{"id":918,"date":"2021-03-22T02:20:56","date_gmt":"2021-03-22T02:20:56","guid":{"rendered":"http:\/\/www.ishygddt.xyz\/~blog\/?p=918"},"modified":"2022-03-08T14:03:56","modified_gmt":"2022-03-08T20:03:56","slug":"python-parse-http-date","status":"publish","type":"post","link":"http:\/\/www.ishygddt.xyz\/~blog\/2021\/03\/python-parse-http-date","title":{"rendered":"Parsing the HTTP \"Date\" header in Python"},"content":{"rendered":"

Per RFC 7231 \u00a77.1.1.1<\/a>:<\/p>\n

A recipient that parses a timestamp value in an HTTP header field MUST accept all three HTTP-date formats.<\/p><\/blockquote>\n

These formats are then described as (with the first being the only<\/span> preferred format; the latter 2 are designated as \"obsolete\"), converted for this post into strftime(3)<\/code><\/a> syntax:<\/p>\n

    \n
  1. %a, %d %b %Y %H:%M:%S %Z<\/code>
    \nwith the timezone always given as<\/em> \"GMT\", but to be interpreted as<\/em> UTC.<\/li>\n
  2. %A, %d-%b-%y %H:%M:%S %Z<\/code>
    \nwhere the timezone may be equal to any of an array of \"standard\" abbreviations from     RFC 850 \u00a72.1.4<\/a>.<\/li>\n
  3. %a %b %-d %H:%M:%S %Y<\/code>
    \nwhere you simply pray\/assume that the remote server is operating in UTC.<\/li>\n<\/ol>\n

    However, Python's strptime<\/code><\/a> function does not support timezones: it eats<\/em> them with %Z<\/code>, but does not actually use<\/em> them. Therefore, we will have to hack this support in ourselves. The pytz<\/code><\/a> module is indispensible<\/span> for parsing them, so we will appreciate\/use it. (We additionally have to crack open re<\/code><\/a> because, depressingly, strptime<\/code> does not even make available to us<\/em> that which it matched as %Z<\/code>.)<\/p>\n

    So, a Python\u00a0 function to parse an HTTP Date header into a datetime object would be something like:<\/p>\n

    from datetime import datetime\nfrom pytz import timezone, utc\nimport re\ndef parse_http_date(date):\n\ttry:\n\t\timf1 = '%a, %d %b %Y %H:%M:%S GMT'\n\t\treturn datetime.strptime(date, imf1).replace(tzinfo=utc)\n\texcept ValueError:\n\t\ttry:\n\t\t\trfc850 = '%A, %d-%b-%y %H:%M:%S %Z'\n\t\t\ttzname = re.fullmatch(r'((\\w+), (\\d+)-(\\w+)-(\\d+) (\\d+):(\\d+):(\\d+)) (.+)', s).group(9)\n\t\t\tif tzname == 'GMT': tzname = 'UTC'\n\t\t\treturn datetime.strptime(date, rfc850).replace(tzinfo=timezone(tzname))\n\t\texcept (ValueError, TypeError):\n\t\t\tpass\n\t\ttry:\n\t\t\tasctime = '%a %b %-d %H:%M:%S %Y'\n\t\t\treturn datetime.strptime(date, asctime).replace(tzinfo=utc)\n\t\texcept ValueError:\n\t\t\tpass\n\t\t# Neither of the "obsolete" formats worked, so re-raise original strptime error from preferred format\n\t\traise\n<\/code><\/pre>\n
    If you don't care about parsing obsolete formats, this can be reduced to:<\/p>\n
    from datetime import datetime\nfrom datetime import timezone as tz\ndef parse_http_date(date):\n\timf1 = '%a, %d %b %Y %H:%M:%S GMT'\n\treturn datetime.strptime(date, imf1).replace(tzinfo=tz.utc)<\/code><\/pre>\n
    \u2026which only uses the standard library!<\/p>\n","protected":false},"excerpt":{"rendered":"
    Per RFC 7231 \u00a77.1.1.1<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[101],"tags":[72,44],"class_list":["post-918","post","type-post","status-publish","format-standard","hentry","category-writeups","tag-parsing","tag-python"],"_links":{"self":[{"href":"http:\/\/www.ishygddt.xyz\/~blog\/wp-json\/wp\/v2\/posts\/918","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.ishygddt.xyz\/~blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.ishygddt.xyz\/~blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.ishygddt.xyz\/~blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/www.ishygddt.xyz\/~blog\/wp-json\/wp\/v2\/comments?post=918"}],"version-history":[{"count":18,"href":"http:\/\/www.ishygddt.xyz\/~blog\/wp-json\/wp\/v2\/posts\/918\/revisions"}],"predecessor-version":[{"id":2088,"href":"http:\/\/www.ishygddt.xyz\/~blog\/wp-json\/wp\/v2\/posts\/918\/revisions\/2088"}],"wp:attachment":[{"href":"http:\/\/www.ishygddt.xyz\/~blog\/wp-json\/wp\/v2\/media?parent=918"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.ishygddt.xyz\/~blog\/wp-json\/wp\/v2\/categories?post=918"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.ishygddt.xyz\/~blog\/wp-json\/wp\/v2\/tags?post=918"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}