{"id":1154,"date":"2020-11-15T09:38:47","date_gmt":"2020-11-15T09:38:47","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=1154"},"modified":"2025-03-27T14:55:01","modified_gmt":"2025-03-27T14:55:01","slug":"python-slicing","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/advanced-python\/python-slicing\/","title":{"rendered":"Python Slicing in Depth"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about Python slicing and how to use it to extract data from and assign data to a sequence.<\/p>\n\n\n\n

Python slicing review #<\/a><\/h2>\n\n\n\n

So far you’ve learned about slicing such as list slicing<\/a>.<\/p>\n\n\n\n

Technically, slicing relies on indexing. Therefore, slicing only works with sequence types<\/a>.<\/p>\n\n\n\n

For mutable sequence types such as lists<\/a>, you can use slicing to extract and assign data. For example:<\/p>\n\n\n

colors = ['red'<\/span>, 'green'<\/span>, 'blue'<\/span>, 'orange'<\/span>]\n\n# extract data<\/span>\nprint(colors[1<\/span>:3<\/span>])\n\n# assign data<\/span>\ncolors[1<\/span>:3<\/span>] = ['pink'<\/span>, 'black'<\/span>]\nprint(colors)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['green', 'blue']\n['red', 'pink', 'black', 'orange']<\/code><\/span>Code language:<\/span> JSON \/ JSON with Comments<\/span> (<\/span>json<\/span>)<\/span><\/small><\/pre>\n\n\n
However, you can use slicing to extract data from immutable sequences. For example:<\/p>\n\n\n
topic = 'Python Slicing'<\/span>\n\n# Extract data<\/span>\nprint(topic[0<\/span>:6<\/span>])<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Python<\/code><\/span><\/pre>\n\n\n
If you attempt to use slicing to assign data to an immutable sequence, you’ll get an error. For example:<\/p>\n\n\n
topic[0<\/span>:6<\/span>] = 'Java'<\/span><\/code><\/span>Code language:<\/span> JavaScript<\/span> (<\/span>javascript<\/span>)<\/span><\/small><\/pre>\n\n\n
Error:<\/p>\n\n\n
TypeError<\/span>: 'str'<\/span> object does not support item assignment<\/code><\/span>Code language:<\/span> JavaScript<\/span> (<\/span>javascript<\/span>)<\/span><\/small><\/pre>\n\n\n
The slicing seq[start:stop]<\/code> returns the elements starting at the index start<\/code> up to the index stop - 1<\/code>. Therefore, it’s easier to visualize that the indexes are between the elements when you slice the sequence:<\/p>\n\n\n
\n
\"Python<\/figure>\n<\/div>\n\n\n
Python slice type #<\/a><\/h2>\n\n\n\n
Everything in Python is an object including the slice. A slice is actually an object of the slice<\/code> type. When you use the slicing notation:<\/p>\n\n\n
seq<\/span>[start:stop]<\/span><\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
The start:stop<\/code> is a slice<\/code> object.<\/p>\n\n\n
slice(start, stop)<\/code><\/span><\/pre>\n\n\n
For example:<\/p>\n\n\n
s = slice(1<\/span>, 3<\/span>)\n\nprint<\/span>(type(s))\nprint<\/span>(s.start, s.stop)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
<class<\/span> 'slice<\/span>'><\/span>\n1 3<\/code><\/span>Code language:<\/span> HTML, XML<\/span> (<\/span>xml<\/span>)<\/span><\/small><\/pre>\n\n\n
So instead of using the slicing notation:<\/p>\n\n\n
colors<\/span>[1:3]<\/span><\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
… you can use the slice object instead:<\/p>\n\n\n
colors = ['red'<\/span>, 'green'<\/span>, 'blue'<\/span>, 'orange'<\/span>]\ns = slice(1<\/span>, 3<\/span>)\n\nprint<\/span>(colors[s])<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['green', 'blue']<\/code><\/span>Code language:<\/span> JSON \/ JSON with Comments<\/span> (<\/span>json<\/span>)<\/span><\/small><\/pre>\n\n\n
Python Slicing: start and stop bounds #<\/a><\/h2>\n\n\n\n
The slice seq[start:stop]<\/code> selects elements starting at the index start<\/code> and stopping at the index stop<\/code> (excluding the element at the index stop<\/code>).<\/p>\n\n\n\n
In other words, it returns all elements of the sequence at the index n<\/code> where n<\/code> satisfies the following expression:<\/p>\n\n\n
start <= n < stop<\/code><\/span><\/pre>\n\n\n
When start<\/code> or stop<\/code> is greater than the length of the sequence:<\/p>\n\n\n
len(seq)<\/code><\/span><\/pre>\n\n\n
… Python uses len(seq)<\/code> for the start<\/code> or stop<\/code>.<\/p>\n\n\n\n
Both start<\/code> and stop<\/code> are optional. The start<\/code> defaults to 0 and stop<\/code> defaults to len(seq)<\/code> when you don’t specify it.<\/p>\n\n\n\n
The following example returns the entire list:<\/p>\n\n\n
colors = ['red'<\/span>, 'green'<\/span>, 'blue'<\/span>, 'orange'<\/span>]\nprint<\/span>(colors[0<\/span>:100<\/span>])<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['red', 'green', 'blue', 'orange']<\/code><\/span>Code language:<\/span> JSON \/ JSON with Comments<\/span> (<\/span>json<\/span>)<\/span><\/small><\/pre>\n\n\n
Since the stop bound is 100, Python uses the len(colors)<\/code> for the stop<\/code> bound.<\/p>\n\n\n\n
The following example returns an empty list:<\/p>\n\n\n
colors = ['red'<\/span>, 'green'<\/span>, 'blue'<\/span>, 'orange'<\/span>]\nprint(colors[10<\/span>:])<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Since the start<\/code> is 10, Python assigns the len(colors)<\/code> to it.<\/p>\n\n\n\n
Negative start and stop bounds #<\/a><\/h3>\n\n\n\n
The slice object also accepts negative start<\/code> and stop<\/code> bounds. The following example uses the negative start and stop bounds to slice a list:<\/p>\n\n\n
colors = ['red'<\/span>, 'green'<\/span>, 'blue'<\/span>, 'orange'<\/span>]\n\nprint(colors[-4<\/span>:-2<\/span>])<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['red', 'green']<\/code><\/span>Code language:<\/span> JSON \/ JSON with Comments<\/span> (<\/span>json<\/span>)<\/span><\/small><\/pre>\n\n\n
To get the 'blue'<\/code> and 'orange'<\/code> elements from the colors<\/code> list, you can combine the negative and positive bounds:<\/p>\n\n\n
colors = ['red'<\/span>, 'green'<\/span>, 'blue'<\/span>, 'orange'<\/span>]\nprint(colors[-2<\/span>:4<\/span>])<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['blue', 'orange']<\/code><\/span>Code language:<\/span> JSON \/ JSON with Comments<\/span> (<\/span>json<\/span>)<\/span><\/small><\/pre>\n\n\n
\"\"<\/figure>\n\n\n\n
The step value #<\/a><\/h2>\n\n\n\n
Slices support the third argument, which is the step value. The step value defaults to 1 if you don’t specify it:<\/p>\n\n\n
seq[star:stop:step]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
It’s equivalent to:<\/p>\n\n\n
s = slice(start, stop, step) \nseq[s]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
 See the following example:<\/p>\n\n\n
colors = ['red'<\/span>, 'green'<\/span>, 'blue'<\/span>, 'orange'<\/span>]\nprint(colors[0<\/span>:4<\/span>:2<\/span>])<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['red', 'blue']<\/code><\/span>Code language:<\/span> JSON \/ JSON with Comments<\/span> (<\/span>json<\/span>)<\/span><\/small><\/pre>\n\n\n
The indices method #<\/a><\/h2>\n\n\n\n
A slice object essentially defines a sequence of indices for selecting elements of a sequence.<\/p>\n\n\n\n
To make it more convenient, the slice<\/code> type has the indices<\/code> method that returns the equivalent range (start, stop, step)<\/code> for any slice of a sequence with a specified length:<\/p>\n\n\n
slice<\/span>(start<\/span>, stop<\/span>, step<\/span>).indices<\/span>(length<\/span>)<\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
It returns a new tuple:<\/p>\n\n\n
(i, j, k)<\/code><\/span><\/pre>\n\n\n
And you can use the values of this tuple to generate a list of indices using the range<\/code> function. For example:<\/p>\n\n\n
colors = ['red'<\/span>, 'green'<\/span>, 'blue'<\/span>, 'orange'<\/span>]\n\ns = slice(0<\/span>, 4<\/span>, 2<\/span>)\nt = s.indices(len(colors))\n\nfor<\/span> index in<\/span> range(*t):\n    print(colors[index])<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
red\nblue<\/code><\/span><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
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Try it<\/a><\/p>\n\n\n\n