Python Time Complexity

January 19, 2024


In the world of programming, particularly in Python, understanding how efficiently a piece of code runs is as crucial as making it functional. One fundamental concept that helps in this understanding is Big O Notation. This blog post aims to demystify Big O Notation, elucidating the common runtime complexities in Python code and providing practical examples for clarity.

What is Big O Notation?

Big O Notation is a mathematical concept used in Computer Science to describe the performance or complexity of an algorithm. Specifically, it measures the worst-case scenario in terms of time (time complexity) or space (space complexity) an algorithm needs to complete its function relative to its input size.

Now let's look at some common runtime complexities in Python.

O(1) - Constant Time

An algorithm is said to have a constant time when it is not dependent on the input size. No matter how large the input, the time it takes to complete a task remains the same. An example is accessing a specific element in an array.

O(n) - Linear Time

The execution time of an algorithm is directly proportional to the size of the input data. As the input size increases, the runtime increases linearly. An example is finding the maximum element in an unsorted list.

O(log n) - Logarithmic Time

This complexity denotes an algorithm that reduces the size of its input data in each step (usually by half). It is much more efficient than linear time. An example is binary search in a sorted array.

O(n^2) - Quadratic Time

In these algorithms, the time of execution is proportional to the square of the input size. They are less efficient and typically used for simple tasks or on small data sets. An example is bubble sort or insertion sort.

O(2^n) - Exponential Time

These algorithms have exponential growth. The runtime doubles with each addition to the input data set. They are often seen in brute-force algorithms. An example is certain recursive algorithms like the Fibonacci sequence.

O(n!) - Factorial Time

The worst-case scenario for complexity. The algorithm's runtime grows with the factorial of the input size. An example is solving the traveling salesman problem via brute-force.

Example: Linear Search - O(n)

def linear_search(arr, x):
    for i in range(len(arr)):
        if arr[i] == x:
            return i
    return -1

Example: Binary Search - O(log n)

def binary_search(arr, low, high, x):
    if high >= low:
        mid = (high + low) // 2
        if arr[mid] == x:
            return mid
        elif arr[mid] > x:
            return binary_search(arr, low, mid - 1, x)
            return binary_search(arr, mid + 1, high, x)
        return -1


Big O Notation provides a valuable lens for examining the efficiency of algorithms in Python. By understanding and applying this concept, developers can optimize their code for better performance, especially crucial in large-scale applications or data-intensive tasks. Remember, the goal is not just to make code that works, but code that works efficiently.