Python List Slicing

In Python, list slicing is a common practice and it is the most used technique for programmers to solve efficient problems. Consider a Python list, in order to access a range of elements in a list, you need to slice a list. One way to do this is to use the simple slicing operator i.e. colon(:). With this operator, one can specify where to start the slicing, where to end, and specify the step. List slicing returns a new list from the existing list.

Python List Slicing Syntax

The format for list slicing is of Python List Slicing is as follows:

Lst[ Initial : End : IndexJump ]

If Lst is a list, then the above expression returns the portion of the list from index Initial to index End, at a step size IndexJump.

Indexing in Python List

Indexing is a technique for accessing the elements of a Python List. There are various ways by which we can access an element of a list.

Positive Indexes

In the case of Positive Indexing, the first element of the list has the index number 0, and the last element of the list has the index number N-1, where N is the total number of elements in the list (size of the list).

Positive Indexing of a Python List

Example:

In this example, we will display a whole list using positive index slicing.

# Initialize list
Lst = [50, 70, 30, 20, 90, 10, 50]

# Display list
print(Lst[::])

Output:

[50, 70, 30, 20, 90, 10, 50]

Negative Indexes

The below diagram illustrates a list along with its negative indexes. Index -1 represents the last element and -N represents the first element of the list, where N is the length of the list.

Negative Indexing of a Python List

Example:

In this example, we will access the elements of a list using negative indexing.

# Initialize list
Lst = [50, 70, 30, 20, 90, 10, 50]

# Display list
print(Lst[-7::1])

Output:

[50, 70, 30, 20, 90, 10, 50]

Slicing

As mentioned earlier list slicing in Python is a common practice and can be used both with positive indexes as well as negative indexes. The below diagram illustrates the technique of list slicing:

Python List Slicing

Example:

In this example, we will transform the above illustration into Python code.

# Initialize list
Lst = [50, 70, 30, 20, 90, 10, 50]

# Display list
print(Lst[1:5])

Output:

[70, 30, 20, 90]

Examples of List Slicing in Python

Let us see some examples which depict the use of list slicing in Python.

Example 1: Leaving any argument like Initial, End, or IndexJump blank will lead to the use of default values i.e. 0 as Initial, length of the list as End, and 1 as IndexJump.

# Initialize list
List = [1, 2, 3, 4, 5, 6, 7, 8, 9]

# Show original list
print("Original List:\n", List)

print("\nSliced Lists: ")

# Display sliced list
print(List[3:9:2])

# Display sliced list
print(List[::2])

# Display sliced list
print(List[::])

Output:

Original List:
 [1, 2, 3, 4, 5, 6, 7, 8, 9]

Sliced Lists: 
[4, 6, 8]
[1, 3, 5, 7, 9]
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Example 2: A reversed list can be generated by using a negative integer as the IndexJump argument. Leaving the Initial and End as blank. We need to choose the Initial and End values according to a reversed list if the IndexJump value is negative. 

# Initialize list
List = ['Geeks', 4, 'geeks !']

# Show original list
print("Original List:\n", List)

print("\nSliced Lists: ")

# Display sliced list
print(List[::-1])

# Display sliced list
print(List[::-3])

# Display sliced list
print(List[:1:-2])

Output:

Original List:
 ['Geeks', 4, 'geeks !']

Sliced Lists: 
['geeks !', 4, 'Geeks']
['geeks !']
['geeks !']

Example 3: If some slicing expressions are made that do not make sense or are incomputable then empty lists are generated.

# Initialize list
List = [-999, 'G4G', 1706256, '^_^', 3.1496]

# Show original list
print("Original List:\n", List)

print("\nSliced Lists: ")

# Display sliced list
print(List[10::2])

# Display sliced list
print(List[1:1:1])

# Display sliced list
print(List[-1:-1:-1])

# Display sliced list
print(List[:0:])

Output:

Original List:
 [-999, 'G4G', 1706256, '^_^', 3.1496]

Sliced Lists: 
[]
[]
[]
[]

Example 4: List slicing can be used to modify lists or even delete elements from a list.

# Initialize list
List = [-999, 'G4G', 1706256, 3.1496, '^_^']

# Show original list
print("Original List:\n", List)


print("\nSliced Lists: ")

# Modified List
List[2:4] = ['Geeks', 'for', 'Geeks', '!']

# Display sliced list
print(List)

# Modified List
List[:6] = []

# Display sliced list
print(List)

Output:

Original List:
 [-999, 'G4G', 1706256, 3.1496, '^_^']

Sliced Lists: 
[-999, 'G4G', 'Geeks', 'for', 'Geeks', '!', '^_^']
['^_^']

Example 5: By concatenating sliced lists, a new list can be created or even a pre-existing list can be modified. 

# Initialize list
List = [1, 2, 3, 4, 5, 6, 7, 8, 9]

# Show original list
print("Original List:\n", List)

print("\nSliced Lists: ")

# Creating new List
newList = List[:3]+List[7:]

# Display sliced list
print(newList)

# Changing existing List
List = List[::2]+List[1::2]

# Display sliced list
print(List)

Output:

Original List:
 [1, 2, 3, 4, 5, 6, 7, 8, 9]

Sliced Lists: 
[1, 2, 3, 8, 9]
[1, 3, 5, 7, 9, 2, 4, 6, 8]

Python List Slicing – FAQs

What does distinct() do in Python?

Python itself does not have a built-in distinct() function. However, the concept of “distinct” is commonly used in SQL databases to retrieve unique values. In Python, you can achieve the same effect using a set to filter out duplicate elements from an iterable.

How do you check if a list contains unique values in Python?

To check if a list contains only unique values, you can compare the length of the list with the length of the set created from the list:

def is_unique(lst):
    return len(lst) == len(set(lst))
# Example usage
my_list = [1, 2, 3, 4, 5]
print(is_unique(my_list))  # Output: True
my_list_with_duplicates = [1, 2, 3, 3, 4, 5]
print(is_unique(my_list_with_duplicates))  # Output: False

What built-in Python data structure can be used to get unique values from a list?

The set data structure is used to get unique values from a list. A set automatically removes duplicates and maintains only unique elements.

my_list = [1, 2, 2, 3, 4, 4, 5]
unique_values = list(set(my_list))
print(unique_values)  # Output: [1, 2, 3, 4, 5]

How do I count unique items in a list in Python?

To count the number of unique items in a list, convert the list to a set and get the length of the set:

my_list = [1, 2, 2, 3, 4, 4, 5]
unique_count = len(set(my_list))
print(unique_count)  # Output: 5

How do I check if a list contains duplicates in Python?

To check if a list contains duplicates, you can compare the length of the list with the length of the set created from the list. If they are not equal, the list contains duplicates:

def contains_duplicates(lst):
    return len(lst) != len(set(lst))
# Example usage
my_list = [1, 2, 3, 3, 4, 5]
print(contains_duplicates(my_list))  # Output: True
my_unique_list = [1, 2, 3, 4, 5]
print(contains_duplicates(my_unique_list))  # Output: False

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