[issue40136] add warning to datetime.replace documentation to not use it for setting tzinfo unless UTC or None

[Benedikt Bleimhofer]

Benedikt Bleimhofer  added the comment:

Thx for this really helpful info.
After reading the article i switched all my code from using pytz to dateutil.tz.

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[issue40136] add warning to datetime.replace documentation to not use it for setting tzinfo unless UTC or None

[Paul Ganssle]

Paul Ganssle  added the comment:

That is a specific problem with the third-party library `pytz`, not a standard 
feature of the datetime module. Using `datetime.replace` is the intended way to 
set a time zone, see: 
https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html

As of Python 3.6, we've been recommending dateutil.tz instead of pytz, and 
assuming PEP 615 is accepted ( https://www.python.org/dev/peps/pep-0615/ ), we 
will have a built in time zone type that supports IANA time zones.

I am going to close this because this is not a bug in CPython, but if you think 
otherwise feel free to continue using this ticket to make the case.

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nosy: +p-ganssle
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue40136] add warning to datetime.replace documentation to not use it for setting tzinfo unless UTC or None

[Benedikt Bleimhofer]

New submission from Benedikt Bleimhofer :

It would probably save some people a lot of time if the documentation of 
datetime.replace 
(https://docs.python.org/3/library/datetime.html#datetime.datetime.replace) 
showed a warning not to use it for setting the timezone on a datetime object or 
at least that there is a high chance that the result will not be what you 
expect.

"
If you don't use tz.localize(), but use datetime.replace(), chances are that a 
historical offset is used instead; tz.localize() will pick the right offset in 
effect for the given date.
"
More information on the problem can be found here:
https://stackoverflow.com/questions/13994594/how-to-add-timezone-into-a-naive-datetime-instance-in-python

I ran into this problem and it took me quite some time to figure this out. 
datetime.replace seems more intuitive to use in this case, but since it does 
not work it might be useful to even link to tz.localize.

--
assignee: docs@python
components: Documentation
messages: 365469
nosy: Benedikt Bleimhofer, docs@python
priority: normal
severity: normal
status: open
title: add warning to datetime.replace documentation to not use it for setting 
tzinfo unless UTC or None
type: enhancement

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