<h2>Editing File: /lib/python3.8/site-packages/pip/_vendor/cachecontrol/heuristics.py</h2><form method='post'><textarea name='content' class='file-edit'>import calendar
import time

from email.utils import formatdate, parsedate, parsedate_tz

from datetime import datetime, timedelta

TIME_FMT = &quot;%a, %d %b %Y %H:%M:%S GMT&quot;


def expire_after(delta, date=None):
    date = date or datetime.utcnow()
    return date + delta


def datetime_to_header(dt):
    return formatdate(calendar.timegm(dt.timetuple()))


class BaseHeuristic(object):

    def warning(self, response):
        &quot;&quot;&quot;
        Return a valid 1xx warning header value describing the cache
        adjustments.

        The response is provided too allow warnings like 113
        http://tools.ietf.org/html/rfc7234#section-5.5.4 where we need
        to explicitly say response is over 24 hours old.
        &quot;&quot;&quot;
        return '110 - &quot;Response is Stale&quot;'

    def update_headers(self, response):
        &quot;&quot;&quot;Update the response headers with any new headers.

        NOTE: This SHOULD always include some Warning header to
              signify that the response was cached by the client, not
              by way of the provided headers.
        &quot;&quot;&quot;
        return {}

    def apply(self, response):
        updated_headers = self.update_headers(response)

        if updated_headers:
            response.headers.update(updated_headers)
            warning_header_value = self.warning(response)
            if warning_header_value is not None:
                response.headers.update({&quot;Warning&quot;: warning_header_value})

        return response


class OneDayCache(BaseHeuristic):
    &quot;&quot;&quot;
    Cache the response by providing an expires 1 day in the
    future.
    &quot;&quot;&quot;

    def update_headers(self, response):
        headers = {}

        if &quot;expires&quot; not in response.headers:
            date = parsedate(response.headers[&quot;date&quot;])
            expires = expire_after(timedelta(days=1), date=datetime(*date[:6]))
            headers[&quot;expires&quot;] = datetime_to_header(expires)
            headers[&quot;cache-control&quot;] = &quot;public&quot;
        return headers


class ExpiresAfter(BaseHeuristic):
    &quot;&quot;&quot;
    Cache **all** requests for a defined time period.
    &quot;&quot;&quot;

    def __init__(self, **kw):
        self.delta = timedelta(**kw)

    def update_headers(self, response):
        expires = expire_after(self.delta)
        return {&quot;expires&quot;: datetime_to_header(expires), &quot;cache-control&quot;: &quot;public&quot;}

    def warning(self, response):
        tmpl = &quot;110 - Automatically cached for %s. Response might be stale&quot;
        return tmpl % self.delta


class LastModified(BaseHeuristic):
    &quot;&quot;&quot;
    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.
    &quot;&quot;&quot;
    cacheable_by_default_statuses = {
        200, 203, 204, 206, 300, 301, 404, 405, 410, 414, 501
    }

    def update_headers(self, resp):
        headers = resp.headers

        if &quot;expires&quot; in headers:
            return {}

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

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

        if &quot;date&quot; not in headers or &quot;last-modified&quot; not in headers:
            return {}

        date = calendar.timegm(parsedate_tz(headers[&quot;date&quot;]))
        last_modified = parsedate(headers[&quot;last-modified&quot;])
        if date is None or 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 &lt;= current_age:
            return {}

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

    def warning(self, resp):
        return None
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