列出“怪癖”在Python中

Question

我在交互式解释器中尝试使用列表进行一些操作，我注意到这一点：

>>> list = range(1, 11)
>>> for i in list:
...     list.remove(i)
...
>>> list
[2, 4, 6, 8, 10]

任何人都可以解释为什么它留下偶数吗？现在这让我很困惑......非常感谢。

Answer 1

修改正在迭代的列表是不安全的。

Answer 2

我的猜测是 for 循环的实现如下：

list = range(1, 11)

i = 0
while i < len(list):
    list.remove(list[i])
    i += 1

print(list)

每次删除一个元素时，“下一个”元素就会滑入其位置，但 i 无论如何都会递增，跳过 2 个元素。

但是，ObscureRobot 是对的，这样做并不安全（这可能是未定义的行为）。

Answer 3

如果要在迭代列表时修改列表，请从后到前进行操作：

lst = range(1, 11)
for i in reversed(lst):
    lst.remove(i)

Answer 4

我发现使用 Python 解释这一点最简单：

>>> for iteration, i in enumerate(lst):
...     print 'Begin iteration', iteration, 'where lst =', str(lst), 'and the value at index', iteration, 'is', lst[iteration]
...     lst.remove(i)
...     print 'End iteration', iteration, 'where lst =', str(lst), 'with', i, 'removed\n'
... 
Begin iteration 0 where lst = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] and the value at index 0 is 1
End iteration 0 where lst = [2, 3, 4, 5, 6, 7, 8, 9, 10] with 1 removed

Begin iteration 1 where lst = [2, 3, 4, 5, 6, 7, 8, 9, 10] and the value at index 1 is 3
End iteration 1 where lst = [2, 4, 5, 6, 7, 8, 9, 10] with 3 removed

Begin iteration 2 where lst = [2, 4, 5, 6, 7, 8, 9, 10] and the value at index 2 is 5
End iteration 2 where lst = [2, 4, 6, 7, 8, 9, 10] with 5 removed

Begin iteration 3 where lst = [2, 4, 6, 7, 8, 9, 10] and the value at index 3 is 7
End iteration 3 where lst = [2, 4, 6, 8, 9, 10] with 7 removed

Begin iteration 4 where lst = [2, 4, 6, 8, 9, 10] and the value at index 4 is 9
End iteration 4 where lst = [2, 4, 6, 8, 10] with 9 removed

请注意，(a) 在迭代列表时修改列表并 (b) 调用列表“列表”是一个坏主意。