BUG: Timedelta.__bool__

Closes pandas-dev#21484

Author: TomAugspurger

doc/source/whatsnew/v0.23.2.txt

@@ -82,4 +82,5 @@ Bug Fixes
 
 **Other**
 
+- Bug in `:class:`Timedelta` where timedeltas shorter than 1 microsecond were considered False (:issue:`21484`)
 -

pandas/_libs/tslibs/timedeltas.pyx

@@ -899,6 +899,9 @@ cdef class _Timedelta(timedelta):
     def __str__(self):
         return self._repr_base(format='long')
 
+    def __bool__(self):
+        return self.value != 0
+
     def isoformat(self):
         """
         Format Timedelta as ISO 8601 Duration like

pandas/tests/scalar/timedelta/test_timedelta.py

@@ -588,3 +588,17 @@ def test_components(self):
         result = s.dt.components
         assert not result.iloc[0].isna().all()
         assert result.iloc[1].isna().all()
+
+
+@pytest.mark.parametrize('value, expected', [
+    (Timedelta('10S'), True),
+    (Timedelta('-10S'), True),
+    (Timedelta(10, unit='ns'), True),
+    (Timedelta(0, unit='ns'), False),
+    (Timedelta(-10, unit='ns'), True),
+    (Timedelta(None), True),
+    (pd.NaT, True),
+])
+def test_truthiness(value, expected):
+    # https://github.com/pandas-dev/pandas/issues/21484
+    assert bool(value) is expected