Sort lists naturally

## Project description

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

natsort comes with a shell script that is desecribed below. You can also execute natsort from the command line with python -m natsort.

There exists another natural sorting package for python called naturalsort. This package does not take into account floats and negatives (which is the default behavior of natsort) and so may be preferred if you wish to only sort version numbers.

## Problem Statement

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

```>>> a = ['a2', 'a8', 'a7', 'a5', 'a9', 'a1', 'a4', 'a10', 'a3', 'a6']
>>> sorted(a)
['a1', 'a10', 'a2', 'a3', 'a4', 'a5', 'a6', 'a7', 'a8', 'a9']
```

Notice that it has the order (‘1’, ‘10’, ‘2’)? This is because the list is being sorted in ASCII order, which sorts numbers like you would letters (i.e. ‘a’, ‘at’, ‘b’). It would be better if you had a sorting algorithm that recognized numbers as numbers and treated them like numbers, not letters.

This is where natsort comes it: it provides a key that helps sorts lists “naturally”. It provides support for ints and floats (including negatives and exponental notation) or you can turn this off to support sort version numbers.

## Synopsis

Using natsort is simple:

```>>> from natsort import natsorted
>>> a = ['a2', 'a8', 'a7', 'a5', 'a9', 'a1', 'a4', 'a10', 'a3', 'a6']
>>> natsorted(a)
['a1', 'a2', 'a3', 'a4', 'a5', 'a6', 'a7', 'a8', 'a9', 'a10']
```

natsort identifies the numbers and sorts them separately from letters.

You can also mix and match int, float, str, and unicode types when you sort:

```>>> a = ['4.5', 6, 2.3, u'5']
>>> sorted(a)
[2.3, 6, '4.5', u'5']
>>> natsorted(a)
[2.3, '4.5', u'5', 6]
```

### The sorting algorithms

Sometimes you want to sort by floats, sometimes by ints, and sometimes simply by digits. natsort supports all three number types. They can be chosen with the number_type argument to natsorted.

#### Sort by floats

By default, natsort searches for floats (even in exponential notation!). This means that it will look for things like negative signs and decimal points when determining a number:

```>>> a = ['a50', 'a51.', 'a50.4', 'a5.034e1', 'a50.300']
>>> sorted(a)
['a5.034e1', 'a50', 'a50.300', 'a50.4', 'a51.']
>>> natsorted(a, number_type=float)
['a50', 'a50.300', 'a5.034e1', 'a50.4', 'a51.']
>>> natsorted(a) # Float is the default behavior
['a50', 'a50.300', 'a5.034e1', 'a50.4', 'a51.']
```

To achieve this, selecting this number type causes natsort to parse the string ‘b-40.2’ into [‘b’, -40.2].

#### Sort by ints

In some cases you don’t want natsort to identify your numbers as floats, particularly if you are sorting version numbers. This is because you want the version ‘1.10’ to come after ‘1.2’, not before. In that case, it is advantageous to sort by ints, not floats:

```>>> a = ['ver1.9.9a', 'ver1.11', 'ver1.9.9b', 'ver1.11.4', 'ver1.10.1']
>>> sorted(a)
['ver1.10.1', 'ver1.11', 'ver1.11.4', 'ver1.9.9a', 'ver1.9.9b']
>>> natsorted(a)
['ver1.10.1', 'ver1.11', 'ver1.11.4', 'ver1.9.9a', 'ver1.9.9b']
>>> natsorted(a, number_type=int)
['ver1.9.9a', 'ver1.9.9b', 'ver1.10.1', 'ver1.11', 'ver1.11.4']
```

To achieve this, selecting this number type causes natsort to parse the string ‘b-40.2’ into [‘b’, -40, ‘.’, 2].

#### Sort by digits

The only difference between sorting by ints and sorting by digits is that sorting by ints may take into account a negative sign, and sorting by digits will not. This may be an issue if you used a ‘-‘ as your separator before the version numbers:

```>>> a = ['ver-2.9.9a', 'ver-1.11', 'ver-2.9.9b', 'ver-1.11.4', 'ver-1.10.1']
>>> natsorted(a, number_type=int)
['ver-2.9.9a', 'ver-2.9.9b', 'ver-1.10.1', 'ver-1.11', 'ver-1.11.4']
>>> natsorted(a, number_type=None)
['ver-1.10.1', 'ver-1.11', 'ver-1.11.4', 'ver-2.9.9a', 'ver-2.9.9b']
```

To achieve this, selecting this number type causes natsort to parse the string ‘b-40.2’ into [‘b-‘, 40, ‘.’, 2].

### Using a sorting key

Like the builtin sorted function, natsorted can accept a key so that you can sort based on a particular item of a list or by an attribute of a class:

```>>> from operator import attrgetter, itemgetter
>>> a = [['num4', 'b'], ['num8', 'c'], ['num2', 'a']]
>>> natsorted(a, key=itemgetter(0))
[['num2', 'a'], ['num4', 'b'], ['num8', 'c']]
>>> class Foo:
...    def __init__(self, bar):
...        self.bar = bar
...    def __repr__(self):
...        return "Foo('{0}')".format(self.bar)
>>> b = [Foo('num3'), Foo('num5'), Foo('num2')]
>>> natsorted(b, key=attrgetter('bar'))
[Foo('num2'), Foo('num3'), Foo('num5')]
```

## API

The natsort package provides three functions: natsort_key, natsorted, and index_natsorted.

### natsorted

natsort.natsorted (sequence, key = lambda x: x, number_type = float)

sequence (iterable)
The sequence to sort.
key (function)
A key used to determine how to sort each element of the sequence.
number_type (None, float, int)
The types of number to sort on: float searches for floating point numbers, int searches for integers, and None searches for digits (like integers but does not take into account negative sign).
returns
The sorted sequence.

Use natsorted just like the builtin sorted:

```>>> from natsort import natsorted
>>> a = ['num3', 'num5', 'num2']
>>> natsorted(a)
['num2', 'num3', 'num5']
```

### natsort_key

natsort.natsort_key (value, number_type = float)

value
The value used by the sorting algorithm
number_type (None, float, int)
The types of number to sort on: float searches for floating point numbers, int searches for integers, and None searches for digits (like integers but does not take into account negative sign).
returns
The modified value with numbers extracted.

Using natsort_key is just like any other sorting key in python:

```>>> from natsort import natsort_key
>>> a = ['num3', 'num5', 'num2']
>>> a.sort(key=natsort_key)
>>> a
['num2', 'num3', 'num5']
```

If you need to call natsort_key with the number_type argument, or get a special attribute or item of each element of the sequence, the easiest way is to make a lambda expression that calls natsort_key:

```>>> from operator import itemgetter
>>> a = [['num4', 'b'], ['num8', 'c'], ['num2', 'a']]
>>> f = itemgetter(0)
>>> a.sort(key=lambda x: natsort_key(f(x), number_type=int))
>>> a
[['num2', 'a'], ['num4', 'b'], ['num8', 'c']]
```

### index_natsorted

natsort.index_natsorted (sequence, key = lambda x: x, number_type = float)

sequence (iterable)
The sequence to sort.
key (function)
A key used to determine how to sort each element of the sequence.
number_type (None, float, int)
The types of number to sort on: float searches for floating point numbers, int searches for integers, and None searches for digits (like integers but does not take into account negative sign).
returns
The ordered indexes of the sequence.

Use index_natsorted if you want to sort multiple lists by the sorting of one list:

```>>> from natsort import index_natsorted
>>> a = ['num3', 'num5', 'num2']
>>> b = ['foo', 'bar', 'baz']
>>> index = index_natsorted(a)
>>> index
[2, 0, 1]
>>> # Sort both lists by the sort order of a
>>> [a[i] for i in index]
['num2', 'num3', 'num5']
>>> [b[i] for i in index]
['baz', 'foo', 'bar']
```

## Shell Script

For your convenience, there is a natsort shell script supplied to you that allows you to call natsort from the command-line. natsort was written to aid in computational chemistry research so that it would be easy to analyze large sets of output files named after the parameter used:

```\$ ls *.out
mode1000.35.out mode1243.34.out mode744.43.out mode943.54.out
```

(Obviously, in reality there would be more files, but you get the idea.) Notice that the shell sorts in ASCII order. This is the behavior of programs like find as well as ls. The problem is, when passing these files to an analysis program causes them not to appear in numerical order, which can lead to bad analysis. To remedy this, use natsort:

```# This won't get you what you want
\$ foo *.out
# This will sort naturally
\$ natsort *.out
mode744.43.out
mode943.54.out
mode1000.35.out
mode1243.34.out
\$ natsort *.out | xargs foo
```

You can also filter out numbers using the natsort command-line script:

```\$ natsort *.out -f 900 1100 # Select only numbers between 900-1100
mode943.54.out
mode1000.35.out
```

If needed, you can exclude specific numbers:

```\$ natsort *.out -e 1000.35 # Exclude 1000.35 from search
mode744.43.out
mode943.54.out
mode1243.34.out
```

For other options, use natsort --help.

It is also helpful to note that natsort accepts pipes, and also will sort each directory in a PATH independently of each other. Files in the current directory are listed before files in subdirectories.

Seth M. Morton

## History

### 10-01-2013 v. 3.0.2

• Made float, int, and digit searching algorithms all share the same base function
• Made the __version__ variable available when importing the module

### 8-15-2013 v. 3.0.1

• Added support for unicode strings.
• Removed extraneous string2int function.
• Fixed empty string removal function.

### 7-13-2013 v. 3.0.0

• Added a number_type argument to the sorting functions to specify how liberal to be when deciding what a number is.
• Reworked the documentation.

### 6-25-2013 v. 2.2.0

• Added key attribute to natsorted and index_natsorted so that it mimics the functionality of the built-in sorted
• Added tests to reflect the new functionality, as well as tests demonstrating how to get similar functionality using natsort_key.

### 12-5-2012 v. 2.1.0

• Reorganized package
• Now using a platform independent shell script generator (entry_points from distribute)
• Can now execute natsort from command line with python -m natsort as well

### 11-30-2012 v. 2.0.2

• Added the use_2to3 option to setup.py
• Added distribute_setup.py to the distribution
• Added dependency to the argparse module (for python2.6)

### 11-21-2012 v. 2.0.1

• Reorganized directory structure
• Added tests into the natsort.py file iteself

### 11-16-2012, v. 2.0.0

• Updated sorting algorithm to support floats (including exponentials) and basic version number support