[issue32195] datetime.strftime with %Y no longer outputs leading zeros

[Ned Deily]

Ned Deily  added the comment:

> Isn't this a duplicate of issue 13305?

Looks like it, thanks, though I think the analysis still applies.  Closing as 
duplicate.

--
resolution: not a bug -> duplicate
stage:  -> resolved
status: open -> closed
superseder:  -> datetime.strftime("%Y") not consistent for years < 1000

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[issue32195] datetime.strftime with %Y no longer outputs leading zeros

[Alexander Belopolsky]

Alexander Belopolsky  added the comment:

Isn't this a duplicate of issue 13305?

--
status: pending -> open

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[issue32195] datetime.strftime with %Y no longer outputs leading zeros

[Ned Deily]

Ned Deily  added the comment:

As documented in the Python Library Reference: "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. To see the 
full set of format codes supported on your platform, consult the strftime(3) 
documentation."

And this appears to be one of those differences, presumably another GNU libc vs 
BSD one, as I see the same behavior as macOS on the FreeBSD system I have 
available.

The Open Group 2008 documentation for the strftime C interface discusses this 
problem a bit here 
(http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html):

"The %Y conversion specification to strftime() was frequently assumed to be a 
four-digit year, but the ISO C standard does not specify that %Y is restricted 
to any subset of allowed values from the tm_year field. Similarly, the %C 
conversion specification was assumed to be a two-digit field and the first part 
of the output from the %F conversion specification was assumed to be a 
four-digit field. With tm_year being a signed 32 or more-bit int and with many 
current implementations supporting 64-bit time_t types in one or more 
programming environments, these assumptions are clearly wrong.

POSIX.1-2008 now allows the format specifications %0xC, %0xF, %0xG, and %0xY 
(where 'x' is a string of decimal digits used to specify printing and scanning 
of a string of x decimal digits) with leading zero fill characters. Allowing 
applications to set the field width enables them to agree on the number of 
digits to be printed and scanned in the ISO 8601:2000 standard expanded 
representation of a year (for %F, %G, and %Y ) or all but the last two digits 
of the year (for %C ). This is based on a feature in some versions of GNU 
libc's strftime(). The GNU version allows specifying space, zero, or no-fill 
characters in strftime() format strings, but does not allow any flags to be 
specified in strptime() format strings. These implementations also allow these 
flags to be specified for any numeric field. POSIX.1-2008 only requires the 
zero fill flag ( '0' ) and only requires that it be recognized when processing 
%C, %F, %G, and %Y specifications when a minimum field width is also specified. 
The '0' flag is the only flag needed to produce and scan the ISO 8601:2000 
standard year fields using the extended format forms. POSIX.1-2008 also allows 
applications to specify the same flag and field width specifiers to be used in 
both strftime() and strptime() format strings for symmetry. Systems may provide 
other flag characters and may accept flags in conjunction with conversion 
specifiers other than %C, %F, %G, and %Y; but portable applications cannot 
depend on such extensions."

Experimenting a bit, it seems that the current Linux glibc implementation 
supports the %0xY extension while the current macOS (and other BSD?) do not.

Python 3.6.3 (default, Oct  3 2017, 21:16:13)
[GCC 7.2.0] on linux
>>> datetime.datetime.min.strftime('%Y')
'1'
>>> datetime.datetime.min.strftime('%02Y')
'01'
>>> datetime.datetime.min.strftime('%04Y')
'0001'

Python 3.6.3 (v3.6.3:2c5fed86e0, Oct  3 2017, 00:32:08)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
>>> datetime.datetime.min.strftime('%Y')
'0001'
>>> datetime.datetime.min.strftime('%02Y')
'2Y'
>>> datetime.datetime.min.strftime('%04Y')
'4Y'

So I'm afraid there's not much we can do about that without changing Python's 
policy about using the platform's native implementations of date and time 
functions.

Alexander, do you agree?

--
resolution:  -> not a bug
status: open -> pending

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[issue32195] datetime.strftime with %Y no longer outputs leading zeros

[Serhiy Storchaka]

Change by Serhiy Storchaka :


--
nosy: +ned.deily, ronaldoussoren

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[issue32195] datetime.strftime with %Y no longer outputs leading zeros

[Dave Challis]

Dave Challis  added the comment:

My mistake, it appears to be related to the OS it's running on rather than the 
version (I just happened to test with different versions on different OSes).

On Mac OS X (with 3.6.2):

>>> import datetime
>>> datetime.datetime.min.strftime('%Y')
'0001'

On Linux (with 3.6.2 again):

>>> import datetime
>>> datetime.datetime.min.strftime('%Y')
'1

--

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[issue32195] datetime.strftime with %Y no longer outputs leading zeros

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

In what version datetime.strftime with %Y did output leading zeros?

--
nosy: +belopolsky, serhiy.storchaka

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[issue32195] datetime.strftime with %Y no longer outputs leading zeros

[Dave Challis]

New submission from Dave Challis :

Tested in python 3.6.2:

>>> import datetime
>>> datetime.datetime.min.strftime('%Y')
'1'

Expected output:

'0001'

This means that strftime and strptime aren't necessarily symmetric, e.g.:

>>> datetime.datetime.strptime(datetime.datetime.min.strftime('%Y'), '%Y')
ValueError: time data '1' does not match format '%Y'

--
components: Library (Lib)
messages: 307389
nosy: davechallis
priority: normal
severity: normal
status: open
title: datetime.strftime with %Y no longer outputs leading zeros
type: behavior
versions: Python 3.6

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