http.py 文件源码

def condition(etag_func=None, last_modified_func=None):
    """
    Decorator to support conditional retrieval (or change) for a view
    function.

    The parameters are callables to compute the ETag and last modified time for
    the requested resource, respectively. The callables are passed the same
    parameters as the view itself. The ETag function should return a string (or
    None if the resource doesn't exist), while the last_modified function
    should return a datetime object (or None if the resource doesn't exist).

    The ETag function should return a complete ETag, including quotes (e.g.
    '"etag"'), since that's the only way to distinguish between weak and strong
    ETags. If an unquoted ETag is returned (e.g. 'etag'), it will be converted
    to a strong ETag by adding quotes.

    This decorator will either pass control to the wrapped view function or
    return an HTTP 304 response (unmodified) or 412 response (precondition
    failed), depending upon the request method. In either case, it will add the
    generated ETag and Last-Modified headers to the response if it doesn't
    already have them.
    """
    def decorator(func):
        @wraps(func, assigned=available_attrs(func))
        def inner(request, *args, **kwargs):
            # Compute values (if any) for the requested resource.
            def get_last_modified():
                if last_modified_func:
                    dt = last_modified_func(request, *args, **kwargs)
                    if dt:
                        return timegm(dt.utctimetuple())

            # The value from etag_func() could be quoted or unquoted.
            res_etag = etag_func(request, *args, **kwargs) if etag_func else None
            res_etag = quote_etag(res_etag) if res_etag is not None else None
            res_last_modified = get_last_modified()

            response = get_conditional_response(
                request,
                etag=res_etag,
                last_modified=res_last_modified,
            )

            if response is None:
                response = func(request, *args, **kwargs)

            # Set relevant headers on the response if they don't already exist.
            if res_last_modified and not response.has_header('Last-Modified'):
                response['Last-Modified'] = http_date(res_last_modified)
            if res_etag and not response.has_header('ETag'):
                response['ETag'] = res_etag

            return response

        return inner
    return decorator


# Shortcut decorators for common cases based on ETag or Last-Modified only