{"id":1133,"date":"2020-11-15T03:03:17","date_gmt":"2020-11-15T03:03:17","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=1133"},"modified":"2025-03-27T14:31:47","modified_gmt":"2025-03-27T14:31:47","slug":"python-sequences","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/advanced-python\/python-sequences\/","title":{"rendered":"Python Sequences"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about the Python sequences and their basic operations.<\/p>\n\n\n\n

Introduction to Python sequences #<\/a><\/h2>\n\n\n\n

A sequence is a positionally ordered collection of items. And you can refer to any item in the sequence by using its index number e.g., s[0]<\/code> and s[1]<\/code>.<\/p>\n\n\n\n

In Python, the sequence index starts at 0, not 1. So the first element is s[0]<\/code> and the second element is s[1]<\/code>. If the sequence s<\/code> has n<\/code> items, the last item is s[n-1]<\/code>.<\/p>\n\n\n\n

Python has the following built-in sequence types: lists<\/a>, bytearrays, strings<\/a>, tuples<\/a>, range, and bytes. Python classifies sequence types as mutable and immutable<\/a>.<\/p>\n\n\n\n

The mutable sequence types are lists and bytearrays while the immutable sequence types are strings, tuples, range, and bytes.<\/p>\n\n\n\n

A sequence can be homogeneous or heterogeneous. In a homogeneous sequence, all elements have the same type. For example, strings are homogeneous sequences where each element is of the same type.<\/p>\n\n\n\n

Lists, however, are heterogeneous sequences where you can store elements of different types including integer, strings, objects, etc.<\/p>\n\n\n\n

In general, homogeneous sequence types are more efficient than heterogeneous in terms of storage and operations.<\/p>\n\n\n\n

Sequence type vs iterable type #<\/a><\/h2>\n\n\n\n

An iterable<\/a> is a collection of objects where you can get each element one by one. Therefore, any sequence is iterable. For example, a list is iterable.<\/p>\n\n\n\n

However, an iterable may not be a sequence type. For example, a set<\/a> is iterable but it’s not a sequence.<\/p>\n\n\n\n

Generally speaking, iterables are more general than sequence types.<\/p>\n\n\n\n

Standard Python sequence methods #<\/a><\/h2>\n\n\n\n

The following explains some standard sequence methods:<\/p>\n\n\n\n

Counting elements of a Python sequence #<\/a><\/h3>\n\n\n\n

To get the number of elements of a sequence, you use the built-in len<\/code> function:<\/p>\n\n\n

len(seq)<\/code><\/span><\/pre>\n\n\n
The following example uses the len<\/code> function to get the number of items in the cities<\/code> list:<\/p>\n\n\n
cities = ['San Francisco'<\/span>, 'New York'<\/span>, 'Washington DC'<\/span>]\n\nprint<\/span>(len(cities))\n<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
3<\/code><\/span><\/pre>\n\n\n
Checking if an item exists in a Python sequence #<\/a><\/h3>\n\n\n\n
To check if an item exists in a sequence, you use the in<\/code> operator:<\/p>\n\n\n
element in seq<\/code><\/span><\/pre>\n\n\n
The following example uses the in<\/code> operator to check if the 'New York'<\/code> is in the cities<\/code> list:<\/p>\n\n\n
cities = ['San Francisco'<\/span>, 'New York'<\/span>, 'Washington DC'<\/span>]\nprint<\/span>('New York'<\/span> in cities)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
True<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
To negate the in<\/code> operator, you use the not<\/code> operator. The following example checks if 'New York'<\/code> is not in the cities<\/code> list:<\/p>\n\n\n
cities = ['San Francisco'<\/span>, 'New York'<\/span>, 'Washington DC'<\/span>]\nprint<\/span>('New York'<\/span> not in cities)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
False<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Finding the index of an item in a Python sequence #<\/a><\/h3>\n\n\n\n
The seq.index(e)<\/code> returns the index of the first occurrence of the item e<\/code> in the sequence seq<\/code>:<\/p>\n\n\n
seq<\/span>.index<\/span>(e<\/span>)<\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
For example:<\/p>\n\n\n
numbers = [1<\/span>, 4<\/span>, 5<\/span>, 3<\/span>, 5<\/span>, 7<\/span>, 8<\/span>, 5<\/span>]\n\nprint<\/span>(numbers.index(5<\/span>))\n<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
2<\/code><\/span><\/pre>\n\n\n
\"\"<\/figure>\n\n\n\n
The index of the first occurrence of number 5 in the numbers list is 2. If the number is not in the sequence, you’ll get an error:<\/p>\n\n\n
numbers = [1<\/span>, 4<\/span>, 5<\/span>, 3<\/span>, 5<\/span>, 7<\/span>, 8<\/span>, 5<\/span>]\nprint<\/span>(numbers.index(10<\/span>))<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Error:<\/p>\n\n\n
ValueError: 10<\/span> is not in list<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
To find the index of the first occurrence of an item at or after a specific index, you use the following form of the index method:<\/p>\n\n\n
seq<\/span>.index<\/span>(e<\/span>, i<\/span>)<\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
The following example returns the index of the first occurrence of the number 5 after the third index:<\/p>\n\n\n
numbers = [1<\/span>, 4<\/span>, 5<\/span>, 3<\/span>, 5<\/span>, 7<\/span>, 8<\/span>, 5<\/span>]\nprint<\/span>(numbers.index(5<\/span>, 3<\/span>))<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
4<\/code><\/span><\/pre>\n\n\n
\"\"<\/figure>\n\n\n\n
The following form of the index method allows you to find the index of the first occurrence of an item at or after the index i<\/code> and before index j<\/code>:<\/p>\n\n\n
seq<\/span>.index<\/span>(e<\/span>, i<\/span>, j<\/span>)<\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
For example:<\/p>\n\n\n
numbers = [1<\/span>, 4<\/span>, 5<\/span>, 3<\/span>, 5<\/span>, 7<\/span>, 8<\/span>, 5<\/span>]\nprint<\/span>(numbers.index(5<\/span>, 3<\/span>, 5<\/span>))<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
4<\/code><\/span><\/pre>\n\n\n
\"\"<\/figure>\n\n\n\n
Slicing a sequence #<\/a><\/h3>\n\n\n\n
To get the slice from the index i<\/code> to (but not including) j<\/code>, you use the following syntax:<\/p>\n\n\n
seq<\/span>[i:j]<\/span><\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
For example:<\/p>\n\n\n
numbers = [1<\/span>, 4<\/span>, 5<\/span>, 3<\/span>, 5<\/span>, 7<\/span>, 8<\/span>, 5<\/span>]\nprint<\/span>(numbers[2<\/span>:6<\/span>])<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
[5<\/span>, 3<\/span>, 5<\/span>, 7<\/span>]<\/code><\/span>Code language:<\/span> JSON \/ JSON with Comments<\/span> (<\/span>json<\/span>)<\/span><\/small><\/pre>\n\n\n
When you slice a sequence, it’s easier to imagine that the sequence indexes locate between two items like this:<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
The extended slice allows you to get a slice from i<\/code> to (but not including j<\/code>) in steps of k<\/code>:<\/p>\n\n\n
seq<\/span>[i:j:k]<\/span><\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
For example:<\/p>\n\n\n
numbers = [1<\/span>, 4<\/span>, 5<\/span>, 3<\/span>, 5<\/span>, 7<\/span>, 8<\/span>, 5<\/span>]\nprint(numbers[2<\/span>:6<\/span>:2<\/span>])<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
[5<\/span>, 5<\/span>]<\/code><\/span>Code language:<\/span> JSON \/ JSON with Comments<\/span> (<\/span>json<\/span>)<\/span><\/small><\/pre>\n\n\n
\"\"<\/figure>\n\n\n\n
Getting min and max items from a Python sequence #<\/a><\/h3>\n\n\n\n
If the ordering between items in a sequence is specified, you can use the built-in min and max functions to find the minimum and maximum items:<\/p>\n\n\n
numbers = [1<\/span>, 4<\/span>, 5<\/span>, 3<\/span>, 5<\/span>, 7<\/span>, 8<\/span>, 5<\/span>]\n\nprint<\/span>(min(numbers))  # 1<\/span>\nprint<\/span>(max(numbers))  # 8<\/span>\n<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
1\n8<\/code><\/span><\/pre>\n\n\n
Concatenating two Python sequences #<\/a><\/h3>\n\n\n\n
To concatenate two sequences into a single sequence, you use the + operator:<\/p>\n\n\n
s3 = s1 + s2<\/code><\/span><\/pre>\n\n\n
The following example concatenates two sequences of strings:<\/p>\n\n\n
east = ['New York'<\/span>, 'New Jersey'<\/span>]\nwest = ['San Diego'<\/span>, 'San Francisco'<\/span>]\n\ncities = east + west\nprint<\/span>(cities)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['New York', 'New Jersey', 'San Diego', 'San Francisco']<\/code><\/span>Code language:<\/span> JSON \/ JSON with Comments<\/span> (<\/span>json<\/span>)<\/span><\/small><\/pre>\n\n\n
It’s quite safe to concatenate immutable sequences. The following example appends one element to the west list. And it doesn’t affect the cities sequence:<\/p>\n\n\n
east = ['New York'<\/span>, 'New Jersey'<\/span>]\nwest = ['San Diego'<\/span>, 'San Francisco'<\/span>]\n\ncities = east + west\n\nwest.append('Sacramento'<\/span>)\n\nprint<\/span>(west)\nprint<\/span>(cities)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['San Diego', 'San Francisco', 'Sacramento']\n['New York', 'New Jersey', 'San Diego', 'San Francisco']<\/code><\/span>Code language:<\/span> JSON \/ JSON with Comments<\/span> (<\/span>json<\/span>)<\/span><\/small><\/pre>\n\n\n
However, you should be aware of concatenations of mutable sequences. The following example shows how to concatenate a list to itself:<\/p>\n\n\n
city = [['San Francisco'<\/span>, 900<\/span>_000]]\ncities = city + city\n\nprint<\/span>(cities)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
[['San Francisco', 1000000<\/span>], ['San Francisco', 1000000<\/span>]]<\/code><\/span>Code language:<\/span> JSON \/ JSON with Comments<\/span> (<\/span>json<\/span>)<\/span><\/small><\/pre>\n\n\n
Since a list is mutable, the memory addresses of the first and second elements from the cities list are the same:<\/p>\n\n\n
print<\/span>(id(cities[0<\/span>]) == id(cities[1<\/span>]))  # True<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
In addition, when you change the value from the original list, the combined list also changes:<\/p>\n\n\n
city[0<\/span>][1<\/span>] = 1<\/span>_000_000\nprint<\/span>(cities)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Putting it all together:<\/p>\n\n\n
city = [['San Francisco'<\/span>, 900<\/span>_000]]\ncities = city + city\n\nprint<\/span>(cities)\nprint<\/span>(id(cities[0<\/span>]) == id(cities[1<\/span>]))  # True<\/span>\n\ncity[0<\/span>][1<\/span>] = 1<\/span>_000_000\nprint<\/span>(cities)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
[['San Francisco'<\/span>, 900000]<\/span>, ['San Francisco'<\/span>, 900000]<\/span>]\nTrue<\/span>\n[['San Francisco'<\/span>, 1000000]<\/span>, ['San Francisco'<\/span>, 1000000]<\/span>]<\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
Repeating a Python sequence #<\/a><\/h3>\n\n\n\n
To repeat a sequence a number of times, you use the multiplication operator (*). The following example repeats the string Python three times:<\/p>\n\n\n
s = 'ha'<\/span>\nprint<\/span>(s*3<\/span>)\n<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
hahaha<\/code><\/span><\/pre>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
    \n
  • Python sequences are positionally ordered collections of items.<\/li>\n<\/ul>\n
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