ENH: support Timedelta division with mismatched resos (#48961)

Author: jbrockmendel

pandas/_libs/tslibs/timedeltas.pxd

@@ -25,3 +25,4 @@ cdef class _Timedelta(timedelta):
     cdef _ensure_components(_Timedelta self)
     cdef inline bint _compare_mismatched_resos(self, _Timedelta other, op)
     cdef _Timedelta _as_reso(self, NPY_DATETIMEUNIT reso, bint round_ok=*)
+    cpdef _maybe_cast_to_matching_resos(self, _Timedelta other)

pandas/_libs/tslibs/timedeltas.pyx

@@ -1537,12 +1537,7 @@ cdef class _Timedelta(timedelta):
     def _as_unit(self, str unit, bint round_ok=True):
         dtype = np.dtype(f"m8[{unit}]")
         reso = get_unit_from_dtype(dtype)
-        try:
-            return self._as_reso(reso, round_ok=round_ok)
-        except OverflowError as err:
-            raise OutOfBoundsTimedelta(
-                f"Cannot cast {self} to unit='{unit}' without overflow."
-            ) from err
+        return self._as_reso(reso, round_ok=round_ok)
 
     @cython.cdivision(False)
     cdef _Timedelta _as_reso(self, NPY_DATETIMEUNIT reso, bint round_ok=True):
@@ -1552,9 +1547,26 @@ cdef class _Timedelta(timedelta):
         if reso == self._reso:
             return self
 
-        value = convert_reso(self.value, self._reso, reso, round_ok=round_ok)
+        try:
+            value = convert_reso(self.value, self._reso, reso, round_ok=round_ok)
+        except OverflowError as err:
+            unit = npy_unit_to_abbrev(reso)
+            raise OutOfBoundsTimedelta(
+                f"Cannot cast {self} to unit='{unit}' without overflow."
+            ) from err
+
         return type(self)._from_value_and_reso(value, reso=reso)
 
+    cpdef _maybe_cast_to_matching_resos(self, _Timedelta other):
+        """
+        If _resos do not match, cast to the higher resolution, raising on overflow.
+        """
+        if self._reso > other._reso:
+            other = other._as_reso(self._reso)
+        elif self._reso < other._reso:
+            self = self._as_reso(other._reso)
+        return self, other
+
 
 # Python front end to C extension type _Timedelta
 # This serves as the box for timedelta64
@@ -1834,11 +1846,7 @@ class Timedelta(_Timedelta):
             if other is NaT:
                 return np.nan
             if other._reso != self._reso:
-                raise ValueError(
-                    "division between Timedeltas with mismatched resolutions "
-                    "are not supported. Explicitly cast to matching resolutions "
-                    "before dividing."
-                )
+                self, other = self._maybe_cast_to_matching_resos(other)
             return self.value / float(other.value)
 
         elif is_integer_object(other) or is_float_object(other):
@@ -1865,11 +1873,7 @@ class Timedelta(_Timedelta):
             if other is NaT:
                 return np.nan
             if self._reso != other._reso:
-                raise ValueError(
-                    "division between Timedeltas with mismatched resolutions "
-                    "are not supported. Explicitly cast to matching resolutions "
-                    "before dividing."
-                )
+                self, other = self._maybe_cast_to_matching_resos(other)
             return float(other.value) / self.value
 
         elif is_array(other):
@@ -1897,11 +1901,7 @@ class Timedelta(_Timedelta):
             if other is NaT:
                 return np.nan
             if self._reso != other._reso:
-                raise ValueError(
-                    "floordivision between Timedeltas with mismatched resolutions "
-                    "are not supported. Explicitly cast to matching resolutions "
-                    "before dividing."
-                )
+                self, other = self._maybe_cast_to_matching_resos(other)
             return self.value // other.value
 
         elif is_integer_object(other) or is_float_object(other):
@@ -1920,6 +1920,7 @@ class Timedelta(_Timedelta):
                 if self._reso != NPY_FR_ns:
                     raise NotImplementedError
                 return _broadcast_floordiv_td64(self.value, other, _floordiv)
+
             elif other.dtype.kind in ['i', 'u', 'f']:
                 if other.ndim == 0:
                     return self // other.item()
@@ -1939,11 +1940,7 @@ class Timedelta(_Timedelta):
             if other is NaT:
                 return np.nan
             if self._reso != other._reso:
-                raise ValueError(
-                    "floordivision between Timedeltas with mismatched resolutions "
-                    "are not supported. Explicitly cast to matching resolutions "
-                    "before dividing."
-                )
+                self, other = self._maybe_cast_to_matching_resos(other)
             return other.value // self.value
 
         elif is_array(other):
@@ -2029,7 +2026,7 @@ cdef _broadcast_floordiv_td64(
     Parameters
     ----------
     value : int64_t; `self.value` from a Timedelta object
-    other : object
+    other : ndarray[timedelta64[ns]]
     operation : function, either _floordiv or _rfloordiv
 
     Returns

pandas/tests/scalar/timedelta/test_timedelta.py

@@ -183,14 +183,23 @@ def test_truediv_timedeltalike(self, td):
         assert (2.5 * td) / td == 2.5
 
         other = Timedelta(td.value)
-        msg = "with mismatched resolutions are not supported"
-        with pytest.raises(ValueError, match=msg):
+        msg = "Cannot cast 106752 days 00:00:00 to unit='ns' without overflow"
+        with pytest.raises(OutOfBoundsTimedelta, match=msg):
             td / other
 
-        with pytest.raises(ValueError, match=msg):
+        with pytest.raises(OutOfBoundsTimedelta, match=msg):
             # __rtruediv__
             other.to_pytimedelta() / td
 
+        # if there's no overflow, we cast to the higher reso
+        left = Timedelta._from_value_and_reso(50, NpyDatetimeUnit.NPY_FR_us.value)
+        right = Timedelta._from_value_and_reso(50, NpyDatetimeUnit.NPY_FR_ms.value)
+        result = left / right
+        assert result == 0.001
+
+        result = right / left
+        assert result == 1000
+
     def test_truediv_numeric(self, td):
         assert td / np.nan is NaT
 
@@ -207,14 +216,22 @@ def test_floordiv_timedeltalike(self, td):
         assert (2.5 * td) // td == 2
 
         other = Timedelta(td.value)
-        msg = "with mismatched resolutions are not supported"
-        with pytest.raises(ValueError, match=msg):
+        msg = "Cannot cast 106752 days 00:00:00 to unit='ns' without overflow"
+        with pytest.raises(OutOfBoundsTimedelta, match=msg):
             td // other
 
         with pytest.raises(ValueError, match=msg):
             # __rfloordiv__
             other.to_pytimedelta() // td
 
+        # if there's no overflow, we cast to the higher reso
+        left = Timedelta._from_value_and_reso(50050, NpyDatetimeUnit.NPY_FR_us.value)
+        right = Timedelta._from_value_and_reso(50, NpyDatetimeUnit.NPY_FR_ms.value)
+        result = left // right
+        assert result == 1
+        result = right // left
+        assert result == 0
+
     def test_floordiv_numeric(self, td):
         assert td // np.nan is NaT
 
@@ -259,15 +276,14 @@ def test_addsub_mismatched_reso(self, td):
         assert result.days == 1 - td.days
 
         other2 = Timedelta(500)
-        # TODO: should be OutOfBoundsTimedelta
-        msg = "value too large"
-        with pytest.raises(OverflowError, match=msg):
+        msg = "Cannot cast 106752 days 00:00:00 to unit='ns' without overflow"
+        with pytest.raises(OutOfBoundsTimedelta, match=msg):
             td + other2
-        with pytest.raises(OverflowError, match=msg):
+        with pytest.raises(OutOfBoundsTimedelta, match=msg):
             other2 + td
-        with pytest.raises(OverflowError, match=msg):
+        with pytest.raises(OutOfBoundsTimedelta, match=msg):
             td - other2
-        with pytest.raises(OverflowError, match=msg):
+        with pytest.raises(OutOfBoundsTimedelta, match=msg):
             other2 - td
 
     def test_min(self, td):