import csv

def group_by_sum(csv_file, group_col, sum_col):
    sums = {}
    with open(csv_file, 'r') as f:
        reader = csv.DictReader(f)
        for row in reader:
            group_val = row[group_col]
            sum_val = float(row[sum_col])
            if group_val in sums:
                sums[group_val] += sum_val
            else:
                sums[group_val] = sum_val
    return sums

示例用法：

>>> group_by_sum('data.csv', 'category', 'amount')
{'cat1': 25.0, 'cat2': 45.5, 'cat3': 30.0}

其中，csv_file 是 CSV 文件的路径，group_col 是分组的列名，sum_col 是需要求和的列名。函数返回一个字典，其中键是分组的值，值是对应组的总和。