httpdate

httpdate is a Python library for parsing and formatting HTTP date fields that are used in headers like Last-Modified and If-Modified-Since. It does so in strict accordance with RFC 9110 Section 5.6.7.

It has only one dependency, which is my tiny leapseconds library that just provides a tuple of official leap seconds.

HTTP date formats

All HTTP dates (eg, in Last-Modified headers) must be sent in this format:

# IMF-fixdate
Sun, 06 Nov 1994 08:49:37 GMT

However, RFC 9110 states that recipients must also accept two other obsolete formats:

# rfc850-date
Sunday, 06-Nov-94 08:49:37 GMT

# asctime-date
Sun Nov  6 08:49:37 1994

RFC 9110 criteria for the HTTP date field includes the following:

• It must represent time as an instance of UTC.
• It must represent weekday names and month names in English.
• It is case-sensitive.
• It must not have any additional whitespace.
• It must be semantically correct (eg, the weekday must be the correct weekday).
• It can include leap seconds (eg, 23:59:60).
• It must represent a year of 1900 or above.

It isn't stated explicitly in the RFCs, but httpdate will only consider a leap second semantically correct if it's an official leap second.

Installation

pip install httpdate

Usage

from httpdate import is_valid_httpdate, httpdate_to_unixtime, unixtime_to_httpdate

# Check if an HTTP date (eg, `Last-Modified` header) is valid:
is_valid_httpdate("Sun, 06 Nov 1994 08:49:37 GMT")

try:
    # Parse an HTTP date:
    httpdate_to_unixtime("Sun, 06 Nov 1994 08:49:37 GMT")
    # Parse an HTTP date (rfc850-date):
    httpdate_to_unixtime("Sunday, 06-Nov-94 08:49:37 GMT")
    # Parse an HTTP date (asctime-date):
    httpdate_to_unixtime("Sun Nov  6 08:49:37 1994")
except ValueError:
    # Not a valid HTTP date string.
    pass

# Format a Unix timestamp as an HTTP date:
try:
    unixtime_to_httpdate(784111777)
except ValueError:
    # Outside the range supported by the operating system.
    pass

• is_valid_httpdate(httpdate):
  • Args
    • httpdate (str): An HTTP date string.
  • Returns
    • bool: True if httpdate is a valid HTTP date string, otherwise False.
  • Raises
    • TypeError if httpdate is not of type str.
• httpdate_to_unixtime(httpdate):
  • Args
    • httpdate (str): An HTTP date string.
  • Returns
    • int: A Unix timestamp (int).
  • Raises
    • TypeError if httpdate is not of type str.
    • ValueError if httpdate is not a valid HTTP date string.
• unixtime_to_httpdate(unixtime):
  • Args
    • unixtime (int): A Unix timestamp.
  • Returns
    • str: An HTTP date string.
  • Raises
    • TypeError if unixtime is not of type int.
    • ValueError if unixtime represents a year before 1900, or if it is outside the range supported by the operating system.

Why Unix timestamps?

Unix timestamps are unambiguous, efficient, and can easily be converted to other date-time formats using only the standard library.

When a Last-Modified header is semantically correct, this conversion — from string to Unix timestamp and back to string — is lossless. (The only exception is leap seconds; for example Sat, 31 Dec 2016 23:59:60 GMT would be returned as Sun, 01 Jan 2017 00:00:00 GMT. Recipients should interpret these as being identical anyway.)

If you want to store the original string instead, just use is_valid_httpdate() to validate before storing.

License

httpdate is distributed under the terms of the MIT license.