[New-bugs-announce] [issue5979] strptime() gives inconsistent exceptions

Ryan Leslie report at bugs.python.org
Sat May 9 21:14:10 CEST 2009

New submission from Ryan Leslie <rylesny at gmail.com>:


>>> from datetime import datetime
>>> datetime.strptime("19951001", "%Y%m%d")
datetime.datetime(1995, 10, 1, 0, 0)
>>> datetime.strptime("19951000", "%Y%m%d") # day = 0, month < 11
ValueError: time data '19951000' does not match format '%Y%m%d'
>>> datetime.strptime("19951100", "%Y%m%d") # day = 0, month >= 11
ValueError: unconverted data remains: 0

The exception messages are not really a serious issue, but note that the
latter one can be confusing for users.

However, there seems to be some underlying issues with the choice to
recognize single digit months with double digit days, which can make
date strings ambiguous:

Consider "19951100" from above with the last '0' removed.

>>> datetime.strptime("1995110", "%Y%m%d")
datetime.datetime(1995, 1, 10, 0, 0)

In this case, strptime has treated the middle '1' as the month,
resulting in 1995-01-10. This hints at why the second exception from
above gives a strange message: with the extra '0' the day portion of
"19951100" (00) is invalid, and strptime falls back on parsing the first
7 characters as above, and then failing due to the remaining '0'.

This seems a little dangerous. For instance:
timestamp = "19951101-23:20:18"
datestamp=timestamp[:7] # Oops, needed to use :8 to get up to index 7.
reallyImportantButWrongDate = datetime.strptime(datestamp, "%Y%m%d")

Note that in this case strptime() from glibc would instead result in an
error, which IMHO is the right thing to do.

I do realize, though, that changing the behavior of strptime() could
devastate some existing code.

components: Library (Lib)
messages: 87505
nosy: ryles
severity: normal
status: open
title: strptime() gives inconsistent exceptions
type: behavior
versions: Python 2.4, Python 2.5, Python 2.6, Python 2.7, Python 3.0, Python 3.1

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