fix: keep naive datetimes naive when building an Interval

`DateTime.diff()` constructs an `Interval`, whose `__init__` normalised
the start and end values via `pendulum.instance()`. That call used the
default `tz=UTC`, so a naive stdlib `datetime` was turned into a
UTC-aware `DateTime`. When the receiver was itself naive, the subsequent
comparison mixed an offset-naive value with an offset-aware one and
raised:

    TypeError: can't compare offset-naive and offset-aware datetimes

`Interval.__new__` already validates that both endpoints share the same
awareness, so forcing a timezone here was both unnecessary and the
source of the bug. Pass each value's own `tzinfo` to `instance()`
instead: naive inputs stay naive and aware inputs keep their timezone
(`DateTime.instance` resolves `tz` as `dt.tzinfo or tz`), so behaviour
for aware datetimes is unchanged.

Fixes #880

Author: mokashang

src/pendulum/interval.py

@@ -122,7 +122,7 @@ def __init__(self, start: _T, end: _T, absolute: bool = False) -> None:
         _start: _T
         if not isinstance(start, pendulum.Date):
             if isinstance(start, datetime):
-                start = cast("_T", pendulum.instance(start))
+                start = cast("_T", pendulum.instance(start, tz=start.tzinfo))
             else:
                 start = cast("_T", pendulum.date(start.year, start.month, start.day))
 
@@ -148,7 +148,7 @@ def __init__(self, start: _T, end: _T, absolute: bool = False) -> None:
         _end: _T
         if not isinstance(end, pendulum.Date):
             if isinstance(end, datetime):
-                end = cast("_T", pendulum.instance(end))
+                end = cast("_T", pendulum.instance(end, tz=end.tzinfo))
             else:
                 end = cast("_T", pendulum.date(end.year, end.month, end.day))
 

tests/datetime/test_diff.py

@@ -198,6 +198,18 @@ def test_diff_in_seconds_with_timezones():
     assert dt_ottawa.diff(dt_vancouver).in_seconds() == 3 * 60 * 60
 
 
+def test_diff_naive_with_naive_stdlib_datetime():
+    # A naive DateTime diffed against a naive stdlib datetime should not
+    # force the latter to UTC, which used to make it offset-aware and raise
+    # "can't compare offset-naive and offset-aware datetimes". See #880.
+    dt = pendulum.naive(2016, 7, 5, 12, 0, 0)
+    other = datetime(2016, 7, 5, 13, 0, 0)
+
+    assert dt.diff(other).in_seconds() == 3600
+    assert dt.diff(other, False).in_seconds() == 3600
+    assert other.tzinfo is None
+
+
 def test_diff_for_humans_now_and_second():
     with pendulum.travel_to(pendulum.datetime(2012, 1, 1, 1, 2, 3), freeze=True):
         assert pendulum.now().diff_for_humans() == "a few seconds ago"