Insert an Element at a Specific Index in Python

In Python, to insert an element at a specific position in a list, you can use the insert() method. Unlike append(), which only adds elements at the end, insert() lets you place an element at any desired index. This tutorial will cover how to use insert() effectively with examples.

Examples

1. Using the insert() Method

The insert() method takes two arguments: the index where the element should be inserted and the element itself.

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# Creating an initial list
numbers = [1, 2, 4, 5]

# Inserting an element at index 2
numbers.insert(2, 3)

# Printing the updated list
print("Updated List:", numbers)

Output:

Updated List: [1, 2, 3, 4, 5]

Here, the number 3 is inserted at index 2, shifting the subsequent elements to the right.

2. Inserting at the Beginning of a List

To insert an element at the beginning of a list, use index 0.

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# Creating an initial list
fruits = ["banana", "cherry"]

# Inserting at the beginning (index 0)
fruits.insert(0, "apple")

# Printing the updated list
print("Updated List:", fruits)

Output:

Updated List: ['apple', 'banana', 'cherry']

The new element is inserted at index 0, making it the first item in the list.

3. Inserting at the End of a List

Although append() is commonly used to add elements at the end, insert() can achieve the same by using len(list) as the index.

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# Creating an initial list
colors = ["red", "blue"]

# Inserting at the end
colors.insert(len(colors), "green")

# Printing the updated list
print("Updated List:", colors)

Output:

Updated List: ['red', 'blue', 'green']

Since len(colors) is the index after the last element, “green” is added at the end.

4. Inserting an Element in a Sorted List

If you need to insert an element into a sorted list while maintaining order, you can use the bisect module.

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import bisect

# Creating a sorted list
sorted_numbers = [10, 20, 30, 40]

# Finding the correct index to insert 25
index = bisect.bisect(sorted_numbers, 25)

# Inserting the number at the right position
sorted_numbers.insert(index, 25)

# Printing the updated list
print("Updated Sorted List:", sorted_numbers)

Output:

Updated Sorted List: [10, 20, 25, 30, 40]

The bisect() function finds the correct index for insertion, keeping the list sorted.

Conclusion

The insert() method provides a flexible way to add elements at specific positions in a list:

  1. Inserting at a specific index: list.insert(index, element)
  2. Inserting at the beginning: Use index 0
  3. Inserting at the end: Use len(list)
  4. Maintaining a sorted list: Use bisect.bisect() to find the correct position