Back
Apr 3, 2011

Sprite cache invalidation

When we use css-sprites it's important to make browser cache them for longest period possible. On other hand, we need to refresh them when they are updated. This is especially visible when all icons are stored in single sprite. When it's outdated - entire site becomes ugly.

To solve this task I've implemented this small script that adds file's hash to url:

background-image: url(images/icons.png);
background-image: url(images/icons.png?a3844c660);
#!/usr/bin/python

import os.path
import re
import hashlib


def file_hash(filename, block_size=2**20):
    md5 = hashlib.md5()
    with open(filename, 'rb') as f:
        for data in iter(lambda: f.read(block_size), ''):
            md5.update(data)
    return md5.hexdigest()


def find_replace(match, basedir):
    match = match.replace("'", '')
    if '?' in match or '#' in match or match.startswith('http://'):
        print 'skipping: ', match
        return None

    filename = os.path.join(basedir, match)
    return '%s?%s' % (match, file_hash(filename)[:10])


def insert_hash(filename, debug=False):
    basedir = os.path.dirname(filename)
    css = open(filename).read()
    matches = re.findall(r"url\((.*?)\)", css)
    replacements = dict([
        (match, find_replace(match, basedir))
        for match in matches
    ])

    if debug:
        for key, value in replacements.iteritems():
            print key, '    =>    ', value
    else:
        for match, repl in replacements.iteritems():
            if repl:
                css = css.replace(match, repl)

        out = open(filename, 'w')
        out.write(css)
        out.close()


if __name__ == '__main__':
    from optparse import OptionParser
    usage = 'usage: %prog filename.css'

    parser = OptionParser(usage=usage)
    parser.add_option("", "--debug", action="store_true", dest="debug", default=False,
                      help="When set, just output list of substitutions.")

    (options, args) = parser.parse_args()

    if len(args) < 1:
        print 'You must provide file name.'

    insert_hash(os.path.realpath(args[0]), options.debug)

Update

This is pretty old article. Now it's better to use something integrated to your build process - grunt, webassets, etc.

Subscribe for the news and updates

More thoughts
Feb 3, 2025Technology
Figma for Developers: What Dev Mode Offers and How to Use It

This article explores Figma’s Dev Mode, a tool that streamlines design-to-code translation by enabling precise inspection, automated code generation, and seamless integration with design systems.

Mar 18, 2024Technology
From boring to exciting: turn learning to code into an adventure

Tired of boring programming courses where you're forced to read thick textbooks and write code that's never used? Need a platform that makes learning fun and exciting? Then you're in the right place!

Jun 1, 2018Technology
Site search organization: basic concepts

Now it's time to get acquainted with Elasticsearch. This NoSQL database is used to store logs, analyze information and - most importantly - search.

Feb 28, 2017Technology
Passing pieces of markup to components in Angular 2 and problems with dynamic content

In this article we'll research how to pass custom markup to Angular components and how to create different types of dynamic components.

Feb 28, 2017Technology
How to write an API in Django

There is such a term as Remote Procedure Call (RPC). In other words, by using this technology, programs can call functions on remote computers. There are many ways to implement RPC.

Jan 28, 2017Technology
Creating a site preview like in slack (using aiohttp)

In this article we will write a small library for pulling metadata and creating a preview for a site just like Slack does.