Sequence in Python

A sequence is one of the most basic data structures in Python.

Each value in a sequence is assigned a position called an index. The first index is 0, the second index is 1, and so on.

Python has six built-in types of sequences, but the most common ones are lists and tuples.

Operations that can be performed on lists include indexing, slicing, adding, multiplying, and checking membership.

Additionally, Python provides built-in methods for determining the length of a sequence, as well as the largest and smallest elements.

Lists are the most widely used data type in Python. They are collections of comma-separated values enclosed within square brackets.

The data items in a list do not need to be of the same type.

To create a list, simply place a comma-separated set of data items inside square brackets, as shown below:

list1 = ['Google', 'Runoob', 1997, 2000]
list2 = [1, 2, 3, 4, 5]
list3 = ["a", "b", "c", "d"]
list4 = ['red', 'green', 'blue', 'yellow', 'white', 'black']

Accessing Values in a List

Just like string indexing, list indices start at 0, the second index is 1, and so on.

You can access, slice, and combine list elements through their indices.

Example

#!/usr/bin/python3

list = ['red', 'green', 'blue', 'yellow', 'white', 'black']
print(list[0])
print(list[1])
print(list[2])

The output of this code is:

red
green
blue

Negative Indexing

Indices can also be negative. The last element has an index of -1, the second-to-last element has an index of -2, and so forth.

Example

#!/usr/bin/python3

list = ['red', 'green', 'blue', 'yellow', 'white', 'black']
print(list[-1])
print(list[-2])
print(list[-3])

The output of this code is:

black
white
yellow

Slicing Lists

In addition to accessing values via indices, you can also slice lists using square brackets.

Example

#!/usr/bin/python3

nums = [10, 20, 30, 40, 50, 60, 70, 80, 90]
print(nums[0:4])

The output of this code is:

[10, 20, 30, 40]

Slicing with Negative Indices

You can also use negative indices for slicing:

Example

#!/usr/bin/python3

list = ['Google', 'Runoob', "Zhihu", "Taobao", "Wiki"]

# Accessing the second element
print("list[1]:", list[1])

# Slicing from the second element (inclusive) to the second-to-last element (exclusive)
print("list[1:-2]:", list[1:-2])

The output of this code is:

list[1]:  Runoob
list[1:-2]:  ['Runoob', 'Zhihu']

Updating a List

You can modify or update the items in a list. You can also use the append() method to add new items to the list, as shown below:

Example (Python 3.0+)

#!/usr/bin/python3

list = ['Google', 'Runoob', 1997, 2000]

print("The third element is: ", list[2])
list[2] = 2001
print("Updated third element is: ", list[2])

list1 = ['Google', 'Runoob', 'Taobao']
list1.append('Baidu')
print("Updated list: ", list1)

Output of the code:

The third element is:  1997
Updated third element is:  2001
Updated list:  ['Google', 'Runoob', 'Taobao', 'Baidu']

Note: We will discuss the usage of the append() method in the upcoming sections.

Deleting List Elements

You can use the del statement to delete elements from a list, as shown in the following example:

Example (Python 3.0+)

#!/usr/bin/python3

list = ['Google', 'Runoob', 1997, 2000]

print("Original list: ", list)
del list[2]
print("After deleting the third element: ", list)

Output of the code:

Original list:  ['Google', 'Runoob', 1997, 2000]
After deleting the third element:  ['Google', 'Runoob', 2000]

Note: We will discuss the usage of the remove() method in the upcoming sections.

Python List Operators

The + and * operators work similarly for lists as they do for strings. The + operator concatenates lists, while the * operator repeats the list.

Here are some examples:

Python List Slicing and Concatenation

Slicing lists in Python is similar to slicing strings, as shown below:

L = ['Google', 'Runoob', 'Taobao']

Example:

>>> L = ['Google', 'Runoob', 'Taobao']
>>> L[2]
'Taobao'
>>> L[-2]
'Runoob'
>>> L[1:]
['Runoob', 'Taobao']

You can also concatenate lists:

>>> squares = [1, 4, 9, 16, 25]
>>> squares += [36, 49, 64, 81, 100]
>>> squares
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

Nested Lists

A nested list refers to creating other lists within a list. For example:

>>> a = ['a', 'b', 'c']
>>> n = [1, 2, 3]
>>> x = [a, n]
>>> x
[['a', 'b', 'c'], [1, 2, 3]]
>>> x[0]
['a', 'b', 'c']
>>> x[0][1]
'b'

List Comparison

To compare lists, you need to introduce the eq method from the operator module (for more details, see the Python operator module):

Example

# Import the operator module
import operator

a = [1, 2]
b = [2, 3]
c = [2, 3]

print("operator.eq(a,b): ", operator.eq(a, b))
print("operator.eq(c,b): ", operator.eq(c, b))

Output of the code:

operator.eq(a,b):  False
operator.eq(c,b):  True

Python List Functions and Methods

Python includes the following functions:

Python also includes the following methods: