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PEP 321 -- Date/Time Parsing and Formatting

Title:Date/Time Parsing and Formatting
Author:A.M. Kuchling <amk at>
Type:Standards Track


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 parsing.

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 [2]
  • ARPA/RFC2822 [1]
  • ctime [4]
  • Formats commonly written by humans such as the American "MM/DD/YYYY", the European "YYYY/MM/DD", and variants such as "DD-Month-YYYY".
  • CVS-style or tar-style dates ("tomorrow", "12 hours ago", etc.)

XXX The Perl [3] 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 functions:

    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 [5] function (Python implementation by Simon Willison at