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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'

FWIW, issue #1777412 added support for years < 1000 to Python 3.3 strftime.

I am not sure this can be fixed without distributing our own implementation of strftime. See issue 3173.

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.

Proposed patch to fix the issue in xmlrpc.client

New changeset 3f025427f02b by Florent Xicluna in branch 'default':
Fix regression due to changeset 2096158376e5 (issue #13305).
http://hg.python.org/cpython/rev/3f025427f02b 

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?

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 

Did your commit fix the issue or not?

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.

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 

other test_time related errors are followed with issue 13309, issue 13312 and issue 13313 

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.

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.

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.

> 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.

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.

New changeset 291d47954618 by Victor Stinner in branch 'default':
Always test datetime.strftime("%4Y")
https://hg.python.org/cpython/rev/291d47954618 

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.

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?

> 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.

>> 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.

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...

New changeset e54224e8d6a9 by Victor Stinner in branch 'default':
Revert change 291d47954618
https://hg.python.org/cpython/rev/e54224e8d6a9 

(See also msg307413 in duplicate Issue32195 for more discussion.)

In issue39103, I filed a bug relating to this issue.
I'd like for Python to provide a portable implementation of strftime instead of just documenting that the version isn't portable.
Given that this ticket assigned to 'docs' suggests that a portable version is out of the question. Can we expand the scope of this issue to actually providing a portable version?

> I'd like for Python to provide a portable implementation of strftime instead of just documenting that the version isn't portable.
If someone wants to do that, I suggest to first start with a project on PyPI. It sounds like a tricky project. Date, time, locales and localization are hard problems!

This also causes an issue when using stftime and strptime together on Linux (Ubuntu 20.04.1 LTS).
When encoding a datetime using strftime and decoding with strptime using the same pattern an error is raised if year < 1000.
Example:
>>> pattern = "%Y-%m-%d"
>>> datetime.strptime(
... datetime(1, 1, 1).strftime(pattern),
... pattern
... )
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/lib/python3.8/_strptime.py", line 568, in _strptime_datetime
 tt, fraction, gmtoff_fraction = _strptime(data_string, format)
 File "/usr/lib/python3.8/_strptime.py", line 349, in _strptime
 raise ValueError("time data %r does not match format %r" %
ValueError: time data '1-01-01' does not match format '%Y-%m-%d'

Has there been any work/progress on this? Alternatively, what suggested work around/mitigations are suggested?

The tempora library implements a [portable strftime](https://tempora.readthedocs.io/en/latest/index.html#tempora.strftime).