BUG: Timestamp.to_pydatetime losing 'fold'
State: closed

- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry


Labels:
name: Bug
name: Timezones
description: Timezone data dtype

Comments:
hmm...things failing
@pganssle Not sure what to make of this test failure.  Is it obvious whether the problem is with the current behavior vs with the patch?

```
    def test_tz_localize_ambiguous_compat(self):
        # validate that pytz and dateutil are compat for dst
        # when the transition happens
        naive = Timestamp("2013-10-27 01:00:00")
    
        pytz_zone = "Europe/London"
        dateutil_zone = "dateutil/Europe/London"
        result_pytz = naive.tz_localize(pytz_zone, ambiguous=0)
        result_dateutil = naive.tz_localize(dateutil_zone, ambiguous=0)
        assert result_pytz.value == result_dateutil.value
        assert result_pytz.value == 1382835600000000000
    
        # fixed ambiguous behavior
        # see gh-14621
        assert result_pytz.to_pydatetime().tzname() == "GMT"
>       assert result_dateutil.to_pydatetime().tzname() == "BST"
E       AssertionError: assert 'GMT' == 'BST'
E         - BST
E         + GMT
```
@jbrockmendel looks blocked here. can revisit for 1.4 if that changes.
> Not sure what to make of this test failure. Is it obvious whether the problem is with the current behavior vs with the patch?

I don't really understand why that test is expecting `dateutil` and `pytz` to have different behaviors for this particular datetime. The [documentation for `tz_localize`](https://pandas.pydata.org/docs/reference/api/pandas.Timestamp.tz_localize.html) seems to indicate that `ambiguous` is used to specify which side of the DST transition you want:

> bool contains flags to determine if time is dst or not (note that this flag is only applicable for ambiguous fall dst dates).

It seems unclear whether this means that `0` == DST and `1` == STD, but if that's the case you will run into the [semantic differences between `is_dst` and `fold`](https://pytz-deprecation-shim.readthedocs.io/en/latest/migration.html#semantic-differences-between-is-dst-and-fold) if you try to map `ambiguous` to a specific version of the flag.

In any case, the test at least seems wrong, because of this part:

```python
        assert result_pytz.value == result_dateutil.value
        assert result_pytz.value == 1382835600000000000

        # fixed ambiguous behavior
        # see gh-14621
        assert result_pytz.to_pydatetime().tzname() == "GMT"
        assert result_dateutil.to_pydatetime().tzname() == "BST"
```

If I understand correctly, `.value` is a fixed offset from the unix epoch in nanoseconds. If it's the same for the `pytz` result and the `dateutil` result, then `tzname()` should also be the same. The correct answer for that timestamp is `GMT`:

```python
>>> import zoneinfo
>>> from datetime import datetime
>>> LON = zoneinfo.ZoneInfo("Europe/London")
datetime.datetime(2013, 10, 27, 1, 0, fold=1, tzinfo=zoneinfo.ZoneInfo(key='Europe/London'))
>>> datetime.fromtimestamp(1382835600, tz=LON).tzname()
'GMT'
```

I recommend adding a corresponding test in a zone with negative DST to make sure you aren't doing the wrong thing somewhere in the chain of logic.
@meeseeksdev backport 1.4.x
any issues closed by this?
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