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num.txt 第一行是逗号分隔的整数 "3,8,2,5"，读取并求和输出。
# num.txt 内容：3,8,2,5
with open("num.txt") as f:
line = f.read().strip()
nums = [int(x) for x in line.____(",")] # 按逗号切分
print(sum(____)) # 对 nums 求和split
nums
split(",") 按逗号切成 ["3","8","2","5"]；转 int 后用 sum(nums) 求和 =18。