New submission from Paul Ganssle <b...@m.ganssle.io>:
As part of bpo-35829, it was suggested that we add the ability to output the
"Z" suffix in `isoformat()`, so that `fromisoformat()` can both be the exact
functional inverse of `isoformat()` and parse datetimes with "Z" outputs. I
think that that's not a particularly compelling motivation for this, but I also
see plenty of examples of `datetime.utcnow().isoformat() + "Z"` out there, so
it seems like this is a feature that we would want to have *anyway*,
particularly if we want to deprecate and remove `utcnow`.
I've spun this off into its own issue so that we can discuss how to implement
the feature. The two obvious questions I see are:
1. What do we call the option? `use_utc_designator`, `allow_Z`, `utc_as_Z`?
2. What do we consider as "UTC"? Is it anything with +00:00? Just
`timezone.utc`? Anything that seems like a fixed-offset zone with 0 offset?
For example, do we want this?
>>> LON = zoneinfo.ZoneInfo("Europe/London")
>>> datetime(2022, 3, 1, tzinfo=LON).isoformat(utc_as_z=True)
2022-03-01T00:00:00Z
>>> datetime(2022, 6, 1, tzinfo=LON).isoformat(utc_as_z=True)
2022-06-01T00:00:00+01:00
Another possible definition might be if the `tzinfo` is a fixed-offset zone
with offset 0:
>>> datetime.timezone.utc.utcoffset(None)
timedelta(0)
>>> zoneinfo.ZoneInfo("UTC").utcoffset(None)
timedelta(0)
>>> dateutil.tz.UTC.utcoffset(None)
timedelta(0)
>>> pytz.UTC.utcoffset(None)
timedelta(0)
The only "odd man out" is `dateutil.tz.tzfile` objects representing fixed
offsets, since all `dateutil.tz.tzfile` objects return `None` when `utcoffset`
or `dst` are passed `None`. This can and will be changed in future versions.
I feel like "If the offset is 00:00, use Z" is the wrong rule to use
conceptually, but considering that people will be opting into this behavior, it
is more likely that they will be surprised by `datetime(2022, 3, 1,
tzinfo=ZoneInfo("Europe/London").isoformat(utc_as_z=True)` returning
`2022-03-01T00:00:00+00:00` than alternation between `Z` and `+00:00`.
Yet another option might be to add a completely separate function,
`utc_isoformat(*args, **kwargs)`, which is equivalent to (in the parlance of
the other proposal) `dt.astimezone(timezone.utc).isoformat(*args, **kwargs,
utc_as_z=True)`. Basically, convert any datetime to UTC and append a Z to it.
The biggest footgun there would be people using it on naïve datetimes and not
realizing that it would interpret them as system local times.
----------
assignee: p-ganssle
components: Library (Lib)
messages: 412384
nosy: belopolsky, brett.cannon, p-ganssle
priority: normal
severity: normal
stage: needs patch
status: open
title: Add option to output UTC datetimes as "Z" in `.isoformat()`
type: enhancement
versions: Python 3.11
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue46614>
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