Simple yet flexible natural sorting in Python.
- Source Code: https://github.com/SethMMorton/natsort
- Downloads: https://pypi.org/project/natsort/
- Documentation: https://natsort.readthedocs.io/
- Quick Description
- Quick Examples
- Optional Dependencies
- How to Run Tests
- How to Build Documentation
- Deprecation Schedule
NOTE: Please see the Deprecation Schedule section for changes in
natsort version 7.0.0.
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
(also see the examples
for a quick start guide, or the
api for complete details).
natsorted is designed to be a drop-in replacement for the
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.
- Sorting Versions
- Sorting by Real Numbers (i.e. Signed Floats)
- Locale-Aware Sorting (or "Human Sorting")
- Further Customizing Natsort
- Sorting Mixed Types
- Handling Bytes on Python 3
- Generating a Reusable Sorting Key and Sorting In-Place
- Other Useful Things
natsort does not actually comprehend version numbers.
It just so happens that the most common versioning schemes are designed to
work with standard natural sorting techniques; these schemes include
YEAR.MONTH.DAY. If your data
conforms to a scheme like this, then it will work out-of-the-box with
natsorted (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 versions that use a more complicated scheme, please see these examples.
Sorting by Real Numbers (i.e. Signed Floats)
This is useful in scientific data analysis (and was
the default behavior of
version < 4.0.0). Use the
>>> 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']
Locale-Aware Sorting (or "Human Sorting")
This is where the non-numeric characters are also 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) # 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 locale issues and the
Optional Dependencies section below before using the
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 the ns enum.
You can also add your own custom transformation functions with the
argument. 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']
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
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
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']
All of the algorithm customizations mentioned in the
Further Customizing Natsort section can also be applied to
natsort_keygen through the alg keyword option.
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.
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.
- 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']
Users can further customize
natsortsorting behavior with the
algoptions (see details in the Further Customizing Natsort section).
The key generated by
natsort_keygenalways returns a
tuple. It does so in the following way (some details omitted for clarity):
- 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
tupleis always returned, a
TypeErrorshould not be common unless one tries to do something odd like sort an
- 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
natsort comes with a shell script called
natsort, or can also be called
from the command line with
python -m natsort.
natsort requires Python 3.5 or greater. Python 3.4 is unofficially supported,
meaning that support has not been removed, but it is no longer tested.
The most efficient sorting can occur if you install the
(version >=2.0.0); 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
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 https://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]
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
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 with the
-e flag, e.g.
tox -e py36. Static analysis
is done with
tox -e flake8. You can see all available testing environments
If you do not wish to use
tox, you can install the testing dependencies with the
dev/requirements.txt file and then run the tests manually using
$ pip install -r dev/requirements.txt $ 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.
How to Build Documentation
If you want to build the documentation for
natsort, it is recommended to
$ tox -e docs
This will place the documentation in
build/sphinx/html. If you do not
which to use
tox, you can do the following:
$ pip install sphinx sphinx_rtd_theme $ python setup.py build_sphinx
Dropped Python 2.7 Support
natsort version 7.0.0 dropped support for Python 2.7.
The version 6.X branch will remain as a "long term support" branch where bug
fixes are applied so that users who cannot update from Python 2.7 will not be
forced to use a buggy
natsort version (bug fixes will need to be requested;
by default only the 7.X branch will be updated).
New features would not be added to version 6.X, only bug fixes.
Dropped Deprecated APIs
natsort version 6.0.0, the following APIs and functions were removed
number_typekeyword argument (deprecated since 3.4.0)
signedkeyword argument (deprecated since 3.4.0)
expkeyword argument (deprecated since 3.4.0)
as_pathkeyword argument (deprecated since 3.4.0)
py3_safekeyword argument (deprecated since 3.4.0)
ns.TYPESAFE(deprecated since version 5.0.0)
ns.DIGIT(deprecated since version 5.0.0)
ns.VERSION(deprecated since version 5.0.0)
versorted()(discouraged since version 4.0.0, officially deprecated since version 5.5.0)
index_versorted()(discouraged since version 4.0.0, officially deprecated since version 5.5.0)
In general, if you want to determine if you are using deprecated APIs you can run your code with the following flag
$ python -Wdefault::DeprecationWarning my-code.py
DeprecationWarnings are not shown, but this will cause them
to be shown. Alternatively, you can just set the environment variable
PYTHONWARNINGS to "default::DeprecationWarning" and then run your code.
Dropped Pipenv for Development
natsort version 6.0.0 no longer uses Pipenv
to install development dependencies.
Dropped Python 2.6 and 3.3 Support
natsort version 6.0.0 dropped support for Python 2.6 and Python 3.3.
Seth M. Morton