BUG: Avoid rounding when specifying unit and integer

Add tests

Add whatsnew

flake8

Author: mroeschke

doc/source/whatsnew/v0.23.0.txt

@@ -716,7 +716,7 @@ Datetimelike
 - Bug in :class:`Timestamp` and :func:`to_datetime` where a string representing a barely out-of-bounds timestamp would be incorrectly rounded down instead of raising ``OutOfBoundsDatetime`` (:issue:`19382`)
 - Bug in :func:`Timestamp.floor` :func:`DatetimeIndex.floor` where time stamps far in the future and past were not rounded correctly (:issue:`19206`)
 - Bug in :func:`to_datetime` where passing an out-of-bounds datetime with ``errors='coerce'`` and ``utc=True`` would raise ``OutOfBoundsDatetime`` instead of parsing to ``NaT`` (:issue:`19612`)
--
+- Bug in :class:`Timedelta`: where a numerical value with a unit would round values (:issue: `12690`)
 
 Timezones
 ^^^^^^^^^

pandas/_libs/tslibs/timedeltas.pyx

@@ -200,22 +200,22 @@ cpdef inline int64_t cast_from_unit(object ts, object unit) except? -1:
 
     if unit == 'D' or unit == 'd':
         m = 1000000000L * 86400
-        p = 6
+        p = 9
     elif unit == 'h':
         m = 1000000000L * 3600
-        p = 6
+        p = 9
     elif unit == 'm':
         m = 1000000000L * 60
-        p = 6
+        p = 9
     elif unit == 's':
         m = 1000000000L
-        p = 6
+        p = 9
     elif unit == 'ms':
         m = 1000000L
-        p = 3
+        p = 6
     elif unit == 'us':
         m = 1000L
-        p = 0
+        p = 3
     elif unit == 'ns' or unit is None:
         m = 1L
         p = 0
@@ -229,10 +229,10 @@ cpdef inline int64_t cast_from_unit(object ts, object unit) except? -1:
     # cast the unit, multiply base/frace separately
     # to avoid precision issues from float -> int
     base = <int64_t> ts
-    frac = ts -base
+    frac = ts - base
     if p:
         frac = round(frac, p)
-    return <int64_t> (base *m) + <int64_t> (frac *m)
+    return <int64_t> (base * m) + <int64_t> (frac * m)
 
 
 cdef inline _decode_if_necessary(object ts):

pandas/tests/scalar/test_timedelta.py

@@ -432,6 +432,15 @@ def test_construction(self):
 
         pytest.raises(ValueError, lambda: Timedelta(u'foo bar'))
 
+    @pytest.mark.parametrize("unit, value, expected", [
+        ('us', 9.999, 9999), ('ms', 9.999999, 9999999),
+        ('s', 9.999999999, 9999999999)])
+    def test_rounding_on_int_unit_construction(self, unit, value, expected):
+        result = Timedelta(value, unit=unit)
+        assert result.value == expected
+        result = Timedelta(str(value) + unit)
+        assert result.value
+
     def test_overflow_on_construction(self):
         # xref https://github.com/statsmodels/statsmodels/issues/3374
         value = pd.Timedelta('1day').value * 20169940