Mapreduce job for blob migration

jobs.py

def migrate_blobstore(entity):
    " Migrate entities with blob fields from another app. "
    import urllib, mimetypes
    from google.appengine.api import files
    
    modified = False
    
    fields = entity.get_blob_fields()
    logging.debug("Fields are: %s" % fields)
    for field in fields:
        if entity.get_blobref_key(field):
            modified = True
            try:
                resp = urlfetch.fetch(
                    'http://blah.appspot.com/admin/migrate/blobs/%s' % urllib.quote(str(entity.get_blobref_key(field))), 
                    deadline=30
                )
            except urlfetch.DownloadError:
                # Try again
                raise
            else:
                if resp.status_code == 404:
                    logging.info("Blob gave 404 %s" % entity.get_blobref_key(field))
                else:
                    filename = '%s_%s_%d%s' % (
                        entity.__class__.__name__,
                        field,
                        entity.key().id(), 
                        mimetypes.guess_extension(resp.headers['Content-Type']).replace('jpe','jpg')
                    )
                    
                    logging.info("Storing blob as %s" % filename)
                    
                    file_name = files.blobstore.create(
                        mime_type=resp.headers['Content-Type'],
                        _blobinfo_uploaded_filename=filename
                    )

                    with files.open(file_name, 'a') as f:
                        f.write(resp.content)

                    files.finalize(file_name)

                    # Get the file's blob key
                    blob_key = files.blobstore.get_blob_key(file_name)
                    
                    setattr(entity, field, blob_key)
                    
    if modified:
        # Enable high performance urls
        entity.fix_serving_urls()
        yield op.db.Put(entity)