想要用python来求算术平均数，但是累加求和那里总写不出来，代码没出啥问题吧？

你的while循环和for循环重复了

def suanshupingjunshu(self, nums):
    sum = 0
    for i in nums:
        sum += i
    return sum / len(nums)

或者

def suanshupingjunshu(self, nums):
    sum = 0
    for i in range(len(nums)):
        sum += nums[i]
    return sum / len(nums)

或者

def suanshupingjunshu(self, nums):
    sum = 0
    i = 0
    while i < len(nums):
        sum += nums[i]
        i += 1
    return sum / len(nums)