Conditional HTTP

Pyramid requests and responses support conditional HTTP requests via the ETag 和 Last-Modified 标题。It is useful to enable this for an entire site to save on bandwidth for repeated requests. 有可能 ETag support for an entire site can be done using a tween:

 1def conditional_http_tween_factory(handler, registry):
 2    def conditional_http_tween(request):
 3        response = handler(request)
 4
 5        # If the Last-Modified header has been set, we want to enable the
 6        # conditional response processing.
 7        if response.last_modified is not None:
 8            response.conditional_response = True
 9
10        # We want to only enable the conditional machinery if either we
11        # were given an explicit ETag header by the view or we have a
12        # buffered response and can generate the ETag header ourself.
13        if response.etag is not None:
14            response.conditional_response = True
15        elif (isinstance(response.app_iter, collections.abc.Sequence) and
16                len(response.app_iter) == 1):
17            response.conditional_response = True
18            response.md5_etag()
19
20        return response
21    return conditional_http_tween

这个tween的作用是首先检查响应以确定它是否已经有了 Last-Modified 或 ETag 标题集。If it does, then it will enable the conditional response processing. If the response does not have an ETag 头集合，然后它将尝试确定响应是否已经完全加载到内存中（以避免将可能非常大的对象加载到内存中）。如果它已经加载到内存中，那么它将生成一个 ETag header from the MD5 digest of the response body, and again enable the conditional response processing.