[New-bugs-announce] [issue44352] Native Windows Python builds running on Europe/Moscow TZ report wrong time from datetime.datetime.now when there is TZ environment variable also set to Europe/Moscow

[Mike Kaganski]

New submission from Mike Kaganski <mikekaganski at hotmail.com>:

On a Windows 10 system, which TZ is set to Moscow (UTC+3), I use a native Windows Python build (as opposed to e.g. one from Cygwin). Specifically, I tested both custom Python build created by LibreOffice project, as well as the Python by Python Software Foundation available from MS Store [1].

1. Open command prompt on Windows (cmd.exe).
2. Execute 'set TZ=Europe/Moscow'
3. Execute 'python'
4. In the Python prompt, execute 'import datetime'
5. Execute 'datetime.datetime.now()'

The result of this is a time that is two hours off, e.g.

> datetime.datetime(2021, 6, 8, 19, 59, 21, 925240)

instead of proper

> datetime.datetime(2021, 6, 8, 21, 59, 21, 925240)

which appears when step #2 is skipped.
Possibly Python takes both system time zone information *and* the environment variable into account when calculating the time. I suppose it should only consider one or the other, not both.

Note that the problem does not happen with Cygwin's Python, which works fine with the same TZ environment variable value.

For a real-life problem that results from this, see case at [2], where unit test is failing only from 00:00 till 02:00 on my local system, obviously telling me that I should sleep at that time, not try to run unit tests :-)

[1] https://www.microsoft.com/en-us/p/python-38/9mssztt1n39l
[2] https://gerrit.libreoffice.org/c/core/+/92217/2#message-f55091795e7cde9d75adc00ddb69451121b644f6

----------
components: Windows
messages: 395355
nosy: mikekaganski, paul.moore, steve.dower, tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: Native Windows Python builds running on Europe/Moscow TZ report wrong time from datetime.datetime.now when there is TZ environment variable also set to Europe/Moscow
type: behavior
versions: Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue44352>
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