# Natural string sorting

Connelly Barnes connellybarnes at yahoo.com
Fri Jun 18 07:33:07 CEST 2004

```Summary:

Sorts strings in a way that seems natural to humans.
If the strings contain integers, then the integers are ordered
numerically.  For example, sorts ['Team 11', 'Team 3', 'Team 1']
into the order ['Team 1', 'Team 3', 'Team 11'].

Code:

#---------------------------------------------------------
# natsort.py: Natural string sorting.
#---------------------------------------------------------

# By Seo Sanghyeon.  Some changes by Connelly Barnes.

def try_int(s):
"Convert to integer if possible."
try: return int(s)
except: return s

def natsort_key(s):
"Used internally to get a tuple by which s is sorted."
import re
return map(try_int, re.findall(r'(\d+|\D+)', s))

def natcmp(a, b):
"Natural string comparison, case sensitive."
return cmp(natsort_key(a), natsort_key(b))

def natcasecmp(a, b):
"Natural string comparison, ignores case."
return natcmp(a.lower(), b.lower())

def natsort(seq, cmp=natcmp):
"In-place natural string sort."
seq.sort(cmp)

def natsorted(seq, cmp=natcmp):
"Returns a copy of seq, sorted by natural string sort."
import copy
temp = copy.copy(seq)
natsort(temp, cmp)
return temp

Examples:

You can use this code to sort tarball filenames:

>>> natsorted(['ver-1.3.12', 'ver-1.3.3', 'ver-1.2.5', 'ver-1.2.15',
'ver-1.2.3', 'ver-1.2.1'])
['ver-1.2.1', 'ver-1.2.3', 'ver-1.2.5', 'ver-1.2.15', 'ver-1.3.3',
'ver-1.3.12']

Chemical elements:

>>> natsorted(['C1H2', 'C1H4', 'C2H2', 'C2H6', 'C2N', 'C3H6'])
['C1H2', 'C1H4', 'C2H2', 'C2H6', 'C2N', 'C3H6']

Teams:

>>> natsorted(['Team 101', 'Team 58', 'Team 30', 'Team 1'])
['Team 1', 'Team 30', 'Team 58', 'Team 101']

Pass natcasecmp as a second argument for case-insensitive sorting:

>>> natsorted(['a5', 'A7', 'a15', 'a9', 'A8'], natcasecmp)
['a5', 'A7', 'A8', 'a9', 'a15']

Enjoy!

```