Added M and Y units deprecated test and whats new note

Author: ryankarlos

doc/source/whatsnew/v0.24.0.rst

@@ -1045,6 +1045,7 @@ Deprecations
 - The ``keep_tz=False`` option (the default) of the ``keep_tz`` keyword of
   :meth:`DatetimeIndex.to_series` is deprecated (:issue:`17832`).
 - Timezone converting a tz-aware ``datetime.datetime`` or :class:`Timestamp` with :class:`Timestamp` and the ``tz`` argument is now deprecated. Instead, use :meth:`Timestamp.tz_convert` (:issue:`23579`)
+- Deprecated the `M` and `Y` `units` parameter of :func: `pandas.to_timedelta`, :func: `pandas.Timedelta` and :func: `pandas.TimedeltaIndex`(:issue:`16344`)
 
 .. _whatsnew_0240.deprecations.datetimelike_int_ops:
 

pandas/core/tools/timedeltas.py

@@ -93,6 +93,10 @@ def to_timedelta(arg, unit='ns', box=True, errors='raise'):
         raise ValueError("errors must be one of 'ignore', "
                          "'raise', or 'coerce'}")
 
+    if unit in ['Y', 'y', 'M']:
+        warnings.warn("M and Y units are deprecated.",
+                      FutureWarning, stacklevel=2)
+
     if arg is None:
         return arg
     elif isinstance(arg, ABCSeries):

pandas/tests/scalar/timedelta/test_timedelta.py

@@ -354,6 +354,14 @@ def test_unit_parser(self, units, np_unit, wrapper):
             result = Timedelta('2{}'.format(unit))
             assert result == expected
 
+    @pytest.mark.parametrize('units', ['Y', 'y', 'M'])
+    def test_unit_M_Y_deprecated(self, units):
+        for unit in units:
+            with tm.assert_produces_warning(FutureWarning):
+                to_timedelta(10, unit)
+                TimedeltaIndex([1, 1, 1], unit)
+                Timedelta(10, unit)
+
     def test_numeric_conversions(self):
         assert ct(0) == np.timedelta64(0, 'ns')
         assert ct(10) == np.timedelta64(10, 'ns')