Sort list by function with multiple arguments

Question

I would like to sort a list using a function that returns a float value. If the function had a single argument, I would simply use

sorted(mylist, key=myfunction)

and be on my merry way. However, this doesn't work for functions with multiple arguments. How can this be done?

This is a small part of a chess engine. The 'Bestmove' function takes in the board position (a list), the depth (an integer), and alpha/beta values and returns a list with two entries: the board evaluation (a float), and the suggested move (a list).

In an effort to optimize the alpha/beta pruning process, I would like to change the order in which moves are evaluated (strong moves evaluated first leads to greater efficiency). To achieve this, I would like to sort the list of moves by the first value in the list returned by the 'Bestmove' function.

What do you anticipate the multiple arguments passed to this function actually being in this case? — miradulo, Apr 20 '18 at 23:42
According to https://docs.python.org/3/howto/sorting.html#operator-module-functions if you use the itemgetter and attrgetter functions you can accomplish sorts of lists of tuples. What is your list composed of and what does your function sort on? — Kulix, Apr 20 '18 at 23:48

jpp · Answer 1 · 2018-04-21T00:11:29.257

There are several ways you can do this. Assume we have a list lst.

Set default values

This works only if you can set in advance the default values for your unused parameters.

def func(x, y=1, z=2):
    return x + y + z

res = sorted(lst, key=func)

Use partial functions

You can use functools.partial to create a function with certain parameters fixed.

from functools import partial

def func(x, y, z):
    return x + y + z

res = sorted(lst, key=partial(func, y=1, z=2))

Pass through lambda

If your additional parameters are dependent on the list item itself, you can pass through an anonymous lambda and include your logic.

def func(x, y, z):
    return x + y + z

res = sorted(lst, key=lambda x: func(x, x+1, x+2))

Sort list by function with multiple arguments

1 Answers1