对此 lambda function 的一些解释

Question

Trying to get my head around how the expression below is evaluated, looking at the returned list:

sorted([1, 2, 3, 4, 5, 6, 7, 8, 9], key=lambda x: abs(2-x))

which gives:

[2, 1, 3, 4, 5, 6, 7, 8, 9]

I'm not sure what value x in abs(2-x) takes. Is it the list elements' position [0,1,2,..,8] or the actual elements' values [1,2,3,.,9]

Can someone explain?

Answer 1

It's the values that are passed to the lambda and it returns the new 'keys' to be used for sorting.

In this case the lambda will return [1, 0, 1, 2, 3, 4, 5, 6, 7] so the value 2 with the key 0 will move to the front, followed by 1 and 3, which both have index 1, and then the rest of the values.

Answer 2

x is the actual element.

when writing:

key=lambda x: abs(2-x)

its exactly such as:

def sort_key(x):
  return abs(2 - x)

but only the body of the function is passed to key when writing key=lambda x: abs(2-x) .

list:

[1, 2, 3, 4, 5, 6, 7, 8, 9]

its sorted positions are influenced by the result of key when calling that lambda .

the actual sort:

1 -> abs(2 - 1) = (1) [position]
2 -> abs(2 - 2) = (0) [position]
3 -> abs(2 - 3) = (1) [position]
4 -> abs(2 - 4) = (2) [position]
5 -> abs(2 - 5) = (3) [position]
6 -> abs(2 - 6) = (4) [position]
7 -> abs(2 - 7) = (5) [position]
8 -> abs(2 - 8) = (6) [position]
9 -> abs(2 - 9) = (7) [position]

explanation:

• element 1 is giving 1 so its positioned on index 1
• element 2 is giving 0 so its positioned on index 0
• element 3 is giving 1 so its positioned on index 1 , but 1 is already occupied, so its positioned next to element on pos 1
• element 4 is giving 2 so its positioned on index 2

....

and so on.

Answer 3

First: Python's built-in sorted function has the signature:

sorted(iterable, *, key=None, reverse=False)

meaning if a value is passed to the key parameter, sorted will sort the iterable according to the value of key .

Second: in this case key takes a function (lambda) and evaluates it. The result of the evaluation is as in @alexzander's answer [1,0,1,2,3..7] . sorted use this as the key to sort the given list in that order (ascending, assuming reverse=False which is the default).