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For example:
E:\>py -3.6 -c "import datetime; datetime.datetime.fromtimestamp(42)"
Traceback (most recent call last):
 File "<string>", line 1, in <module>
OSError: [Errno 22] Invalid argument
Works fine at least with Python 2.6-2.7 and 3.3-3.5. Only tested on Windows (missing deadsnakes for easy Linux testing).
I was also surprised to get OSError in general, but apparently fromtimestamp uses that instead of ValueError nowadays in some error situations.

This doesn't seem likely that it's anything to do with datetime, could you try reproducing it in repl?

Can recreate successfully on windows, but not on linux:
> C:\Python36\python.exe -c "import datetime; datetime.datetime.fromtimestamp(42)"
Traceback (most recent call last):
 File "<string>", line 1, in <module>
OSError: [Errno 22] Invalid argument
> C:\Python36\python.exe
Python 3.6.0 (v3.6.0:41df79263a11, Dec 23 2016, 08:06:12) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime; datetime.datetime.fromtimestamp(42)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
OSError: [Errno 22] Invalid argument

Cannot reproduce this with the tip on linux

Sounds like it really is an OSError (that is, that the Windows OS is the source of the error). Whether or not we can or should do something about that is a separate question, though.

The 86,399 upperbound comes from this line (max_fold_seconds=86400):
https://github.com/python/cpython/blob/master/Modules/_datetimemodule.c#L4277-L4278
The bug is introduced as part of the fold detection in this commit: https://github.com/python/cpython/commit/56376cacd2a0f6af834fe4d2a20b1a1095afb26f
Window's locatime_s function gives an EINVAL error when time is less than 0. Which is where the OSError comes from.
https://msdn.microsoft.com/en-us/library/a442x3ye.aspx 

I just ran the following script to check if there are any folds from timestamps [0, 86399] in any timezone.
 import datetime
 import pytz
 for tz in pytz.all_timezones:
 tz = pytz.timezone(tz)
 for i in range(86400):
 if datetime.datetime.fromtimestamp(i, tz).fold == 1:
 print(str(tz)) 
and it turns out there aren't any. I highly doubt any timezone is going to retroactively enable/disable DST during the epoch, so a potentially hacky fix is to just have a windows specific check for this value range.
I'm adding the people involved in PEP 495 to the nosy list so they can give their input.

I think we should just clip the negative lower probe value to 0 on Windows before passing it to local(). Also, if we still care about platforms with 32-bit time_t, we should check for over/under flow *before* calling local().

I've attached two patches that fix this behavior, one by simply foregoing the fold detection for this time range on windows (the patch that I'd argue is simpler and more readable)
And one that truncates the passed values to local to not be negative. This one would have been simple but unfortunately there's the complication that there are two local calls and additionally, the second one cannot be truncated to exactly 0 because then we incorrectly detect a fold for the timestamp 0.

> I just ran the following script to check if there are any folds from timestamps [0, 86399] in any timezone.
Ammar, I am not sure pytz timezones support PEP 495 conventions. Can you repeat your experiment using the latest dateutil.tz or test.datetimetester.ZoneInfo from Python 3.6 test suit?
See <http://dateutil.readthedocs.io/en/stable/tz.html>.

Hey, sorry for the late response. I just ran:
 import datetime
 from dateutil.zoneinfo import get_zonefile_instance
 import dateutil.tz
 zonenames = list(get_zonefile_instance().zones)
 for tz in zonenames:
 tz = dateutil.tz.gettz(tz)
 for i in range(86400):
 if datetime.datetime.fromtimestamp(i, tz).fold == 1:
 print(str(tz))
tz uses your OS's zone info so also posting my distro version:
Debian 8.8 - 3.16.0-4-amd64 #1 SMP Debian 3.16.43-2 (2017年04月30日) x86_64 GNU/Linux
And I got the same result, no timezone with folds in these range.

Created a github pull request for the recommended patch along with a test: https://github.com/python/cpython/pull/2385 

Anything holding up the PR? Looks good at a glance, would be nice to get this landed in time for 3.7.

New changeset 96d1e69a12ed8ab80203277e1abdaf573457a964 by Alexander Belopolsky (Ammar Askar) in branch 'master':
bpo-29097: Forego fold detection on windows for low timestamp values (GH-2385)
https://github.com/python/cpython/commit/96d1e69a12ed8ab80203277e1abdaf573457a964

New changeset 973649342cee3869409f341ff0f0e3d2b1547c2a by Alexander Belopolsky (Miss Islington (bot)) in branch '3.7':
bpo-29097: Forego fold detection on windows for low timestamp values (GH-2385) (GH-8466)
https://github.com/python/cpython/commit/973649342cee3869409f341ff0f0e3d2b1547c2a

New changeset 6ea8a3a0ebf840ca57b6dba9cad26fbb0ddaa5d4 by Alexander Belopolsky (Ammar Askar) in branch '3.6':
[3.6] bpo-29097: Forego fold detection on windows for low timestamp values (GH-2385) (GH-8498)
https://github.com/python/cpython/commit/6ea8a3a0ebf840ca57b6dba9cad26fbb0ddaa5d4