-- this is wrong since you are resetting everytime
        if len(self.nums) < self.size:
            self.running_sum += val
            self.nums.append(val)
            return (self.running_sum*1.0)/(len(self.nums))
        else:
            self.running_sum = self.running_sum + val - self.nums[0]
            self.nums = self.nums[1:]
            self.nums.append(val)
            return self.running_sum/(self.size*1.0)