BUG: pd.DateOffset handle milliseconds

doc/source/whatsnew/v2.0.0.rst

@@ -799,6 +799,7 @@ Datetimelike
 - Bug in :class:`Timestamp` was showing ``UserWarning``, which was not actionable by users, when parsing non-ISO8601 delimited date strings (:issue:`50232`)
 - Bug in :func:`to_datetime` was showing misleading ``ValueError`` when parsing dates with format containing ISO week directive and ISO weekday directive (:issue:`50308`)
 - Bug in :func:`to_datetime` was not raising ``ValueError`` when invalid format was passed and ``errors`` was ``'ignore'`` or ``'coerce'`` (:issue:`50266`)
+- Bug in :class:`DateOffset` when constructing with milliseconds and another super-daily argument. A ``TypeError`` was raised due to misuse of underlying :class:`datetime.timedelta` and :class:`dateutil.relativedelta.relativedelta` (:issue:`49897`)
 -
 
 Timedelta

pandas/_libs/tslibs/offsets.pyx

@@ -298,43 +298,54 @@ _relativedelta_kwds = {"years", "months", "weeks", "days", "year", "month",
 
 
 cdef _determine_offset(kwds):
-    # timedelta is used for sub-daily plural offsets and all singular
-    # offsets, relativedelta is used for plural offsets of daily length or
-    # more, nanosecond(s) are handled by apply_wraps
-    kwds_no_nanos = dict(
-        (k, v) for k, v in kwds.items()
-        if k not in ("nanosecond", "nanoseconds")
-    )
-    # TODO: Are nanosecond and nanoseconds allowed somewhere?
-
-    _kwds_use_relativedelta = ("years", "months", "weeks", "days",
-                               "year", "month", "week", "day", "weekday",
-                               "hour", "minute", "second", "microsecond",
-                               "millisecond")
-
-    use_relativedelta = False
-    if len(kwds_no_nanos) > 0:
-        if any(k in _kwds_use_relativedelta for k in kwds_no_nanos):
-            if "millisecond" in kwds_no_nanos:
-                raise NotImplementedError(
-                    "Using DateOffset to replace `millisecond` component in "
-                    "datetime object is not supported. Use "
-                    "`microsecond=timestamp.microsecond % 1000 + ms * 1000` "
-                    "instead."
-                )
-            offset = relativedelta(**kwds_no_nanos)
-            use_relativedelta = True
-        else:
-            # sub-daily offset - use timedelta (tz-aware)
-            offset = timedelta(**kwds_no_nanos)
-    elif any(nano in kwds for nano in ("nanosecond", "nanoseconds")):
-        offset = timedelta(days=0)
-    else:
-        # GH 45643/45890: (historically) defaults to 1 day for non-nano
-        # since datetime.timedelta doesn't handle nanoseconds
-        offset = timedelta(days=1)
-    return offset, use_relativedelta
+    if not kwds:
+        # GH 45643/45890: (historically) defaults to 1 day
+        return timedelta(days=1), False
+
+    nanos = {"nanosecond", "nanoseconds"}
+
+    # nanos are handled by apply_wraps
+    if all(k in nanos for k in kwds):
+        return timedelta(days=0), False
 
+    kwds_no_nanos = {k: v for k, v in kwds.items() if k not in nanos}
+
+    if "millisecond" in kwds_no_nanos:
+        raise NotImplementedError(
+            "Using DateOffset to replace `millisecond` component in "
+            "datetime object is not supported. Use "
+            "`microsecond=timestamp.microsecond % 1000 + ms * 1000` "
+            "instead."
+        )
+
+    kwds_use_relativedelta = {
+        "year", "month", "day", "hour", "minute",
+        "second", "microsecond", "weekday", "years", "months", "weeks", "days",
+        "hours", "minutes", "seconds", "microseconds"
+    }
+
+    # "weeks" and "days" are left out despite being valid args for timedelta,
+    # because (historically) timedelta is used only for sub-daily.
+    kwds_use_timedelta = {
+        "seconds", "microseconds", "milliseconds", "minutes", "hours",
+    }
+
+    if all(k in kwds_use_timedelta for k in kwds_no_nanos):
+        # Sub-daily offset - use timedelta (tz-aware)
+        # This also handles "milliseconds" (plur): see GH 49897
+        return timedelta(**kwds_no_nanos), False
+
+    # convert milliseconds to microseconds, so relativedelta can parse it
+    if "milliseconds" in kwds_no_nanos:
+        micro = kwds_no_nanos.pop("milliseconds") * 1000
+        kwds_no_nanos["microseconds"] = kwds_no_nanos.get("microseconds", 0) + micro
+
+    if all(k in kwds_use_relativedelta for k in kwds_no_nanos):
+        return relativedelta(**kwds_no_nanos), True
+
+    raise ValueError(
+        f"Invalid argument/s or bad combination of arguments: {list(kwds.keys())}"
+    )
 
 # ---------------------------------------------------------------------
 # Mixins & Singletons
@@ -1163,7 +1174,6 @@ cdef class RelativeDeltaOffset(BaseOffset):
 
     def __init__(self, n=1, normalize=False, **kwds):
         BaseOffset.__init__(self, n, normalize)
-
         off, use_rd = _determine_offset(kwds)
         object.__setattr__(self, "_offset", off)
         object.__setattr__(self, "_use_relativedelta", use_rd)

pandas/tests/tseries/offsets/test_offsets.py

@@ -739,6 +739,28 @@ def test_eq(self):
 
         assert DateOffset(milliseconds=3) != DateOffset(milliseconds=7)
 
+    @pytest.mark.parametrize(
+        "offset_kwargs, expected_arg",
+        [
+            ({"microseconds": 1, "milliseconds": 1}, "2022-01-01 00:00:00.001001"),
+            ({"seconds": 1, "milliseconds": 1}, "2022-01-01 00:00:01.001"),
+            ({"minutes": 1, "milliseconds": 1}, "2022-01-01 00:01:00.001"),
+            ({"hours": 1, "milliseconds": 1}, "2022-01-01 01:00:00.001"),
+            ({"days": 1, "milliseconds": 1}, "2022-01-02 00:00:00.001"),
+            ({"weeks": 1, "milliseconds": 1}, "2022-01-08 00:00:00.001"),
+            ({"months": 1, "milliseconds": 1}, "2022-02-01 00:00:00.001"),
+            ({"years": 1, "milliseconds": 1}, "2023-01-01 00:00:00.001"),
+        ],
+    )
+    def test_milliseconds_combination(self, offset_kwargs, expected_arg):
+        # GH 49897
+        offset = DateOffset(**offset_kwargs)
+        ts = Timestamp("2022-01-01")
+        result = ts + offset
+        expected = Timestamp(expected_arg)
+
+        assert result == expected
+
 
 class TestOffsetNames:
     def test_get_offset_name(self):