How to implement binary search in Python

Standalone binary search function that returns the index of a target in a sorted list, or -1 if not found.

Easy Python 3.9+ Aug 9, 2026 Functions & basics 13 views 0 copies

binary search algorithms search function

Python code

21 lines

Python 3.9+

def binary_search(arr, target):
    left, right = 0, len(arr) - 1
    
    while left <= right:
        mid = (left + right) // 2
        if arr[mid] == target:
            return mid
        elif arr[mid] < target:
            left = mid + 1
        else:
            right = mid - 1
    
    return -1

if __name__ == "__main__":
    test_array = [1, 3, 5, 7, 9, 11, 13, 15]
    targets = [7, 13, 2, 15, 1]
    
    for target in targets:
        result = binary_search(test_array, target)
        print(f"Target {target}: index {result}")

Output

stdout

Target 7: index 3
Target 13: index 6
Target 2: index -1
Target 15: index 7
Target 1: index 0

How it works

The function uses two pointers (left and right) to narrow the search interval. By comparing the middle element with the target, it halves the search space each iteration. The loop runs while left <= right, ensuring all elements are considered. Returning -1 when the loop ends indicates the target is not present. This algorithm runs in O(log n) time.

Common mistakes

Forgetting that the input list must be sorted for binary search to work
Using `left < right` instead of `left <= right`, which can miss edge elements
Not updating `mid` correctly with integer division in Python 2 style

Variations

Use the `bisect` module from the standard library (e.g., `bisect_left`) for a built-in binary search.
Recursive implementation that calls itself with a reduced range.

Real-world use cases

Finding a user record by ID in a sorted database index before performing a full fetch.
Locating a configuration key in a large sorted settings list for fast lookup.
Searching for a timestamp in a sorted log array to determine event spans.

How to implement binary search in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

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How it works

Common mistakes

Variations

Real-world use cases

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