How to sort an alpha numeric list in python?

Question

How to sort list by alphanumeric values

a = ['v1_0005.jpg', 'v1_00015.jpg', 'v2_0007.jpg', 'v2_0002.jpg']

while sorting list using a.sort() I get

['v1_0005.jpg', 'v1_00015.jpg', 'v2_0007.jpg', 'v2_0002.jpg']

which means sorting order is wrong, I expect list is sort by alphanumeric value in python

Expected list to be :

['v2_0002.jpg','v1_0005.jpg','v2_0007.jpg', 'v1_00015.jpg']

Answer 1

You have a problem: your list format is quite wrong - instead of v1_00015.jpg it should be v1_0015.jpg (no extra 0 ). Without that zero Python built-ins .sort() and sorted() work correctly. To transform the list, you can use this:

for i,e in enumerate(a):
    while len(a[i]) > 11:
        a[i] = e.replace('v1_0', 'v1_')

It replaces v1_0 to v1_ in every a item till item length == 11 . Then just use a.sort() :

a.sort()
print(a)

And it prints:

['v1_0001.jpg', 'v1_0002.jpg', 'v1_0003.jpg', 'v1_0015.jpg', 'v1_0017.jpg']

Answer 2

You have to some techniques inorder to get the result that you want, ie you have to write a sorting function with the conditions.

So here you can make use of regex and lambda in python inorder to sort as you wish

import re
def sorted_nicely( l ):
    """ 
    Sorts the given iterable in the way that is expected.
 
    Required arguments:
    l -- The iterable to be sorted.
 
    """
    convert = lambda text: int(text) if text.isdigit() else text
    alphanum_key = lambda key: [convert(c) for c in re.split('([0-9]+)', key)]
    return sorted(l, key = alphanum_key)
              
# Driver code
a = ["v1_0001.jpg","v1_0002.jpg","v1_0003.jpg","v1_00017.jpg","v1_00015.jpg"]
print(sorted_nicely(a))

Output

['v1_0001.jpg', 'v1_0002.jpg', 'v1_0003.jpg', 'v1_00015.jpg', 'v1_00017.jpg']

Answer 3

I'm assuming you want to use the numeric values. So you need to parse them out...
This can be done with a one-liner as so -

a = ['v1_0001.jpg', 'v1_00015.jpg', 'v1_00017.jpg', 'v1_0002.jpg', 'v1_0003.jpg']
a.sort(key=lambda x: int(x.split('v1_')[1].split('.jpg')[0]))
print(a)
# ['v1_0001.jpg', 'v1_0002.jpg', 'v1_0003.jpg', 'v1_00015.jpg', 'v1_00017.jpg']