Python analog of PHP's natsort function (sort a list using a "natural order" algorithm)

From my answer to Natural Sorting algorithm:

import re
def natural_key(string_):
    """See https://blog.codinghorror.com/sorting-for-humans-natural-sort-order/"""
    return [int(s) if s.isdigit() else s for s in re.split(r'(\d+)', string_)]

Example:

>>> L = ['image1.jpg', 'image15.jpg', 'image12.jpg', 'image3.jpg']
>>> sorted(L)
['image1.jpg', 'image12.jpg', 'image15.jpg', 'image3.jpg']
>>> sorted(L, key=natural_key)
['image1.jpg', 'image3.jpg', 'image12.jpg', 'image15.jpg']

To support Unicode strings, .isdecimal() should be used instead of .isdigit(). See example in @phihag's comment. Related: How to reveal Unicodes numeric value property.

.isdigit() may also fail (return value that is not accepted by int()) for a bytestring on Python 2 in some locales e.g., '\xb2' ('²') in cp1252 locale on Windows.

You can check out the third-party natsort library on PyPI:

>>> import natsort
>>> l = ['image1.jpg', 'image15.jpg', 'image12.jpg', 'image3.jpg']
>>> natsort.natsorted(l)
['image1.jpg', 'image3.jpg', 'image12.jpg', 'image15.jpg']

Full disclosure, I am the author.

This function can be used as the key= argument for sorted in Python 2.x and 3.x:

def sortkey_natural(s):
    return tuple(int(part) if re.match(r'[0-9]+$', part) else part
                for part in re.split(r'([0-9]+)', s))