DEPR: enforce deprecation on floats with unit=M or Y (#48671)

Author: jbrockmendel

Diff for: doc/source/whatsnew/v2.0.0.rst

@@ -138,6 +138,15 @@ Deprecations
 -
 -
 
+.. ---------------------------------------------------------------------------
+
+.. _whatsnew_200.prior_deprecations:
+
+Removal of prior version deprecations/changes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+- Disallow passing non-round floats to :class:`Timestamp` with ``unit="M"`` or ``unit="Y"`` (:issue:`47266`)
+-
+
 .. ---------------------------------------------------------------------------
 .. _whatsnew_200.performance:
 

Diff for: pandas/_libs/tslibs/conversion.pyx

@@ -1,9 +1,5 @@
-import warnings
-
 import numpy as np
 
-from pandas.util._exceptions import find_stack_level
-
 cimport numpy as cnp
 from numpy cimport (
     int32_t,
@@ -281,11 +277,8 @@ cdef _TSObject convert_to_tsobject(object ts, tzinfo tz, str unit,
                     # GH#47267 it is clear that 2 "M" corresponds to 1970-02-01,
                     #  but not clear what 2.5 "M" corresponds to, so we will
                     #  disallow that case.
-                    warnings.warn(
-                        "Conversion of non-round float with unit={unit} is ambiguous "
-                        "and will raise in a future version.",
-                        FutureWarning,
-                        stacklevel=find_stack_level(),
+                    raise ValueError(
+                        f"Conversion of non-round float with unit={unit} is ambiguous."
                     )
 
             ts = cast_from_unit(ts, unit)

Diff for: pandas/tests/scalar/timestamp/test_constructors.py

@@ -42,10 +42,11 @@ def test_constructor_int_float_with_YM_unit(self, typ):
     def test_constructor_float_not_round_with_YM_unit_deprecated(self):
         # GH#47267 avoid the conversions in cast_from-unit
 
-        with tm.assert_produces_warning(FutureWarning, match="ambiguous"):
+        msg = "Conversion of non-round float with unit=[MY] is ambiguous"
+        with pytest.raises(ValueError, match=msg):
             Timestamp(150.5, unit="Y")
 
-        with tm.assert_produces_warning(FutureWarning, match="ambiguous"):
+        with pytest.raises(ValueError, match=msg):
             Timestamp(150.5, unit="M")
 
     def test_constructor_datetime64_with_tz(self):