On 1/28/19, David Raymond <david.raym...@tomtom.com> wrote:
>
> In the docs there is
> "0 is a legal argument for any position in the time tuple; if it is normally
> illegal the value is forced to a correct one."
The POSIX specification for strftime only states that "[i]f any of the
specified values are outside the normal range, the characters stored
are unspecified" [1]. Python tries to bring some semblance of order
across the various implementations of this function.
For tm_year, it was eventually decided circa Python 3.2 to support the
full signed range on most platforms, even if isn't a normal date, such
as year 0 and negative year values. For example, in 3.6.7 in Linux:
>>> time.strftime("%Y = %C * 100 + %y", (0, 0, 0, 0, 0, 0, 0, 0, 0))
'0 = 0 * 100 + 00'
>>> time.strftime("%Y = %C * 100 + %y", (-10, 0, 0, 0, 0, 0, 0, 0, 0))
'-10 = -1 * 100 + 90'
2.7.15 in Linux has different behavior, especially if
time.accept2dyear is true (set by default unless the environment
variable PYTHONY2K is defined).
>>> time.accept2dyear
1
>>> time.strftime("%Y = %C * 100 + %y", (0, 0, 0, 0, 0, 0, 0, 0, 0))
'2000 = 20 * 100 + 00'
>>> time.strftime("%Y = %C * 100 + %y", (-10, 0, 0, 0, 0, 0, 0, 0, 0))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: year out of range
The magical year value of 2000 comes from code to support pre-Y2K
scripts that assume years 69-99 are 1969-1999 and years 0-68 are
2000-2068. If we force time.accept2dyear to false, we see that Python
2 actually requires the year to be at least 1900.
>>> time.accept2dyear = 0
>>> time.strftime("%Y = %C * 100 + %y", (0, 0, 0, 0, 0, 0, 0, 0, 0))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: year >= 1900 required
> and yet if given 0 for the year/first item in the tuple it raises a
> ValueError.
>
> Is that a bug in the code, the documentation, or my understanding?
It's not a bug. However, in recent versions of the Windows C runtime,
"%y" does support year values down to 0 (but not negative years), so
the check [2] can be relaxed down to 0 (whatever year 0 is supposed to
be), and the additional check for "%y" can be removed [3].
[1]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html
[2]: https://github.com/python/cpython/blob/v3.7.2/Modules/timemodule.c#L721
[3]: https://github.com/python/cpython/blob/v3.7.2/Modules/timemodule.c#L760
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