feat(rust, python): disallow with_time_zone from/to tz-naive

[MarcoGorelli]

closes #6658

[ritchie46]

Thanks for the PR, Could you help me again in the rationale of this? I don't fully understand.

[MarcoGorelli]

Sure! It's that if people start with a tz-naive datetime, and they want to set a timezone, then they're probably not looking at doing any conversion - they just want to set the time zone

I can imagine many users doing something like:

In [8]: tz_naive
Out[8]:
shape: (1,)
Series: '' [datetime[μs]]
[
        2020-01-01 03:00:00
]

In [9]: tz_naive.dt.with_time_zone('America/Barbados')
Out[9]:
shape: (1,)
Series: '' [datetime[μs, America/Barbados]]
[
        2019-12-31 23:00:00 AST
]

# "hmm wait, that's not what I wanted..."

In [10]: tz_naive.dt.cast_time_zone('America/Barbados')
Out[10]: 
shape: (1,)
Series: '' [datetime[μs, America/Barbados]]
[
        2020-01-01 03:00:00 AST
]

# "ah, that's more like it"

And that's if they look at their output - if they assume that the timezone was set, then it's potentially a hidden bug.

If someone really wants a conversion from UTC, then I think it's better to force them to be explicit about it, and write

tz_naive.dt.cast_time_zone('UTC').dt.with_time_zone('America/Barbados')`

Like this, teaching time zones would be easy:

• with cast_time_zone, you set/unset/change the timezone
• with with_time_zone, you convert from one timezone to another

Whereas currently it's a bit more confusing, because with_time_zone also converts from naive as if it were in UTC

---

For comparison, pandas disallows tz_convert on tz-naive:

In [8]: ser.dt.tz_convert('America/Barbados')
---------------------------------------------------------------------------
TypeError: Cannot convert tz-naive timestamps, use tz_localize to localize

[ritchie46]

Right, makes sense. Thanks!