您可以在列表理解中嵌套多少个 for 循环是否有限制？

Question

我想知道为什么以下列表理解失败 (UnboundLocalError: local variable 'y_value' referenced before assignment)。如果我在一个三重嵌套的 for 循环中逐字地拼出 for 循环，我会很好地得到我想要的结果。

for y_value_group in collinear_y_values:
    all_y_values = []
    for y_value in y_value_group:
        for line_id in horizontal_lines[y_value]:
            for p in LINES[line_id].pts:
                all_y_values.append(p.y)
    print(all_y_values)
    all_y_values = [p.y for p in LINES[line_id].pts for line_id in horizontal_lines[y_value] for y_value in y_value_group]
    print(all_y_values)

给出以下输出：

[-0.01447138307529966, 0.22089181280929138, 0.22089181280929138, 0.19409634248390767]
---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
<ipython-input-85-62411ee08ee6> in <module>
     24     print(all_y_values)
---> 25     all_y_values = [p.y for p in LINES[line_id].pts for line_id in horizontal_lines[y_value] for y_value in y_value_group]
     26     print(all_y_values)

<ipython-input-85-62411ee08ee6> in <listcomp>(.0)
     24     print(all_y_values)
---> 25     all_y_values = [p.y for p in LINES[line_id].pts for line_id in horizontal_lines[y_value] for y_value in y_value_group]
     26     print(all_y_values)

UnboundLocalError: local variable 'y_value' referenced before assignment

Answer 1

有点令人困惑，在列表理解中你应该把外循环放在第一位：

>>> [x for x in y for y in np.random.random((2,2))]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'y' is not defined
>>> [x for y in np.random.random((2,2)) for x in y]
[0.5656047153549479, 0.19139220091114273, 0.10286775868807774, 0.3230695608882298]

所以只需更改顺序：

[
    p.y 
    for y_value in y_value_group
    for line_id in horizontal_lines[y_value]
    for p in LINES[line_id].pts
]