File hash GET parameters for Django staticfiles

README.md

About

A class for the static files app in Django that invalidates outdated browser cache.

How to use

You add the line

STATICFILES_STORAGE = 'path.to.hashpathstaticfilesstorage.HashPathStaticFilesStorage'

to your settings.py and everywhere you use the 'static' templatetag it will append a hash calculated form the contents of the file as a GET parameter at the end of the URL for the file. Example:

{% static "path/to/file.txt" %} -> /static/path/to/file.txt?4e1243

This guarantees that every time you update your static files, whether it is an image, a CSS file or anything else, all browsers fetch the new version of the file instead of using their cached versions.

Remember to change the import path to match your setup (replace the 'path.to' with your actual import path).

Caching

You can reduce the time it takes to return the URL by caching the hashes when they are first calculated. The cache framework that ships with Django is used to store the hashes, so make sure you configure that before caching your hashes.

When you start using the cache, and every time after that when you have to invalidate the cache, you simply create an object of the type HashPathStaticFilesStorage and call the method 'invalidate_cache' on that object:

h = HashPathStaticFilesStorage()
h.invalidate_cache()

This should happen every time you alter or update a file in one of your /static folders.

Configuration

This class introduces two new variables that you can put in your settings.py to configure it's behaviour:

• STATICFILES_HASH_ACCURACY An integer that controls the number of characters used in the hash. This should not be set below 3, as that increases the possibility of a collision. The hash can be at most 40 characters long so values above 40 have the same effect as 40. Defaults to 6
• STATICFILES_HASH_KEY_PREFIX A prefix for they keys in the cache. Defaults to 'staticfiles_hash'.

Requirements

Django 1.4 or newer is required.

hashpathstaticfilesstorage.py

# -*- coding: utf-8 -*-

import time

from hashlib import sha1

from django.conf import settings
from django.core.cache import cache
from django.contrib.staticfiles.storage import StaticFilesStorage


try:
    ACCURACY = settings.STATICFILES_HASH_ACCURACY
except AttributeError:
    ACCURACY = 6

try:
    KEY_PREFIX = settings.STATICFILES_HASH_KEY_PREFIX
except AttributeError:
    KEY_PREFIX = 'staticfiles_hash'

class HashPathStaticFilesStorage(StaticFilesStorage):
    """A static file storage that returns a unique url based on the contents
    of the file. When a static file is changed the url will also change,
    forcing all browsers to download the new version of the file.
    
    The uniqueness of the url is a GET parameter added to the end of it. It
    contains the first 6 characters of the SHA1 sum of the contents of the
    file.
    
    Example: {% static "image.jpg" %} -> /static/image.jpg?4e1243
    
    The accuracy of the hash (number of characters used) can be set in
    settings.py with STATICFILES_HASH_ACCURACY. Setting this value too low
    (1 or 2) can cause different files to get the same hash and is not
    recommended. SHA1 hashes are 40 characters long so all accuracy values
    above 40 have the same effect as 40.
    
    The values can be cached for faster performance. All keys in the cache have
    the prefix specified in STATICFILES_HASH_KEY_PREFIX in setings.py. This
    value defaults to 'staticfiles_hash'
    """

    @property
    def prefix_key(self):
        return "%s:%s" % (KEY_PREFIX, 'prefix')

    def invalidate_cache(self, nocache=False):
        """Invalidates the cache. Run this when one or more static files change.
        If called with nocache=True the cache will not be used.
        """
        value = int(time.time())
        if nocache:
            value = None
        cache.set(self.prefix_key, value)

    def get_cache_key(self, name):
        hash_prefix = cache.get(self.prefix_key)
        if not hash_prefix:
            return None
        key = "%s:%s:%s" % (KEY_PREFIX, hash_prefix, name)
        return key

    def set_cached_hash(self, name, the_hash):
        key = self.get_cache_key(name)
        if key:
            cache.set(key, the_hash)

    def get_cached_hash(self, name):
        key = self.get_cache_key(name)
        if not key:
            return None
        the_hash = cache.get(key)
        return the_hash

    def calculate_hash(self, name):
        path = self.path(name)
        try:
            the_file = open(path, 'r')
            the_hash = sha1(the_file.read()).hexdigest()[:ACCURACY]
            the_file.close()
        except IOError:
            return ""
        return the_hash

    def get_hash(self, name):
        the_hash = self.get_cached_hash(name)
        if the_hash:
            return the_hash
        the_hash = self.calculate_hash(name)
        self.set_cached_hash(name, the_hash)
        return the_hash

    def url(self, name):
        base_url = super(HashPathStaticFilesStorage, self).url(name)
        the_hash = self.get_hash(name)
        if "?" in base_url:
            return "%s&%s" % (base_url, the_hash)
        return "%s?%s" % (base_url, the_hash)

Thank you for this snippet, it's been very useful! I just wanted to share a small edit I made on my side at line 79. I needed to load images using the static template tag. I added the b flag to open files as binaries instead of relying on the environment's encoding (which can and most likely will break for non-textual files): the_file = open(path, 'rb').