Natural sorting for python.
- Source Code: https://github.com/SethMMorton/natsort
- Downloads: https://pypi.python.org/pypi/natsort
- Documentation: http://pythonhosted.org//natsort/
natsort was initially created for sorting scientific output filenames that contained floating point numbers in the names. There was a serious lack of algorithms out there that could perform a natural sort on floats but plenty for ints; check out this StackOverflow question and its answers and links therein, this ActiveState forum, and of course this great article on natural sorting from CodingHorror.com for examples of what I mean. natsort was created to fill in this gap. It has since grown and can now sort version numbers (which seems to be the most common use case based on user feedback) as well as some other nice features.
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 versorted():
>>> 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')
'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 A Note For Bugs With Locale-Aware Sorting and the Installation 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 Examples and Recipes for a quick start guide, or the natsort API for more details.
Installation of natsort is ultra-easy. Simply execute from the command line:
easy_install natsort
or, if you have pip (preferred over easy_install):
pip install natsort
Both of the above commands will download the source for you.
You can also download the source from http://pypi.python.org/pypi/natsort, or browse the git repository at https://github.com/SethMMorton/natsort.
If you choose to install from source, you can unzip the source archive and enter the directory, and type:
python setup.py install
If you wish to run the unit tests, enter:
python setup.py test
If you want to build this documentation, enter:
python setup.py build_sphinx
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.
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.
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.
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.