Implement file hash computation as a separate method of staticfiles' CachedFilesMixin

[mkai]

I'm currently extending django-cumulus (Rackspace Cloud Files storage backend) and S3BotoStorage (Amazon S3 storage backend) to use hashed file names using the CachedFilesMixin of contrib.staticfiles. That came down to (as intended, I suppose!):

class MyCachedCloudFilesStorage(CachedFilesMixin, CloudFilesStorage):
    pass

Now I noticed that both Amazon and Rackspace include file hashes for each file in their API responses. It the response (e. g. a file list) is cached appropriately, the post-processing of the CachedFilesMixin can be sped up dramatically by just getting the hash from the cached response instead of calculating it from the file (which is expensive because it reads the file over the network).

I propose the attached patch so that backend authors can implement their own method of getting the hash for a file:

diff --git a/django/contrib/staticfiles/storage.py b/django/contrib/staticfiles/storage.py
index c000f97..e59b987 100644
--- a/django/contrib/staticfiles/storage.py
+++ b/django/contrib/staticfiles/storage.py
@@ -65,6 +65,13 @@ class CachedFilesMixin(object):
                 compiled = re.compile(pattern)
                 self._patterns.setdefault(extension, []).append(compiled)
 
+    def get_file_hash(self, name, content=None):
+        """Gets the MD5 hash of the file."""
+        md5 = hashlib.md5()
+        for chunk in content.chunks():
+            md5.update(chunk)
+        return md5.hexdigest()[:12]
+
     def hashed_name(self, name, content=None):
         parsed_name = urlsplit(unquote(name))
         clean_name = parsed_name.path.strip()
@@ -79,13 +86,9 @@ class CachedFilesMixin(object):
                 return name
         path, filename = os.path.split(clean_name)
         root, ext = os.path.splitext(filename)
-        # Get the MD5 hash of the file
-        md5 = hashlib.md5()
-        for chunk in content.chunks():
-            md5.update(chunk)
-        md5sum = md5.hexdigest()[:12]
+        file_hash = self.get_file_hash(clean_name, content)
         hashed_name = os.path.join(path, u"%s.%s%s" %
-                                   (root, md5sum, ext))
+                                   (root, file_hash, ext))

[Preston Holmes]

Is there a reason you wouldn't just subclass the mixin and make your changes there?

[mkai]

Yes, of course, and I'm already doing this. I needed to copy/paste the get_hash_name method in full, though, so my storage class was blown up from the above (2 lines) to 45 lines of duplicated code.

I figure that when overwriting the mixin, customizing the file hash computation would be a common change, especially when one is using a slow, remote storage and wants to avoid having to open each file (which takes about a second for each file on Rackspace Cloud Files, for example).

[Jannis Leidel]

Sounds like a good idea to me.

[Jannis Leidel]

FWIW, the content parameter to the method needs to be checked before iterating over it.

[Jannis Leidel]

Fixed in 5f75ac91df2ef1c19946f65e08aa50165f061cd4.