def data(self):
        if not hasattr(self, '_data'):
            request = URLRequest(self.url)

            # Look in the cache for etag / last modified headers to use
            # TODO: "expires" header could be supported
            if self.env and self.env.cache:
                headers = self.env.cache.get(
                    ('url', 'headers', self.url))
                if headers:
                    etag, lmod = headers
                    if etag: request.add_header('If-None-Match', etag)
                    if lmod: request.add_header('If-Modified-Since', lmod)

            # Make a request
            try:
                response = urlopen(request)
            except __HOLE__ as e:
                if e.code != 304:
                    raise
                    # Use the cached version of the url
                self._data = self.env.cache.get(('url', 'contents', self.url))
            else:
                with contextlib.closing(response):
                    self._data = response.read()

                # Cache the info from this request
                if self.env and self.env.cache:
                    self.env.cache.set(
                        ('url', 'headers', self.url),
                        (response.headers.get("ETag"),
                         response.headers.get("Last-Modified")))
                    self.env.cache.set(('url', 'contents', self.url), self._data)
        return self._data
dataset/ETHPy150Open miracle2k/webassets/src/webassets/merge.py/UrlHunk.data