classification
Title: datetime.strftime("%Y") not consistent for years < 1000
Type: behavior Stage: needs patch
Components: Documentation, Library (Lib) Versions: Python 3.6
process
Status: open Resolution:
Dependencies: Superseder: external strftime for Python?
View: 3173
Assigned To: docs@python Nosy List: Elena.Oat, belopolsky, docs@python, ezio.melotti, flox, martin.panter, ned.deily, p-ganssle, python-dev, vstinner
Priority: normal Keywords: easy, patch

Created on 2011-10-31 20:12 by flox, last changed 2018-07-05 15:41 by p-ganssle.

Files
File name Uploaded Description Edit
issue13305_xmlrpc_patch.diff flox, 2011-10-31 23:01 review
doc.patch acucci, 2016-01-02 16:40 review
issue13305.diff Elena.Oat, 2016-03-11 15:41
Messages (25)
msg146740 - (view) Author: Florent Xicluna (flox) * (Python committer) Date: 2011-10-31 20:12
See msg146725 on issue 13291.

on linux
>>> datetime(1,  2, 10, 11, 41, 23).strftime("%Y")
'1'

on osx
>>> datetime(1,  2, 10, 11, 41, 23).strftime("%Y")
'0001'


>>> datetime.strptime('0001', '%Y')
datetime.datetime(1, 1, 1, 0, 0)

>>> datetime.strptime('1', '%Y')
ValueError: time data '1' does not match format '%Y'
msg146743 - (view) Author: Florent Xicluna (flox) * (Python committer) Date: 2011-10-31 20:54
FWIW, issue #1777412 added support for years < 1000 to Python 3.3 strftime.
msg146745 - (view) Author: Alexander Belopolsky (belopolsky) * (Python committer) Date: 2011-10-31 21:04
I am not sure this can be fixed without distributing our own implementation of strftime.  See issue 3173.
msg146750 - (view) Author: Florent Xicluna (flox) * (Python committer) Date: 2011-10-31 22:59
There's many discrepancies between OS X and Linux about time formatting...

OS X
>>> from datetime import datetime
>>> datetime(1900, 1, 1).strftime("%6Y")
'6Y'

Linux
>>> from datetime import datetime
>>> datetime(1900, 1, 1).strftime("%6Y")
'001900'

BTW, these discrepancies are already mentioned:
http://docs.python.org/dev/library/datetime.html#strftime-strptime-behavior

“The full set of format codes supported varies across platforms, because Python calls the platform C library’s strftime() function, and platform variations are common.”

We should had an asterisk to the "%Y" saying that the padding is not consistent across platforms.
msg146751 - (view) Author: Florent Xicluna (flox) * (Python committer) Date: 2011-10-31 23:01
Proposed patch to fix the issue in xmlrpc.client
msg146752 - (view) Author: Roundup Robot (python-dev) (Python triager) Date: 2011-10-31 23:07
New changeset 3f025427f02b by Florent Xicluna in branch 'default':
Fix regression due to changeset 2096158376e5 (issue #13305).
http://hg.python.org/cpython/rev/3f025427f02b
msg146765 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2011-11-01 10:04
Le 01/11/2011 00:07, Roundup Robot a écrit :
>
> Roundup Robot<devnull@psf.upfronthosting.co.za>  added the comment:
>
> New changeset 3f025427f02b by Florent Xicluna in branch 'default':
> Fix regression due to changeset 2096158376e5 (issue #13305).
> http://hg.python.org/cpython/rev/3f025427f02b

I don't like this hack. If there is a bug in time.strftime(), we need to 
fix time.strftime(), not xmlrpclib.

Is there a test for the hack?
msg146767 - (view) Author: Roundup Robot (python-dev) (Python triager) Date: 2011-11-01 11:56
New changeset 230f0956aaa3 by Florent Xicluna in branch 'default':
Strengthen the tests for format '%Y', in relation with issue #13305.
http://hg.python.org/cpython/rev/230f0956aaa3
msg146967 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2011-11-03 19:57
Did your commit fix the issue or not?
msg146972 - (view) Author: Florent Xicluna (flox) * (Python committer) Date: 2011-11-03 20:20
I understand that the issue is because the C standard does not specify the length of the string returned by '%Y'.

The changeset 230f0956aaa3 adds a test to verify that either '%Y' or '%4Y' returns a 4-digits value usable to produce ISO-8601 representations.

Now it is a documentation issue: add a comment to the "%Y" specifier saying that the padding is not applicable to all platforms and in such case the "%4Y" specifier should return the 4-digit value.
msg146976 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2011-11-03 20:30
Since the changeset 55a3b563f0dbed04af317f632f7f3c0f6abe175b, test_strptime is failing on "AMD64 Gentoo Wide 3.x" buildbot:

======================================================================
FAIL: test_strptime (test.test_time.TimeTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.ochtman-gentoo-amd64/build/Lib/test/test_time.py", line 159, in test_strptime
    time.strptime(strf_output, format)
ValueError: time data 'LMT' does not match format '%Z'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.ochtman-gentoo-amd64/build/Lib/test/test_time.py", line 162, in test_strptime
    (format, strf_output))
AssertionError: conversion specifier '%Z' failed with 'LMT' input.

----------------------------------------------------------------------

http://www.python.org/dev/buildbot/all/builders/AMD64%20Gentoo%20Wide%203.x/builds/2661
msg146978 - (view) Author: Florent Xicluna (flox) * (Python committer) Date: 2011-11-03 20:39
other test_time related errors are followed with issue 13309, issue 13312 and issue 13313
msg252165 - (view) Author: Alexander Belopolsky (belopolsky) * (Python committer) Date: 2015-10-02 21:43
Can someone recap the status of this issue?  It is classified as a documentation bug, but I don't see a clear statement of what is wrong with the documentation.
msg257271 - (view) Author: Ezio Melotti (ezio.melotti) * (Python committer) Date: 2016-01-01 04:47
See msg146972 -- AFAIU the padding of %Y is inconsistent for years <1000 (e.g. 0042 vs 42), and this is not documented in the note (2) of https://docs.python.org/3/library/datetime.html#strftime-and-strptime-behavior
Also, using %4Y seems to consistently produce zero-padded values across different platforms.
msg257383 - (view) Author: Martin Panter (martin.panter) * (Python committer) Date: 2016-01-02 22:55
At the bottom of <https://docs.python.org/dev/library/time.html#time.strftime> it suggests that this four-digit field width specifier is not portable. Does that also hold for the datetime version? It seems like a bad idea to recommend an unportable workaround for a portability problem.

Also, in the patch it is not clear if you are referring to strptime(), strftime(), or both. On Linux, datetime.strptime(..., "%4Y") fails for me.
msg257397 - (view) Author: Ezio Melotti (ezio.melotti) * (Python committer) Date: 2016-01-03 06:20
> Also, using %4Y seems to consistently produce zero-padded values across
> different platforms.

Actually I was wrong.  Based on some more testing I did on Linux and Windows 8 and the previous comments in this thread, it seems that, with 1 as year:
* '%4Y' does produces '0001' on Linux but gives a value error on Windows and '4Y' on OS X;
* '%Y' produces '1' on Linux and '0001' on Windows and OS X;

So the problem is only on Linux and only with years <1000, where to have a zero-padded value %4Y can be used instead.
I don't know if these results are true for all versions of Linux/Win/OSX, so the note could just say that:
1) some platforms (e.g. Linux) don't zero-pad years <1000;
2) some platforms (e.g. Linux) support %4Y to add padding, but this doesn't work on other platforms.
msg261577 - (view) Author: Elena Oat (Elena.Oat) * Date: 2016-03-11 15:41
I submitted a patch for documentation changes related to strftime "%Y" directive inconsistencies. I am not sure that specifying the OS is correct, because I haven't tested the inconsistencies on all Linux PCs, Windows versions or OS X. I still left them there though, so let me know what's your opinion on it.
msg261607 - (view) Author: Roundup Robot (python-dev) (Python triager) Date: 2016-03-11 21:36
New changeset 291d47954618 by Victor Stinner in branch 'default':
Always test datetime.strftime("%4Y")
https://hg.python.org/cpython/rev/291d47954618
msg261608 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2016-03-11 21:37
About Elena Oat's patch issue13305.diff: I'm not sure that strftime("%4Y") works on all platforms, so I enabled the test on strftime("%4Y"). I will check our buildbots.
msg261609 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2016-03-11 21:43
acucci's patch doc.patch suggests to use format "%4Y". Problem: I tried it on Linux, and it looks like it doesn't work.

>>> datetime.datetime.strptime("1980", "%Y")
datetime.datetime(1980, 1, 1, 0, 0)
>>> datetime.datetime.strptime("1980", "%4Y")
Traceback (most recent call last):
  ...
ValueError: '4' is a bad directive in format '%4Y'

Or maybe I misunderstood the doc change. Do you suggest to use %4Y format with strptime(), with strftime() or with both?
msg261637 - (view) Author: Ezio Melotti (ezio.melotti) * (Python committer) Date: 2016-03-12 06:33
> About Elena Oat's patch issue13305.diff: I'm not sure that
> strftime("%4Y") works on all platforms

It doesn't, that's what the patch is trying to document.
AFAICT it works on my Linux (but not on yours), it gives '4Y' on Mac, and raises an error on Windows.  This should depend on the libc used by system, so the same OS can give different results depending on which libc it uses, and the only sure thing is that %4Y is not a portable solution.
FWIW the patch LGTM (except a couple minor nits -- zero-pad should be hyphenated and there should be a double space after the dot), but I'm not sure if it's better mentioning platforms at all or just being vague and say that it works on some platforms but not others and leave up to the user figuring out where.
msg261638 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2016-03-12 07:10
>> About Elena Oat's patch issue13305.diff: I'm not sure that
>> strftime("%4Y") works on all platforms

> It doesn't, that's what the patch is trying to document.

Oh. I confirm: %4Y gives "4Y" for any year on FreeBSD and Mac OS X, it raises ValueError("Invalid format string") on Windows, etc.
msg261639 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2016-03-12 07:14
Elena's issue13305.diff:

"Zero padding can be forced on some platforms by using e.g. ``%4Y``"

Sorry, I still doesn't understand the change. %4Y doesn't work with strptime() and it only work with strftime() on very few platforms.

Can you explain when %4Y should be used? strftime() and/or strptime()? Which platform?

To me, it looks wrong to document %4Y since it almost never works...
msg261640 - (view) Author: Roundup Robot (python-dev) (Python triager) Date: 2016-03-12 07:17
New changeset e54224e8d6a9 by Victor Stinner in branch 'default':
Revert change 291d47954618
https://hg.python.org/cpython/rev/e54224e8d6a9
msg307417 - (view) Author: Ned Deily (ned.deily) * (Python committer) Date: 2017-12-01 23:02
(See also msg307413 in duplicate Issue32195 for more discussion.)
History
Date User Action Args
2018-07-05 15:41:41p-gansslesetnosy: + p-ganssle
2017-12-01 23:02:32ned.deilysetnosy: + ned.deily
messages: + msg307417
2017-12-01 23:01:16ned.deilylinkissue32195 superseder
2016-03-12 07:17:11python-devsetmessages: + msg261640
2016-03-12 07:14:52vstinnersetmessages: + msg261639
2016-03-12 07:10:52vstinnersetmessages: + msg261638
2016-03-12 06:33:03ezio.melottisetmessages: + msg261637
2016-03-11 21:43:52vstinnersetmessages: + msg261609
2016-03-11 21:37:44vstinnersetmessages: + msg261608
2016-03-11 21:36:40python-devsetmessages: + msg261607
2016-03-11 15:41:57Elena.Oatsetfiles: + issue13305.diff
nosy: + Elena.Oat
messages: + msg261577

2016-01-03 06:20:49ezio.melottisetmessages: + msg257397
2016-01-02 22:55:38martin.pantersetnosy: + martin.panter
messages: + msg257383
2016-01-02 16:40:37acuccisetfiles: + doc.patch
2016-01-01 04:47:09ezio.melottisetkeywords: + easy

messages: + msg257271
2015-10-02 21:43:06belopolskysetmessages: + msg252165
versions: + Python 3.6, - Python 3.3
2011-11-03 20:39:43floxsetmessages: + msg146978
2011-11-03 20:30:16vstinnersetmessages: + msg146976
2011-11-03 20:20:48floxsetassignee: docs@python
dependencies: - external strftime for Python?
superseder: external strftime for Python?
components: + Documentation

nosy: + docs@python
messages: + msg146972
2011-11-03 19:57:05vstinnersetmessages: + msg146967
2011-11-01 11:56:50python-devsetmessages: + msg146767
2011-11-01 10:04:08vstinnersetmessages: + msg146765
2011-10-31 23:07:19python-devsetnosy: + python-dev
messages: + msg146752
2011-10-31 23:01:06floxsetfiles: + issue13305_xmlrpc_patch.diff
keywords: + patch
messages: + msg146751
2011-10-31 22:59:35floxsetmessages: + msg146750
2011-10-31 21:04:59belopolskysetdependencies: + external strftime for Python?
messages: + msg146745
2011-10-31 20:59:01ezio.melottisetnosy: + ezio.melotti
2011-10-31 20:54:42floxsetversions: - Python 2.7, Python 3.2
nosy: + belopolsky, vstinner

messages: + msg146743

stage: test needed -> needs patch
2011-10-31 20:12:37floxcreate