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Sort lists naturally

Project description

Natural sorting for python.

Quick Description

When you try to sort a list of strings that contain numbers, the normal python sort algorithm sorts lexicographically, so you might not get the results that you expect:

>>> a = ['a2', 'a9', 'a1', 'a4', 'a10']
>>> sorted(a)
['a1', 'a10', 'a2', 'a4', 'a9']

Notice that it has the order (‘1’, ‘10’, ‘2’) - this is because the list is being sorted in lexicographical order, which sorts numbers like you would letters (i.e. ‘b’, ‘ba’, ‘c’).

natsort provides a function natsorted that helps sort lists “naturally”, either as real numbers (i.e. signed/unsigned floats or ints), or as versions. Using natsorted is simple:

>>> from natsort import natsorted
>>> a = ['a2', 'a9', 'a1', 'a4', 'a10']
>>> natsorted(a)
['a1', 'a2', 'a4', 'a9', 'a10']

natsorted identifies numbers anywhere in a string and sorts them naturally. Here are some other things you can do with natsort (please see the examples for a quick start guide, or the api for more details).

Sorting Versions

This is handled properly by default (as of natsort version >= 4.0.0):

>>> a = ['version-1.9', 'version-2.0', 'version-1.11', 'version-1.10']
>>> natsorted(a)
['version-1.9', 'version-1.10', 'version-1.11', 'version-2.0']

If you need to sort release candidates, please see for a useful hack.

Sorting by Real Numbers (i.e. Signed Floats)

This is useful in scientific data analysis and was the default behavior of natsorted for natsort version < 4.0.0. Use the realsorted function:

>>> from natsort import realsorted, ns
>>> a = ['num5.10', 'num-3', 'num5.3', 'num2']
>>> natsorted(a)
['num2', 'num5.3', 'num5.10', 'num-3']
>>> natsorted(a, alg=ns.REAL)
['num-3', 'num2', 'num5.10', 'num5.3']
>>> realsorted(a)  # shortcut for natsorted with alg=ns.REAL
['num-3', 'num2', 'num5.10', 'num5.3']

Locale-Aware Sorting (or “Human Sorting”)

This is where the non-numeric characters are ordered based on their meaning, not on their ordinal value, and a locale-dependent thousands separator is accounted for in the number. This can be achieved with the humansorted function:

>>> a = ['Apple', 'apple15', 'Banana', 'apple14,689', 'banana']
>>> natsorted(a)
['Apple', 'Banana', 'apple14,689', 'apple15', 'banana']
>>> import locale
>>> locale.setlocale(locale.LC_ALL, 'en_US.UTF-8')
>>> natsorted(a, alg=ns.LOCALE)
['apple15', 'apple14,689', 'Apple', 'banana', 'Banana']
>>> from natsort import humansorted
>>> humansorted(a)  # shortcut for natsorted with alg=ns.LOCALE
['apple15', 'apple14,689', 'Apple', 'banana', 'Banana']

You may find you need to explicitly set the locale to get this to work (as shown in the example). Please see and the Installation section below before using the humansorted function.

Sorting Mixed Types

You can mix and match int, float, and str (or unicode) types when you sort:

>>> a = ['4.5', 6, 2.0, '5', 'a']
>>> natsorted(a)
[2.0, '4.5', '5', 6, 'a']
>>> # On Python 2, sorted(a) would return [2.0, 6, '4.5', '5', 'a']
>>> # On Python 3, sorted(a) would raise an "unorderable types" TypeError

Handling Bytes on Python 3

natsort does not officially support the bytes type on Python 3, but convenience functions are provided that help you decode to str first:

>>> from natsort import as_utf8
>>> a = [b'a', 14.0, 'b']
>>> # On Python 2, natsorted(a) would would work as expected.
>>> # On Python 3, natsorted(a) would raise a TypeError (bytes() < str())
>>> natsorted(a, key=as_utf8) == [14.0, b'a', 'b']
>>> a = [b'a56', b'a5', b'a6', b'a40']
>>> # On Python 2, natsorted(a) would would work as expected.
>>> # On Python 3, natsorted(a) would return the same results as sorted(a)
>>> natsorted(a, key=as_utf8) == [b'a5', b'a6', b'a40', b'a56']

Generating a Reusable Sorting Key

All of the *sorted functions from the natsort are convenience functions around the something similar to the following:

>>> from natsort import natsort_keygen
>>> natsort_key = natsort_keygen()
>>> a = ['a2', 'a9', 'a1', 'a4', 'a10']
>>> natsorted(a) == sorted(a, key=natsort_key)

You can use this key for your own use (such as passing to list.sort). You can also customize the key with the ns enum (see the ns enum).

Other Useful Things

Shell script

natsort comes with a shell script called natsort, or can also be called from the command line with python -m natsort.


natsort requires Python version 2.6 or greater or Python 3.3 or greater. It may run on (but is not tested against) Python 3.2.

Optional Dependencies


The most efficient sorting can occur if you install the fastnumbers package (version >=0.7.1); it helps with the string to number conversions. natsort will still run (efficiently) without the package, but if you need to squeeze out that extra juice it is recommended you include this as a dependency. natsort will not require (or check) that fastnumbers is installed at installation.


It is recommended that you install PyICU if you wish to sort in a locale-dependent manner, see for an explanation why.


Seth M. Morton


These are the last three entries of the changelog. See the package documentation for the complete changelog.

06-04-2016 v. 5.0.1

  • The ns enum attributes can now be imported from the top-level namespace.
  • Fixed a bug with the from natsort import * mechanism.
  • Fixed bug with using natsort with python -OO.

05-08-2016 v. 5.0.0

  • ns.LOCALE/humansorted now accounts for thousands separators.
  • Refactored entire codebase to be more functional (as in use functions as units). Previously, the code was rather monolithic and difficult to follow. The goal is that with the code existing in smaller units, contributing will be easier.
  • Deprecated ns.TYPESAFE option as it is now always on (due to a new iterator-based algorithm, the typesafe function is now cheap).
  • Increased speed of execution (came for free with the new functional approach because the new factory function paradigm eliminates most if branches during execution).
    • For the most cases, the code is 30-40% faster than version 4.0.4.
    • If using ns.LOCALE or humansorted, the code is 1100% faster than version 4.0.4.
  • Improved clarity of documentaion with regards to locale-aware sorting.
  • Added a new chain_functions function for convenience in creating a complex user-given key from several existing functions.

11-01-2015 v. 4.0.4

  • Improved coverage of unit tests.
  • Unit tests use new and improved hypothesis library.
  • Fixed compatibility issues with Python 3.5

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