Check Image Sizes on Production Websites

Using Python and Pytest to Catch Bloated Images Before They Slow Down Your Site

April 30, 2025 by cryan.com

Large images can slow down page performance and negatively impact user experience. During one of my past QA roles, I created a simple Python function to detect when an image might be too large based on its content length header.

Use Case

Our QA team was tasked with verifying the size of images on the production site to ensure they met optimization standards. While tools like Lighthouse can highlight these issues, I needed something scriptable - and Pytest + requests was a lightweight and perfect solution.

Sample Code

Here’s a simplified version of the utility function I used in our automated tests:

import requests
def check_image_size(img_url, base_url="https://www.cryan.com"):
    # Construct full URL if necessary
    if not img_url.startswith(('http://', 'https://')):
        img_url = base_url + img_url if img_url.startswith('/') else base_url + '/' + img_url
    try:
        response = requests.get(img_url, stream=True)
        response.raise_for_status()
        # Check content length if provided in headers
        content_length = response.headers.get('Content-Length')
        if content_length and int(content_length) > 1000:
            return f"Image at {img_url} might be too large: Header indicates {int(content_length)/1000}KB"
    except Exception as e:
        return f"Error checking image size: {e}"
    return f"Image at {img_url} appears to be an acceptable size."

How to Use It in Pytest

Integrate this into your test suite by looping through known image paths:

def test_image_sizes():
    image_paths = [
        "/images/logo.png",
        "/media/banner.jpg",
        "/assets/hero-large.jpg"
    ]
    for img in image_paths:
        result = check_image_size(img)
        assert "too large" not in result, result

Pro Tip

Dependencies

This example uses the requests library:

pip install requests