New submission from Paul Ganssle <[email protected]>:
Between Python 2 and Python 3, the meaning of a naive datetime underwent a
subtle change. Previously a naive datetime was mostly treated as an abstract
datetime in the same way a unitless number is treated as an abstract quantity
(this is reflected in the current datetime documentation). In Python 3, though,
it became more concrete in the sense that rather than just throwing an error
whenever you try to do an operation that requires knowing the absolute datetime
(e.g. convert between time zones, convert to timestamp, etc), certain
operations will succeed with the assumption that naive times represent system
local times. This makes `utcnow()` and `utcfromtimestamp()` dangerous, because
they create a naive datetime as if the naive datetime represents UTC, but this
context is then lost and if you ever use one of these "aware-only" operations
(.astimezone, .timestamp, etc), you'll get an unexpected answer.
For example, see this script, executed this with `TZ=America/New_York` on a
machine with IANA data installed:
>>> from datetime import datetime
>>> dt = datetime.utcfromtimestamp(0)
>>> dt
datetime.datetime(1970, 1, 1, 0, 0)
>>> dt.timestamp()
18000
This happens because EST is 18000s behind UTC, and `.timestamp()` gives a
number of seconds after UTC (a concrete, not an abstract time). You get the
same gotchas with `utcnow()`.
I'm not sure if actually deprecating `utcnow` and `utcfromtimestamp` is worth
it at the moment, but we can definitely start by adding a warning box to
`utcnow()` and `utcfromtimestamp()` suggesting that you use the timezone-aware
versions of these instead. We may also want to adjust the opening phrasing for
the "naive objects" portion of the datetime documentation as well
(https://docs.python.org/3.7/library/datetime.html), to reflect the fact that
naive datetimes are no longer purely abstract quantities.
----------
assignee: docs@python
components: Documentation
messages: 347148
nosy: belopolsky, docs@python, p-ganssle
priority: normal
severity: normal
stage: needs patch
status: open
title: Document the "gotcha" behaviors in utcnow() and utcfromtimestamp()
type: enhancement
versions: Python 3.7, Python 3.8, Python 3.9
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Python tracker <[email protected]>
<https://bugs.python.org/issue37488>
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