Lists in Python: Methods and Basic Operations

Lists in Python are ordered mutable sets of objects, numbered from 0. In this case, objects can be different – from integers to strings. Lists can also store lists in themselves.

In this article, we will understand the basic principles of lists in Python, and also consider the methods of working with them. If you’re learning Python from scratch, we suggest you also check out the Beginner’s Roadmap .

1. Memory storage
2. List creation
3. Slices
4. Simple operations
5. List methods

Memory storage

When creating a list, an empty area is reserved in memory. On the one hand, this is no different from creating any other data type, but the difference is that the contents of the list can change:

numbers = [1, 2]
numbers[1] = 3
# обновлённый список: [1, 3]

Before replacing an element of the sequence, it print(numbers[1])will output 2, and after replacing it – 3.

Creating a list in Python

This can be done in several ways, for example by listing the elements of the list in square brackets:

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9]

In this case, the unit will be at position 0, that is, it print(numbers[0])will display 1.

You can also use a function to handle an iterable object list(). Let us have some line, then:

list('tproger')
# ['t', 'p', 'r', 'o', 'g', 'e', 'r']

There are also list generators that let you apply a given expression to each element in a sequence. Let’s say you need to create a list consisting of numbers from 1 to 5 inclusive:

numbers = [i for i in range(1,6)]
# [1, 2, 3, 4, 5]

Slices of the list

Slices allow you to get a subset of values. The following code will return a list with elements starting at index 0 and excluding index 2 and above:

numbers = [1, 5, 9, 6]
print(numbers[0:2])
# вывод [1, 5]

Next, let’s display everything, except for the element at position 3:

print(numbers[:3])
# вывод [1, 5, 9]

And now from index 1 to the end:

print(numbers[1:])
# вывод [5, 9, 6]

Python List Operations

• x in l– trueif the item xis in the list l;
• x not in l– trueif the element is xabsent in l;
• l1 + l2 – combining two lists;
• l * n , n * l– copies the list of ntimes;
• len(l)– the number of elements in l;
• min(l) – the smallest element;
• max(l) – the largest element;
• sum(l) – the sum of the numbers in the list;
• for i in list() – iterates over elements from left to right.

Python list methods

Index

Returns the position of the first matched item. The search for a match occurs from left to right. Example:

numbers = [1, 5, 9, 6, 1, 2, 1]
print(numbers.index(1))
# вывод 0: первая найденная единица на позиции 0

Count

This method counts how many times the specified value appears in the Python list:

numbers = [1, 5, 9, 6, 1, 2, 1]
print(numbers.count(1))
# вывод 3, потому что единица встречается 3 раза

Append

Adds the specified value to the end:

numbers = [1, 5, 9, 6]
numbers.append(3)
# обновлённый список: [1, 5, 9, 6, 3]

Sort

Sorts the list in Python. By default, from smallest to largest:

numbers = [1, 5, 9, 6]
numbers.sort()
# обновлённый список: [1, 5, 6, 9]

You can also sort the sequence of elements from largest to smallest:

numbers = [1, 5, 9, 6]
numbers.sort(reverse = true)
# обновлённый список: [9, 6, 5, 1]

Insert

Inserts an element before the specified index:

numbers = [1, 5, 9, 6]
numbers.insert(3, [2, 3])
# обновлённый список: [1, 5, 9, [2, 3], 6]

Remove

Removes the first occurrence of an item in the Python list:

numbers = [1, 5, 9, 6, 1, 2, 1]
numbers.remove(1)
# обновлённый список: [5, 9, 6, 1, 2, 1]

Extend

Similar to the method append(), it adds elements, but the advantage of the method extend()is that it also allows you to add lists:

numbers = [1, 5, 9, 6]
numbers.extend([2, 3])
# обновлённый список: [1, 5, 9, 6, 2, 3]

Pop

And this method removes the element at the specified index, and also displays the deleted element. If no index is specified, the default method will remove the last element:

numbers = [1, 5, 9, 6]
numbers.pop(1)
# получаем:
# 5
# [1, 9, 6]

Join

Converts the list to a string. The element separator is written in quotes before the method, and the Python list itself must consist of strings:

mylist = ['сайт', 'типичный', 'программист']
print(', '.join(mylist))
# вывод 'сайт, типичный, программист'