Simple yet flexible natural sorting in Python.
natsort is a general utility for sorting lists naturally; the definition
of “naturally” is not well-defined, but the most common definition is that numbers
contained within the string should be sorted as numbers and not as you would
other characters. If you need to present sorted output to a user, you probably
want to sort it naturally.
natsort was initially created for sorting scientific output filenames that
contained signed floating point numbers in the names. There was a 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, but has since expanded to handle
just about any definition of a number, as well as other sorting customizations.
1.1. 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 = ['2 ft 7 in', '1 ft 5 in', '10 ft 2 in', '2 ft 11 in', '7 ft 6 in'] >>> sorted(a) ['1 ft 5 in', '10 ft 2 in', '2 ft 11 in', '2 ft 7 in', '7 ft 6 in']
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” (“naturally” is rather ill-defined, but in general it means
sorting based on meaning and not computer code point)..
natsorted() is simple:
>>> from natsort import natsorted >>> a = ['2 ft 7 in', '1 ft 5 in', '10 ft 2 in', '2 ft 11 in', '7 ft 6 in'] >>> natsorted(a) ['1 ft 5 in', '2 ft 7 in', '2 ft 11 in', '7 ft 6 in', '10 ft 2 in']
natsorted() identifies numbers anywhere in a string and sorts them
naturally. Below are some other things you can do with
(please see the Examples and Recipes for a quick start guide, or the natsort API
for more details).
natsorted() is designed to be a drop-in replacement for the built-in
sorted() function. Like
does not sort in-place. To sort a list and assign the output to the
same variable, you must explicitly assign the output to a variable:
>>> a = ['2 ft 7 in', '1 ft 5 in', '10 ft 2 in', '2 ft 11 in', '7 ft 6 in'] >>> natsorted(a) ['1 ft 5 in', '2 ft 7 in', '2 ft 11 in', '7 ft 6 in', '10 ft 2 in'] >>> print(a) # 'a' was not sorted; "natsorted" simply returned a sorted list ['2 ft 7 in', '1 ft 5 in', '10 ft 2 in', '2 ft 11 in', '7 ft 6 in'] >>> a = natsorted(a) # Now 'a' will be sorted because the sorted list was assigned to 'a' >>> print(a) ['1 ft 5 in', '2 ft 7 in', '2 ft 11 in', '7 ft 6 in', '10 ft 2 in']
Please see Generating a Reusable Sorting Key and Sorting In-Place for an alternate way to sort in-place naturally.
1.2.1. 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 Sorting with Alpha, Beta, and Release Candidates for a useful hack.
1.2.2. Sorting by Real Numbers (i.e. Signed Floats)¶
>>> from natsort import realsorted, ns >>> # Note that when interpreting as signed floats, the below numbers are >>> # +5.10, -3.00, +5.30, +2.00 >>> a = ['position5.10.data', 'position-3.data', 'position5.3.data', 'position2.data'] >>> natsorted(a) ['position2.data', 'position5.3.data', 'position5.10.data', 'position-3.data'] >>> natsorted(a, alg=ns.REAL) ['position-3.data', 'position2.data', 'position5.10.data', 'position5.3.data'] >>> realsorted(a) # shortcut for natsorted with alg=ns.REAL ['position-3.data', 'position2.data', 'position5.10.data', 'position5.3.data']
1.2.3. 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 and decimal
separator is accounted for in the number.
This can be achieved with the
>>> 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') 'en_US.UTF-8' >>> natsorted(a, alg=ns.LOCALE) ['apple15', 'apple14,689', 'Apple', 'banana', 'Banana'] >>> from natsort import humansorted >>> humansorted(a) ['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 Possible Issues with humansorted() or ns.LOCALE and the Installation section
below before using the
1.2.4. Further Customizing Natsort¶
If you need to combine multiple algorithm modifiers (such as
ns.IGNORECASE), you can combine the options using the
bitwise OR operator (
|). For example,
>>> a = ['Apple', 'apple15', 'Banana', 'apple14,689', 'banana'] >>> natsorted(a, alg=ns.REAL | ns.LOCALE | ns.IGNORECASE) ['Apple', 'apple15', 'apple14,689', 'Banana', 'banana'] >>> # The ns enum provides long and short forms for each option. >>> ns.LOCALE == ns.L True >>> # You can also customize the convenience functions, too. >>> natsorted(a, alg=ns.REAL | ns.LOCALE | ns.IGNORECASE) == realsorted(a, alg=ns.L | ns.IC) True >>> natsorted(a, alg=ns.REAL | ns.LOCALE | ns.IGNORECASE) == humansorted(a, alg=ns.R | ns.IC) True
All of the available customizations can be found in the documentation for
You can also add your own custom transformation functions with the
These can be used with
alg if you wish:
>>> a = ['apple2.50', '2.3apple'] >>> natsorted(a, key=lambda x: x.replace('apple', ''), alg=ns.REAL) ['2.3apple', 'apple2.50']
1.2.5. Sorting Mixed Types¶
You can mix and match
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
1.2.6. 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'] True >>> 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'] True
1.2.7. Generating a Reusable Sorting Key and Sorting In-Place¶
Under the hood,
natsorted() works by generating a custom sorting
natsort_keygen() and then passes that to the built-in
sorted(). You can use the
natsort_keygen() function yourself to
generate a custom sorting key to sort in-place using the
>>> from natsort import natsort_keygen >>> natsort_key = natsort_keygen() >>> a = ['2 ft 7 in', '1 ft 5 in', '10 ft 2 in', '2 ft 11 in', '7 ft 6 in'] >>> natsorted(a) == sorted(a, key=natsort_key) True >>> a.sort(key=natsort_key) >>> a ['1 ft 5 in', '2 ft 7 in', '2 ft 11 in', '7 ft 6 in', '10 ft 2 in']
1.2.8. Other Useful Things¶
- How do I debug
The best way to debug
natsorted()is to generate a key using
natsort_keygen()with the same options being passed to
natsorted(). One can take a look at exactly what is being done with their input using this key - it is highly recommended to look at this issue describing how to debug for how to debug, and also to review the How Does Natsort Work? page for why
natsortis doing that to your data.
If you are trying to sort custom classes and running into trouble, please take a look at https://github.com/SethMMorton/natsort/issues/60. In short, custom classes are not likely to be sorted correctly if one relies on the behavior of
__lt__and the other rich comparison operators in their custom class - it is better to use a
natsort, or use the
natsortkey as part of your rich comparison operator definition.
- How does
If you don’t want to read How Does Natsort Work?, here is a quick primer.
natsortprovides a key function that can be passed to list.sort() or sorted() in order to modify the default sorting behavior. This key is generated on-demand with the key generator
natsort.natsorted()is essentially a wrapper for the following code:
>>> from natsort import natsort_keygen >>> natsort_key = natsort_keygen() >>> sorted(['1', '10', '2'], key=natsort_key) ['1', '2', '10']
- Assume the input is a string, and attempt to split it into numbers and
non-numbers using regular expressions. Numbers are then converted into
- If the above fails because the input is not a string, assume the input
is some other sequence (e.g.
tuple), and recursively apply the key to each element of the sequence.
- If the above fails because the input is not iterable, assume the input
float, and just return the input in a
- Assume the input is a string, and attempt to split it into numbers and non-numbers using regular expressions. Numbers are then converted into either
natsortgave me results I didn’t expect, and it’s a terrible library!
- Did you try to debug using the above advice? If so, and you still cannot figure out the error, then please file an issue.
1.4. Shell script¶
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.
1.6. Optional Dependencies¶
The most efficient sorting can occur if you install the
(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
It is recommended that you install PyICU if you wish to sort in a locale-dependent manner, see http://natsort.readthedocs.io/en/master/locale_issues.html for an explanation why.
$ pip install natsort
# Install both optional dependencies. $ pip install natsort[fast,icu] # Install just fastnumbers $ pip install natsort[fast]
1.8. How to Run Tests¶
Please note that
natsort is NOT set-up to support
python setup.py test.
The recommended way to run tests is with tox.
tox, running tests is as simple as executing the following in the
tox will create virtual a virtual environment for your tests and install all the
needed testing requirements for you. You can specify a particular python version
-e flag, e.g.
tox -e py36.
If you do not wish to use
tox, you can install the testing dependencies and run the
tests manually using pytest -
Pipfile for use with pipenv that
makes it easy for you to install the testing dependencies:
$ pipenv install --skip-lock --dev $ pipenv run python -m pytest
Note that above I invoked
python -m pytest instead of just
pytest - this is because
the former puts the CWD on sys.path.