Python Lists

Python Lists are sequential data types.

Python lists are enclosed with square brackets([]) and their elements are separated by comma(,).

Python Lists are mutable i.e. we can change any element(s) of lists if we want to change.

We can access list elements using the index value of the element.

Like strings, there is both side indexing in Python Lists i.e. from the front side, indexing starts with “0” and from the backside, indexing starts with “-1“.

myList = [1, 2, 3, 4, "Hello", "World"]  # defining a list
print(myList)  # printing a list
print(myList[0])  # printing first element
print(myList[1])  # printing 2nd element
print(myList[5])  # printing last element
print(myList[-1])  # printing last element
print(myList[-2])  # printing last 2nd element

[1, 2, 3, 4, 'Hello', 'World']
1
2
World
World
Hello

List Slicing

List Slicing is very easy, just type the range of indexes to get the elements inside that range.

myList = [1, 2, 3, 4, "Hello", "World"]  # defining a list
print(myList)  # printing a list
print(myList[0:5])  # python automatically subtracts 
                                    # 2nd value with 1
print(myList[0:6])  # 6-1 = 5
print(myList[1:4])  # 4-1 = 3
print(myList[-4:-1])  # -1-1 = -2
print(myList[-3:-2])  # -2-1 = -3
print(myList[:6])  # python automatically assigns 0 
                       # if we don't put some value
print(myList[0:])  # python automatically assigns length
                   # of list if we don't put some value

[1, 2, 3, 4, 'Hello', 'World']
[1, 2, 3, 4, 'Hello']
[1, 2, 3, 4, 'Hello', 'World']
[2, 3, 4]
[3, 4, 'Hello']
[4]
[1, 2, 3, 4, 'Hello', 'World']
[1, 2, 3, 4, 'Hello', 'World']

Changing Element Value

To change the element of the list, type the list name with the index in square brackets([]), and assign a different value like assigning a value to the variable.

myList = [1, 2, 3, 4, "Hello", "World"]  # defining a list
print("List before change is-->", myList)
myList[0] = 0 # assigning different values
myList[1] = 1
myList[-1] = "Answer"
myList[-2] = "Question"
print("List after change is-->", myList)

List before change is--> [1, 2, 3, 4, 'Hello', 'World']
List after change is--> [0, 1, 3, 4, 'Question', 'Answer']

List Methods/Functions

There are built-in List methods/functions provided by Python which you can use.

The len() function

Using len() function, we can find the length of a list i.e. the number of elements present in the list.

myList = [34, 12, 10, 50, 45, 13]
print(len(myList))

6

The sort() method

Using the sort() method, we can sort any list but the elements in the list should be of the same datatypes like int or string.

myList = [34, 12, 10, 50, 45, 13]
myList.sort()
print(myList)
myList2 = ["d", "c", "a", "b", "B", "A"]
myList2.sort()
print(myList2)

[10, 12, 13, 34, 45, 50]
['A', 'B', 'a', 'b', 'c', 'd']

The min() method

Using the min() method, we can find the minimum value from the list but the elements in the list should be of the same datatypes like int or string.

myList = [34, 12, 10, 50, 45, 13]
print(min(myList))
myList1 = ["d", "c", "a", "b", "B", "A"]
print(min(myList1))

10
A

The max() method

Using the max() method, we can find the maximum value from the list but the elements in the list should be of the same datatypes like int or string.

myList = [34, 12, 10, 50, 45, 13]
print(max(myList))
myList1 = ["d", "c", "a", "b", "B", "A"]
print(max(myList1))

50
d

The list() method

Using the list() method, we can convert any sequence datatype into lists. Datatypes like Tuples, Strings, Sets, etc can be converted to Python Lists.

myTuple = (1, 2, 3, 4)
myList1 = list(myTuple)
myString = "Hello"
myList2 = list(myString)
mySet = {1, 2, 3, 4}
myList3 = list(mySet)
print(myList1)
print(myList2)
print(myList3)

[1, 2, 3, 4]
['H', 'e', 'l', 'l', 'o']
[1, 2, 3, 4]

The append() method

Using the append() method, we can add an element at the end of a Python List.

myList = [1, 2, 3, 4]
myList.append(5)
print(myList)

[1, 2, 3, 4, 5]

The clear() method

Using the clear() method, we can clear all elements from a Python List i.e. we can delete all elements.

myList = [1, 2, 3, 4]
myList.clear()
print(myList)

[]

The copy() method

Using the copy() method, we can copy one Python list to another Python list.

myList1 = [1, 2, 3, 4]
myList2 = myList1.copy()
print(myList2)

[1, 2, 3, 4]

Note:- You can not copy a list just by using the “=” operator as it will be called as assigning one list to another not copying. If you do that, then any changes you will make in the first list will also apply to the second list.

myList1 = [1, 2, 3, 4]
myList2 = myList1  # referring one list to another
print(myList1)
print(myList2)
myList1.append(5)  # appending 5 in myList1
print(myList1)
print(myList2)  # changes will appear in myList2 too

[1, 2, 3, 4]
[1, 2, 3, 4]
[1, 2, 3, 4, 5]
[1, 2, 3, 4, 5]

The reverse() method

Using the reverse() method, we can reverse the order of the position of elements in a Python List.

myList = [1, 2, 3, 4]
myList.reverse()
print(myList)

[4, 3, 2, 1]

The sum() function

Using the sum() function, we can get the sum of Python’s iterable objects like Lists, Tuple, and dictionary. It takes two arguments, the first one is the iterable object and the second one is the number you want to add with the numbers present in the iterable object. The sum() function adds only numbers else it will give an error.

myList = [1, 2, 3, 4, 5, 6, 7]
addition1 = sum(myList)
print(addition1)
addition2 = sum(myList, 3)
print(addition2)

28
31

The range() function

Using the range() function, we can get a list that contains a sequence of numbers, starting from 0 by default, increments by 1 (by default), and stops before the specified number, since using the range() function we can create a list of the desired sequence which will save our time and work.

myList1 = [range(15)]
myList2 = [range(4, 15)]
myList3 = [range(4, 15, 3)]
print(myList1)
print(myList2)
print(myList3)

[range(0, 15)]
[range(4, 15)]
[range(4, 15, 3)]

Surprised, yes this is not what we wanted, actually, we need to unpack the result of the range() function using the argument-unpacking operator i.e. *.

myList1 = [*range(15)]
myList2 = [*range(4, 15)]
myList3 = [*range(4, 15, 3)]
print(myList1)
print(myList2)
print(myList3)

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
[4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
[4, 7, 10, 13]

The pop() function

Using the pop() function, we can remove the last value of the list. If we want to remove any other value from the list using the pop() function, then we need to pass the index number to the pop() function, or else the last value will be removed. The pop() function always returns the removed value.

myList = [4, 5, 6, 7, 8, 9, 10, 11] 
print(myList.pop())  # 11
print(myList)
print(myList.pop(3))  # 7
print(myList)
print(myList.pop(0))  # 4
print(myList)
print(myList.pop())  # 10
print(myList)

11
[4, 5, 6, 7, 8, 9, 10]
7
[4, 5, 6, 8, 9, 10]
4
[5, 6, 8, 9, 10]
10
[5, 6, 8, 9]

The remove() function

Using the remove() function, we can remove any element from the list. To remove an element, pass the element as an argument to the function. The remove() function does not return any value and gives an error if the value is not present in the list.

myList = [4, 5, 6, 7, 8, 9, 10, 11]
print(myList.remove(4))  # removes 4 and returns nothing
print(myList)
myList.remove(7)
print(myList)
myList.remove(11)
print(myList)
print(myList.remove(3))  # 3 not present

None
[5, 6, 7, 8, 9, 10, 11]
[5, 6, 8, 9, 10, 11]
[5, 6, 8, 9, 10]
Traceback (most recent call last):
  File "main.py", line 8, in 
    print(myList.remove(3))  # 3 not present
ValueError: list.remove(x): x not in list

What if one value is repeated? In that case, the remove() function removes the first one only and leaves other values at the same place where they were before.

myList = [4, 5, 6, 7, 4, 8, 9, 4, 10, 11]
myList.remove(4)
print(myList)

[5, 6, 7, 4, 8, 9, 4, 10, 11]

Now, let’s see a one-liner to remove all the similar elements from the list.

myList = [4, 5, 6, 7, 4, 8, 9, 4, 10, 11]
while 4 in myList: myList.remove(4)
print(myList)

[5, 6, 7, 8, 9, 10, 11]

The index() function

Using the index() function, we can find/search for an element in any list. As soon as the element is found, it returns the index of the element. If there are two or more elements with similar values in the list, it will return the index of the element with the lowest index number.

We can pass two more arguments to the index() function, start(from where we want to start searching the element in the list), and end(where we want to end searching the element in the list). These two arguments are optional.

listname.index(element, start, end)

Now, let’s see an example

myList = [4, 5, 6, 7, 4, 8, 9]
print(myList.index(5))
print(myList.index(4))
print(myList.index(4, 2, 5))
print(myList.index(4, 2, 3))

1
0
4
Traceback (most recent call last):
  File "main.py", line 5, in 
    print(myList.index(4, 2, 3))
ValueError: 4 is not in list

The insert() function

Using the insert() function, we can insert an element in the list on our desired location/index.

listname.insert(index, element)

If the index value is greater than or equal to the len(listname), then the element is inserted at the end of the list. It does not return any value and gives an error if it is used with other objects instead of the list.

myList = [4, 5, 6, 7, 4, 8, 9]
myList.insert(0, 1)
print(myList)
myList.insert(1, 2)
print(myList)
myList.insert(10, 12)
print(myList)

[1, 4, 5, 6, 7, 4, 8, 9]
[1, 2, 4, 5, 6, 7, 4, 8, 9]
[1, 2, 4, 5, 6, 7, 4, 8, 9, 12]

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