Class LastModified

cachecontrol/heuristics.py:97–157

If there is no Expires header already, fall back on Last-Modified using the heuristic from http://tools.ietf.org/html/rfc7234#section-4.2.2 to calculate a reasonable value. Firefox also does something like this per https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching_F

class LastModified(BaseHeuristic):
    """
    If there is no Expires header already, fall back on Last-Modified
    using the heuristic from
    http://tools.ietf.org/html/rfc7234#section-4.2.2
    to calculate a reasonable value.

    Firefox also does something like this per
    https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching_FAQ
    http://lxr.mozilla.org/mozilla-release/source/netwerk/protocol/http/nsHttpResponseHead.cpp#397
    Unlike mozilla we limit this to 24-hr.
    """

    cacheable_by_default_statuses = {
        200,
        203,
        204,
        206,
        300,
        301,
        404,
        405,
        410,
        414,
        501,
    }

    def update_headers(self, resp: HTTPResponse) -> dict[str, str]:
        headers: Mapping[str, str] = resp.headers

        if "expires" in headers:
            return {}

        if "cache-control" in headers and headers["cache-control"] != "public":
            return {}

        if resp.status not in self.cacheable_by_default_statuses:
            return {}

        if "date" not in headers or "last-modified" not in headers:
            return {}

        time_tuple = parsedate_tz(headers["date"])
        assert time_tuple is not None
        date = calendar.timegm(time_tuple[:6])
        last_modified = parsedate(headers["last-modified"])
        if last_modified is None:
            return {}

        now = time.time()
        current_age = max(0, now - date)
        delta = date - calendar.timegm(last_modified)
        freshness_lifetime = max(0, min(delta / 10, 24 * 3600))
        if freshness_lifetime <= current_age:
            return {}

        expires = date + freshness_lifetime
        return {"expires": time.strftime(TIME_FMT, time.gmtime(expires))}