python - python 中的组合 filter() 无法按预期工作

Question

据我了解，以下代码应输出[['b']]。相反，它输出 [['a', 'exclude'], ['b']]。

这是Python中的一个错误，还是我误解了什么？

lists_to_filter = [
    ['a', 'exclude'],
    ['b']
]
# notice that when 'exclude' is the last element, the code returns the expected result
for exclude_label in ['exclude', 'something']:
    lists_to_filter = (labels_list for labels_list in lists_to_filter if exclude_label not in labels_list)
    # notice that changing the line above to the commented line below (i.e. expanding the generator to a list) 
    # will make the code output the expected result, 
    # i.e. the issue is only when using filter on another filter, and not on a list
    # lists_to_filter = [labels_list for labels_list in lists_to_filter if exclude_label not in labels_list]
lists_to_filter = list(lists_to_filter)
print(lists_to_filter)

Answer 1

发生这种情况是因为 lists_of_filter 仅在循环外迭代。在循环之外，您有 exclude_label == 'something'，这就是您得到意外结果的原因。要检查它，您可以输入一行 exclude_label = 'exclude':

lists_to_filter = [
    ['a', 'exclude'],
    ['b']
]

for exclude_label in ['exclude', 'something']:
    lists_to_filter = (labels_list for labels_list in lists_to_filter if exclude_label not in labels_list)

exclude_label = 'exclude'
lists_to_filter = list(lists_to_filter)
print(lists_to_filter)  # [['b']]

doc for generator expressions表示“不能在封闭范围内评估后续 for 子句和最左侧 for 子句中的任何过滤条件，因为它们可能依赖于从最左侧迭代获取的值。”。在您的情况下，过滤条件 if except_label ... 取决于从 for excex_label in ... 循环中获取的值。