classification
Title: Ambiguous datetime comparisons should use == rather than 'is' for tzinfo comparison
Type: behavior Stage: needs patch
Components: Library (Lib) Versions: Python 3.7, Python 3.6
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: belopolsky Nosy List: belopolsky, p-ganssle, tim.peters
Priority: normal Keywords:

Created on 2016-11-03 18:37 by p-ganssle, last changed 2016-11-03 18:46 by belopolsky.

Messages (2)
msg280003 - (view) Author: Paul Ganssle (p-ganssle) * Date: 2016-11-03 18:37
According to PEP495 (https://www.python.org/dev/peps/pep-0495/#aware-datetime-equality-comparison) datetimes are considered not equal if they are an ambiguous time and have different zones. However, currently "interzone comparison" is defined / implemented as the zones being the same object rather than the zones comparing equal.

One issue with this is that it actually breaks backwards compatibility of the language, because there doesn't seem to be a way to provide a (backwards-compatible) class that implements folding behavior and has equivalent dates compare equal. An example using python-dateutil:

```
from datetime import datetime
from dateutil import tz

NYC = tz.gettz('America/New_York')
ET = tz.gettz('US/Eastern')

dt = datetime(2011, 11, 6, 5, 30, tzinfo=tz.tzutc()) # This is 2011-11-06 01:30 EDT-4

dt_edt = dt.astimezone(ET)
dt_nyc = dt.astimezone(NYC)

print(dt_nyc == dt_edt)
```

In Python 3.5 that will return True, in Python 3.6 it will return False, even though 'US/Eastern' and 'America/New_York' are the same zone. In this case, I might be able to enforce that these time zones are singletons so that `is` always returns True (though this may have other negative consequences for utility), but even that solution would fall apart for things like `tzrange` and `tzstr`, where you can know that the `dt.utcoffset()`s are going to be identical for ALL values of `dt`, but you can't force the objects to be identical.

I would suggest that it be changed to use `__eq__` to determine whether two `tzinfo` objects are the same zone, as this will allow tzinfo providers to create `tzinfo` objects with a consistent behavior between versions in this edge case.
msg280005 - (view) Author: Alexander Belopolsky (belopolsky) * (Python committer) Date: 2016-11-03 18:46
See Datetime-SIG thread <https://mail.python.org/pipermail/datetime-sig/2016-November/001010.html>.
History
Date User Action Args
2016-11-03 18:46:38belopolskysetnosy: + tim.peters
messages: + msg280005

assignee: belopolsky
stage: needs patch
2016-11-03 18:37:19p-gansslecreate