DEPR: YyM units for Timedelta and friends (pandas-dev#30118)

doc/source/whatsnew/v1.0.0.rst

@@ -539,6 +539,7 @@ or ``matplotlib.Axes.plot``. See :ref:`plotting.formatters` for more.
 - Changed the the default value of `inplace` in :meth:`DataFrame.set_index` and :meth:`Series.set_axis`. It now defaults to ``False`` (:issue:`27600`)
 - Removed the previously deprecated :attr:`Series.cat.categorical`, :attr:`Series.cat.index`, :attr:`Series.cat.name` (:issue:`24751`)
 - :func:`to_datetime` no longer accepts "box" argument, always returns :class:`DatetimeIndex` or :class:`Index`, :class:`Series`, or :class:`DataFrame` (:issue:`24486`)
+- :func:`to_timedelta`, :class:`Timedelta`, and :class:`TimedeltaIndex` no longer allow "M", "y", or "Y" for the "unit" argument (:issue:`23264`)
 - Removed the previously deprecated ``time_rule`` keyword from (non-public) :func:`offsets.generate_range`, which has been moved to :func:`core.arrays._ranges.generate_range` (:issue:`24157`)
 - :meth:`DataFrame.loc` or :meth:`Series.loc` with listlike indexers and missing labels will no longer reindex (:issue:`17295`)
 - :meth:`DataFrame.to_excel` and :meth:`Series.to_excel` with non-existent columns will no longer reindex (:issue:`17295`)

pandas/_libs/tslibs/timedeltas.pyx

@@ -1,6 +1,5 @@
 import collections
 import textwrap
-import warnings
 
 import cython
 
@@ -1204,9 +1203,10 @@ class Timedelta(_Timedelta):
                                  "milliseconds, microseconds, nanoseconds]")
 
         if unit in {'Y', 'y', 'M'}:
-            warnings.warn("M and Y units are deprecated and "
-                          "will be removed in a future version.",
-                          FutureWarning, stacklevel=1)
+            raise ValueError(
+                "Units 'M' and 'Y' are no longer supported, as they do not "
+                "represent unambiguous timedelta values durations."
+            )
 
         if isinstance(value, Timedelta):
             value = value.value

pandas/core/indexes/timedeltas.py

@@ -1,6 +1,5 @@
 """ implement the TimedeltaIndex """
 from datetime import datetime
-import warnings
 
 import numpy as np
 
@@ -201,11 +200,9 @@ def __new__(
             )
 
         if unit in {"Y", "y", "M"}:
-            warnings.warn(
-                "M and Y units are deprecated and "
-                "will be removed in a future version.",
-                FutureWarning,
-                stacklevel=2,
+            raise ValueError(
+                "Units 'M' and 'Y' are no longer supported, as they do not "
+                "represent unambiguous timedelta values durations."
             )
 
         if isinstance(data, TimedeltaArray):

pandas/core/tools/timedeltas.py

@@ -2,8 +2,6 @@
 timedelta support tools
 """
 
-import warnings
-
 import numpy as np
 
 from pandas._libs.tslibs import NaT
@@ -100,10 +98,9 @@ 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 and will be removed in a future version.",
-            FutureWarning,
-            stacklevel=2,
+        raise ValueError(
+            "Units 'M' and 'Y' are no longer supported, as they do not "
+            "represent unambiguous timedelta values durations."
         )
 
     if arg is None:

pandas/tests/indexes/timedeltas/test_timedelta.py

@@ -1,5 +1,4 @@
 from datetime import timedelta
-import re
 
 import numpy as np
 import pytest
@@ -326,11 +325,10 @@ def test_freq_conversion(self):
         tm.assert_index_equal(result, expected)
 
     @pytest.mark.parametrize("unit", ["Y", "y", "M"])
-    def test_unit_m_y_deprecated(self, unit):
-        with tm.assert_produces_warning(FutureWarning) as w:
+    def test_unit_m_y_raises(self, unit):
+        msg = "Units 'M' and 'Y' are no longer supported"
+        with pytest.raises(ValueError, match=msg):
             TimedeltaIndex([1, 3, 7], unit)
-        msg = r".* units are deprecated .*"
-        assert re.match(msg, str(w[0].message))
 
 
 class TestTimeSeries:

pandas/tests/scalar/timedelta/test_timedelta.py

@@ -1,6 +1,5 @@
 """ test the scalar Timedelta """
 from datetime import timedelta
-import re
 
 import numpy as np
 import pytest
@@ -318,12 +317,9 @@ def test_nat_converters(self):
         assert result.dtype.kind == "M"
         assert result.astype("int64") == iNaT
 
-    @pytest.mark.filterwarnings("ignore:M and Y units are deprecated")
     @pytest.mark.parametrize(
         "units, np_unit",
         [
-            (["Y", "y"], "Y"),
-            (["M"], "M"),
             (["W", "w"], "W"),
             (["D", "d", "days", "day", "Days", "Day"], "D"),
             (
@@ -426,19 +422,16 @@ def test_unit_parser(self, units, np_unit, wrapper):
             assert result == expected
 
     @pytest.mark.parametrize("unit", ["Y", "y", "M"])
-    def test_unit_m_y_deprecated(self, unit):
-        with tm.assert_produces_warning(FutureWarning) as w1:
+    def test_unit_m_y_raises(self, unit):
+        msg = "Units 'M' and 'Y' are no longer supported"
+        with pytest.raises(ValueError, match=msg):
             Timedelta(10, unit)
-        msg = r".* units are deprecated .*"
-        assert re.match(msg, str(w1[0].message))
-        with tm.assert_produces_warning(FutureWarning, check_stacklevel=False) as w2:
+
+        with pytest.raises(ValueError, match=msg):
             to_timedelta(10, unit)
-        msg = r".* units are deprecated .*"
-        assert re.match(msg, str(w2[0].message))
-        with tm.assert_produces_warning(FutureWarning, check_stacklevel=False) as w3:
+
+        with pytest.raises(ValueError, match=msg):
             to_timedelta([1, 2], unit)
-        msg = r".* units are deprecated .*"
-        assert re.match(msg, str(w3[0].message))
 
     def test_numeric_conversions(self):
         assert Timedelta(0) == np.timedelta64(0, "ns")