# Sorting by item_in_another_list

Cameron Walsh cameron.walsh at gmail.com
Tue Oct 24 06:12:16 CEST 2006

```Hi,

I have two lists, A and B, such that B is a subset of A.

I wish to sort A such that the elements in B are at the beginning of A,
and keep the existing order otherwise, i.e. stable sort.  The order of
elements in B will always be correct.

for example:

A = [0,1,2,3,4,5,6,7,8,9,10]
B = [2,3,7,8]

desired_result = [2,3,7,8,0,1,4,5,6,9,10]

At the moment I have defined a comparator function:

def sort_by_in_list(x,y):
ret = 0
if x in B:
ret -= 1
if y in B:
ret += 1
return ret

and am using:

A.sort(sort_by_in_list)

which does produce the desired results.

I do now have a few questions:

1.)  Is this the most efficient method for up to around 500 elements?
If not, what would be better?
2.)  This current version does not allow me to choose a different list
for B.  Is there a bind_third function of some description that I could
use to define a new function with 3 parameters, feed it the third (the
list to sort by), and have the A.sort(sort_by_in_list) provide the other
2 variables?

Regards to all,

Cameron.

```