Sorting elements in order while leaving some intact

Question

Hi everyone I'm quite new to Python and I'm trying to figure out how to sort the elements in this list in a particular order

What I have:

lst = ['1. [a] -1  +2, 2. [b] ++3  +4  -1, 3. [c] +5  -E1, 4. -E1', '1. [d] +6  -7  ++8, 2. [e] -E1  +9']

I want to keep the order of the first elements and only change around the order of the numbers, in the order of

(++ + - -- )

So what I'm after is:

lst = ['1. [a] +2  -1, 2. [b] ++3  +4  -1, 3. [c] +5  -E1, 4. -E1', '1. [d] ++8 +6  -7, 2. [e] +9 -E1']

I've tried using the sort and sorted method, to no avail

I have no idea how to go about this, some help would be appreciated

Please go easy on me this is my first post and I'm new to Python

Answer 1

The sort and sorted functions take an additional parameter called key that lets you specify the sort order.

For your example - the sorting order is defined by a precedence list that you have defined - i.e.

precedence = ['++', '+', '-', '--']

You need to define a function that uses this precedence list to return a "sort rank" - for example

def sorting_func(s):
    if s[:2] in precedence:
        return precedence.index(s[:2]) + 1
    elif s[:1] in precedence:
        return precedence.index(s[:1]) + 1
    return 0

And apply this function to your elements -

sorted_lst = []
for element in lst:
    first_split = element.split(',')
    second_split = [_.split() for _ in first_split]
    sorted_second_split = [sorted(_, key = sorting_func) for _ in second_split]
    first_rejoin = [' '.join(_) for _ in sorted_second_split]
    second_rejoin = ', '.join(first_rejoin)
    sorted_lst.append(second_rejoin)

Output

['1. [a] +2 -1, 2. [b] ++3 +4 -1, 3. [c] +5 -E1, 4. -E1',
 '1. [d] ++8 +6 -7, 2. [e] +9 -E1']