PEP 321: Date/Time Parsing and Formatting

Gerrit Holl gerrit at
Mon Nov 17 17:17:52 CET 2003

Posted with permission from the author.
I have some comments on this PEP, see the (coming) followup to this message.

PEP: 321
Title: Date/Time Parsing and Formatting
Version: $Revision: 1.3 $
Last-Modified: $Date: 2003/10/28 19:48:44 $
Author: A.M. Kuchling <amk at>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Python-Version: 2.4
Created: 16-Sep-2003


Python 2.3 added a number of simple date and time types in the
``datetime`` module.  There's no support for parsing strings in various
formats and returning a corresponding instance of one of the types.  
This PEP proposes adding a family of predefined parsing function for
several commonly used date and time formats, and a facility for generic 

The types provided by the ``datetime`` module all have
``.isoformat()`` and ``.ctime()`` methods that return string
representations of a time, and the ``.strftime()`` method can be used
to construct new formats.  There are a number of additional
commonly-used formats that would be useful to have as part of the
standard library; this PEP also suggests how to add them.

Input Formats

Useful formats to support include:

* `ISO8601`_
* ARPA/`RFC2822`_
* `ctime`_
* Formats commonly written by humans such as the American
  "MM/DD/YYYY", the European "YYYY/MM/DD", and variants such as
* CVS-style or tar-style dates ("tomorrow", "12 hours ago", etc.)

XXX The Perl ``_ module supports many different input formats,
both absolute and relative.  Should we try to support them all?


1) Add functions to the ``datetime`` module::

	import datetime
	d = datetime.parse_iso8601("2003-09-15T10:34:54")
2) Add class methods to the various types.  There are already various 
   class methods such as ``.now()``, so this would be pretty natural.::

	import datetime
	d ="2003-09-15T10:34:54")
3) Add a separate module (possible names: date, date_parse, parse_date)
   or subpackage (possible names: datetime.parser) containing parsing 
   	import datetime
   	d = datetime.parser.parse_iso8601("2003-09-15T10:34:54")
Unresolved questions:

* Naming convention to use.
* What exception to raise on errors?  ValueError, or a specialized exception?
* Should you know what type you're expecting, or should the parsing figure
  it out?  (e.g. ``parse_iso8601("yyyy-mm-dd")`` returns a ``date`` instance,
  but parsing "yyyy-mm-ddThh:mm:ss" returns a ``datetime``.)  Should 
  there be an option to signal an error if a time is provided where
  none is expected, or if no time is provided?
* Anything special required for I18N?  For time zones?

Generic Input Parsing

Is a strptime() implementation that returns ``datetime`` types sufficient?

XXX if yes, describe strptime here.  Can the existing pure-Python
implementation be easily retargeted?

Output Formats

Not all input formats need to be supported as output formats, because it's 
pretty trivial to get the ``strftime()`` argument right for simple things 
such as YYYY/MM/DD.   Only complicated formats need to be supported; RFC2822
is currently the only one I can think of.


1) Provide predefined format strings, so you could write this::

	import datetime
	d = datetime.datetime(...)
	print d.strftime(d.RFC2822_FORMAT) # or datetime.RFC2822_FORMAT?
2) Provide new methods on all the objects::
	d = datetime.datetime(...)
	print d.rfc822_time()

Relevant functionality in other languages includes the `PHP date`_
function (Python implementation by Simon Willison at


.. _RFC2822:

.. _ISO8601:


.. _ctime:

.. _PHP date:

Other useful links:


This document has been placed in the public domain.


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