How to Use bisect Module in Python for Sorted Lists

The bisect module in Python helps you insert items into sorted lists while keeping them sorted. Use bisect.bisect() to find the insertion point and bisect.insort() to insert an element in the correct position.

Syntax

The bisect module provides two main functions:

• bisect.bisect(list, item, lo=0, hi=len(list)): Returns the position where item should be inserted to keep the list sorted.
• bisect.insort(list, item, lo=0, hi=len(list)): Inserts item into the list at the correct position to maintain sorting.

Parameters lo and hi define the slice of the list to consider.

python

import bisect

# bisect function syntax
pos = bisect.bisect(sorted_list, item, lo=0, hi=len(sorted_list))

# insort function syntax
bisect.insort(sorted_list, item, lo=0, hi=len(sorted_list))

Example

This example shows how to find the insertion point for a number in a sorted list and how to insert it while keeping the list sorted.

python

import bisect

numbers = [10, 20, 30, 40, 50]

# Find position to insert 35
pos = bisect.bisect(numbers, 35)
print(f"Insert 35 at position: {pos}")

# Insert 35 into the list
bisect.insort(numbers, 35)
print("List after insertion:", numbers)

Output

Insert 35 at position: 3
List after insertion: [10, 20, 30, 35, 40, 50]

Common Pitfalls

Common mistakes include:

• Using bisect on unsorted lists, which gives incorrect results.
• Confusing bisect.bisect() (which returns insertion point) with bisect.insort() (which inserts the item).
• Not specifying lo and hi when you want to limit the search range.

python

import bisect

numbers = [10, 30, 20, 40, 50]  # Not sorted
pos = bisect.bisect(numbers, 25)
print(f"Wrong position in unsorted list: {pos}")  # Wrong result

# Correct way: sort first
numbers.sort()
pos = bisect.bisect(numbers, 25)
print(f"Correct position after sorting: {pos}")

Output

Wrong position in unsorted list: 2
Correct position after sorting: 2

Quick Reference

Function	Purpose	Returns/Inserts
bisect.bisect(list, item, lo=0, hi=len(list))	Find insertion point for item	Index position (int)
bisect.insort(list, item, lo=0, hi=len(list))	Insert item into list	None (modifies list)

Key Takeaways

Use bisect.bisect() to find where to insert an item in a sorted list without changing it.

Use bisect.insort() to insert an item into a sorted list while keeping it sorted.

Always ensure the list is sorted before using bisect functions to get correct results.

You can limit the search range with lo and hi parameters for efficiency.

bisect module is useful for maintaining sorted lists without full sorting after each insertion.