[backport 3.0.x] BUG: fix to_timedelta ignoring unit for mixed round/non-round floats (#65170)

doc/source/whatsnew/v3.0.3.rst

@@ -15,6 +15,7 @@ Fixed regressions
 ~~~~~~~~~~~~~~~~~
 
 - Fixed reading of Parquet files with timezone-aware timestamps or localizing of a timeseries with a ``pytz`` timezone when an older version of ``pytz`` was installed (:issue:`64978`)
+- Fixed regression in :func:`to_timedelta` ignoring the ``unit`` argument for round float values when mixed with non-round floats in a list (:issue:`65150`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_303.bug_fixes:

pandas/_libs/tslibs/timedeltas.pyx

@@ -484,13 +484,15 @@ def array_to_timedelta64(
                 if item == NPY_NAT:
                     ival = NPY_NAT
                 else:
-                    ival = _numeric_to_td64ns(item, parsed_unit, int_reso)
+                    # GH#65150 match the pattern in tslib.pyx: update creso
+                    #  first, then convert using creso as the output resolution.
                     item_reso = int_reso
-
                     state.update_creso(item_reso)
                     if infer_reso:
                         creso = state.creso
 
+                    ival = _numeric_to_td64ns(item, parsed_unit, creso)
+
             elif is_float_object(item):
                 int_item = int(item)
                 if item == int_item:
@@ -501,21 +503,20 @@ def array_to_timedelta64(
                     if item == NPY_NAT:
                         ival = NPY_NAT
                     else:
-                        ival = _numeric_to_td64ns(item, parsed_unit, int_reso)
                         item_reso = int_reso
-
                         state.update_creso(item_reso)
                         if infer_reso:
                             creso = state.creso
-                else:
-                    ival = _numeric_to_td64ns(item, parsed_unit, NPY_FR_ns)
 
+                        ival = _numeric_to_td64ns(item, parsed_unit, creso)
+                else:
                     item_reso = NPY_FR_ns
-                    int_reso = NPY_FR_ns
                     state.update_creso(item_reso)
                     if infer_reso:
                         creso = state.creso
 
+                    ival = _numeric_to_td64ns(item, parsed_unit, creso)
+
             else:
                 raise TypeError(f"Invalid type for timedelta scalar: {type(item)}")
 

pandas/tests/tools/test_to_timedelta.py

@@ -358,6 +358,18 @@ def test_to_timedelta_unit_non_round_floats(self):
         result2 = to_timedelta(arr2, unit="s")
         assert result2.unit == "ns"
 
+    def test_to_timedelta_unit_mixed_round_and_non_round_floats(self):
+        # GH#65150 - round floats mixed with non-round floats should
+        # respect the unit for all values
+        expected = to_timedelta(["0 days 00:00:01", "0 days 00:00:01.01"]).as_unit("ns")
+
+        result = to_timedelta([1.0, 1.01], unit="s")
+        tm.assert_index_equal(result, expected)
+
+        # Also test integers mixed with non-round floats
+        result2 = to_timedelta([1, 1.01], unit="s")
+        tm.assert_index_equal(result2, expected)
+
 
 def test_from_numeric_arrow_dtype(any_numeric_ea_dtype):
     # GH 52425