I’ve used comparator functions while sorting in python quite often. I had a complex object today that i had to sort on using non-standard comparator functions.
My object was a dictionary that contained a list of 2 dictionaries that in turn containd lists of objects. I wanted to sort the first dictinary based on the len() of the objects of the lists at the other end of the hierarchy.
using a cmp function i had to do something like:
def comparator(x,y):
#4 lines do stuff to get the len() of data inside x
#4 lines do stuff to get the len() of data inside y
if len_x > len_y:
return 1
elif len_x == len_y:
return 0
else:
return -1
However i came upon the sorting howto on wiki.python.org and a way to do sorting that i did not know. Using the key funciton.
Instead of sorted(iterable,cmp=my_cmp_func) you can do sorted(iterable,key=my_key_func).
my_key_func is called with each element of the iterable as argument. It must returns a value that is used in sorting (I assume using standard cmp).
This meant that instead of my cmp function i could do only:
def my_key_func(x):
#4 lines do stuff to get the len() of data inside x
return len_x
It makes more sense because basically we are not sorting x and y based on their own value, but we are sorting x and y based on some remote data connected to x and y.
Certainly nicer.