在 Python 中为 list[:] = [...] 赋值的冒号有什么作用
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What does colon at assignment for list[:] = [...] do in Python
提问by sinθ
I came accross the following code:
我遇到了以下代码：
# O(n) space
def rotate(self, nums, k):
deque = collections.deque(nums)
k %= len(nums)
for _ in xrange(k):
deque.appendleft(deque.pop())
nums[:] = list(deque) # <- Code in question
What does nums[:] =do that nums =does not? For that matter, what does nums[:]do that numsdoes not?
什么nums[:] =不做nums =？就此而言，什么nums[:]不做nums呢？
采纳答案by Ryan Haining
This syntax is a slice assignment. A slice of [:]means the entire list. The difference between nums[:] =and nums =is that the latter doesn't replace elements in the original list. This is observable when there are two references to the list
此语法是切片分配。切片[:]意味着整个列表。nums[:] =和之间的区别在于nums =后者不会替换原始列表中的元素。当有两个对列表的引用时，这是可观察到的
>>> original = [1, 2, 3]
>>> other = original
>>> original[:] = [0, 0] # changes the contents of the list that both
# original and other refer to
>>> other # see below, now you can see the change through other
[0, 0]
To see the difference just remove the [:]from the assignment above.
要查看差异，只需[:]从上面的分配中删除。
>>> original = [1, 2, 3]
>>> other = original
>>> original = [0, 0] # original now refers to a different list than other
>>> other # other remains the same
[1, 2, 3]
To take the title of your question literally, if listis a variable name and not the builtin, it will replace the length of the sequence with an ellipsis
从字面上看问题的标题，如果list是变量名而不是内置变量，它将用省略号替换序列的长度
>>> list = [1,2,3,4]
>>> list[:] = [...]
>>> list
[Ellipsis]
回答by sinθ
nums = foorebinds the name numsto refer to the same object that foorefers to.
nums = foo重新绑定名称nums以引用所引用的同一对象foo。
nums[:] = fooinvokes slice assignment on the object that numsrefers to, thus making the contents of the original object a copy of the contents of foo.
nums[:] = foo对引用的对象调用切片赋值nums，从而使原始对象的内容成为 的内容的副本foo。
Try this:
尝试这个：
>>> a = [1,2]
>>> b = [3,4,5]
>>> c = a
>>> c = b
>>> print(a)
[1, 2]
>>> c = a
>>> c[:] = b
>>> print(a)
[3, 4, 5]
