[j for i in ann["segmentation"] for j in i]

今天遇到一个有意思的代码 [j for i in ann["segmentation"] for j in i]
不太常见，列表推导式里有两个 for

ann=dict()
ann["segmentation"] = [[1,2],[2,3]]
 
[j for i in ann["segmentation"] for j in i]  # [1, 2, 2, 3]

其实效果相当于：

result = []
for i in ann["segmentation"]:
    for j in i:
        result.append(j)

展平列表了。