Python - 返回 lst 中仅包含整数的列表列表？

Question

def only_evens(lst):
    """
    Return a list of the lists in lst that contain only even integers. 

    >>> only_evens([[1, 2, 4], [4, 0, 6], [22, 4, 3], [2]])
    [[4, 0, 6], [2]]
    """
    even_lists = []
    condition = True 

    for sublist in lst:
        for num in sublist: 
            if num % 2 != 0: 
                condition = condition and False
        if condition == True: 
            even_lists.append(sublist)

    return even_lists

我不明白为什么这会一直返回空字符串？直观上看，这有道理吗？非常感谢您的帮助。我已经在这个问题上坚持太久了。

编辑:非常感谢大家!我现在明白了:)。

Answer 1

您正在循环外初始化条件。应该为每个子列表重新初始化，并且 condition = condition and False 将始终评估为 False，应该是 condition = False

for sublist in lst:
    condition = True
    for num in sublist: 
        if num % 2 != 0: 
            condition = False
            break

这可以通过 all 函数来完成，如下所示

return [sublist for sublist in lst if all(item % 2 == 0 for item in sublist)]

示例运行

def only_evens(lst):
    return [sublist for sublist in lst if all(item % 2 == 0 for item in sublist)]

print only_evens([[1, 2, 4], [4, 0, 6], [22, 4, 3], [2]])
# [[4, 0, 6], [2]]