Swapping elements in lists in python

问题:

I have a list and I need to swap the 1st element in the list with the maximum element in the list.

But why does code 1 work while code 2 doesn't:

code 1:

a = list.index(max(list))
list[0], list[a] = list[a], list[0]

code 2:

list[0], list[list.index(max(list))] = list[list.index(max(list))], list[0]

I thought Python would first evaluate the right-hand side before assigning it to the names on the left?

回答1:

Python stores results in multiple targets from left to right, executing the assignment target expression in that order.

So your second version essentially comes down to this:

temp = list[list.index(max(list))],list[0]
list[0] = temp[0]
list[list.index(max(list))] = temp[1]

Note that the list.index(max(list)) is executed after list[0] was altered, and that's where you just stored the maximum value.

This is documented in the Assignment statements documenation:

• If the target list is a comma-separated list of targets: The object must be an iterable with the same number of items as there are targets in the target list, and the items are assigned, from left to right, to the corresponding targets.

From there on out it is as if each target is a single target, so the documentation that follows applies from left to right to each target:

Assignment of an object to a single target is recursively defined as follows.

[...]

• If the target is a subscription: The primary expression in the reference is evaluated. It should yield either a mutable sequence object (such as a list) or a mapping object (such as a dictionary). Next, the subscript expression is evaluated.

If you changed the order of assignments your code would work:

list[list.index(max(list))], list[0] = list[0], list[list.index(max(list))]

because now list[list.index(max(list))] is assigned to first.