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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 real numbers anywhere in a string and sorts them naturally.

Sorting version numbers is just as easy with the versorted function:

>>> from natsort import versorted
>>> a = ['version-1.9', 'version-2.0', 'version-1.11', 'version-1.10']
>>> versorted(a)
['version-1.9', 'version-1.10', 'version-1.11', 'version-2.0']
>>> natsorted(a)  # natsorted tries to sort as signed floats, so it won't work
['version-2.0', 'version-1.9', 'version-1.11', 'version-1.10']

You can also perform locale-aware sorting (or “human sorting”), where the non-numeric characters are ordered based on their meaning, not on their ordinal value; this can be achieved with the humansorted function:

>>> a = ['Apple', 'Banana', 'apple', 'banana']
>>> natsorted(a)
['Apple', 'Banana', 'apple', 'banana']
>>> import locale
>>> locale.setlocale(locale.LC_ALL, 'en_US.UTF-8')
>>> from natsort import humansorted
>>> humansorted(a)
['apple', '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 the following caveat and the “Optional Dependencies” section below before using the humansorted function.

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

The natsort algorithm does other fancy things like

  • recursively descend into lists of lists
  • control the case-sensitivity
  • sort file paths correctly
  • allow custom sorting keys
  • exposes a natsort_key generator to pass to list.sort

Please see the package documentation for more details, including examples and recipes.

Shell script

natsort comes with a shell script called natsort, or can also be called from the command line with python -m natsort. The command line script is only installed onto your PATH if you don’t install via a wheel. There is apparently a known bug with the wheel installation process that will not create entry points.


natsort requires python version 2.6 or greater (this includes python 3.x). To run version 2.6, 3.0, or 3.1 the argparse module is required.

Optional Dependencies


The most efficient sorting can occur if you install the fastnumbers package (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.


On some systems, Python’s locale library can be buggy (I have found this to be the case on Mac OS X), so natsort will use PyICU under the hood if it is installed on your computer; this will give more reliable results. natsort will not require (or check) that PyICU is installed at installation.

Depreciation Notices

  • In natsort version 4.0.0, the number_type, signed, exp, as_path, and py3_safe options will be removed from the (documented) API, in favor of the alg option and ns enum. They will remain as keyword-only arguments after that (for the foreseeable future).
  • In natsort version 4.0.0, the natsort_key function will be removed from the public API. All future development should use natsort_keygen in preparation for this.
  • In natsort version 3.1.0, the shell script changed how it interpreted input; previously, all input was assumed to be a filepath, but as of 3.1.0 input is just treated as a string. For most cases the results are the same.
    • As of natsort version 3.4.0, a --path option has been added to force the shell script to interpret the input as filepaths.


Seth M. Morton


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

09-02-2014 v. 3.5.0

  • Added the ‘alg’ argument to the ‘natsort’ functions. This argument accepts an enum that is used to indicate the options the user wishes to use. The ‘number_type’, ‘signed’, ‘exp’, ‘as_path’, and ‘py3_safe’ options are being depreciated and will become (undocumented) keyword-only options in natsort version 4.0.0.
  • The user can now modify how ‘natsort’ handles the case of non-numeric characters.
  • The user can now instruct ‘natsort’ to use locale-aware sorting, which allows ‘natsort’ to perform true “human sorting”.
    • The humansorted convenience function has been included to make this easier.
  • Updated shell script with locale functionality.

08-12-2014 v. 3.4.1

  • ‘natsort’ will now use the ‘fastnumbers’ module if it is installed. This gives up to an extra 30% boost in speed over the previous performance enhancements.
  • Made documentation point to more ‘natsort’ resources, and also added a new example in the examples section.

07-19-2014 v. 3.4.0

  • Fixed a bug that caused user’s options to the ‘natsort_key’ to not be passed on to recursive calls of ‘natsort_key’.
  • Added a ‘natsort_keygen’ function that will generate a wrapped version of ‘natsort_key’ that is easier to call. ‘natsort_key’ is now set to depreciate at natsort version 4.0.0.
  • Added an ‘as_path’ option to ‘natsorted’ & co. that will try to treat input strings as filepaths. This will help yield correct results for OS-generated inputs like ['/p/q/o.x', '/p/q (1)/o.x', '/p/q (10)/o.x', '/p/q/o (1).x'].
  • Massive performance enhancements for string input (1.8x-2.0x), at the expense of reduction in speed for numeric input (~2.0x).
    • This is a good compromise because the most common input will be strings, not numbers, and sorting numbers still only takes 0.6x the time of sorting strings. If you are sorting only numbers, you would use ‘sorted’ anyway.
  • Added the ‘order_by_index’ function to help in using the output of ‘index_natsorted’ and ‘index_versorted’.
  • Added the ‘reverse’ option to ‘natsorted’ & co. to make it’s API more similar to the builtin ‘sorted’.
  • Added more unit tests.
  • Added auxiliary test code that helps in profiling and stress-testing.
  • Reworked the documentation, moving most of it to PyPI’s hosting platform.
  • Added support for
  • Entire codebase is now PyFlakes and PEP8 compliant.

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