REF: consolidate cast_from_unit checks (#50852)

Author: jbrockmendel

pandas/_libs/tslib.pyx

@@ -220,6 +220,19 @@ def format_array_from_datetime(
     return result
 
 
+cdef int64_t _wrapped_cast_from_unit(object val, str unit) except? -1:
+    """
+    Call cast_from_unit and re-raise OverflowError as OutOfBoundsDatetime
+    """
+    # See also timedeltas._maybe_cast_from_unit
+    try:
+        return cast_from_unit(val, unit)
+    except OverflowError as err:
+        raise OutOfBoundsDatetime(
+            f"cannot convert input {val} with the unit '{unit}'"
+        ) from err
+
+
 def array_with_unit_to_datetime(
     ndarray[object] values,
     str unit,
@@ -261,13 +274,10 @@ def array_with_unit_to_datetime(
         bint is_raise = errors=="raise"
         ndarray[int64_t] iresult
         object tz = None
-        bint is_ym
         float fval
 
     assert is_ignore or is_coerce or is_raise
 
-    is_ym = unit in "YM"
-
     if unit == "ns":
         result, tz = array_to_datetime(
             values.astype(object, copy=False),
@@ -292,19 +302,7 @@ def array_with_unit_to_datetime(
                 if val != val or val == NPY_NAT:
                     iresult[i] = NPY_NAT
                 else:
-                    if is_ym and is_float_object(val) and not val.is_integer():
-                        # Analogous to GH#47266 for Timestamp
-                        raise ValueError(
-                            f"Conversion of non-round float with unit={unit} "
-                            "is ambiguous"
-                        )
-
-                    try:
-                        iresult[i] = cast_from_unit(val, unit)
-                    except OverflowError:
-                        raise OutOfBoundsDatetime(
-                            f"cannot convert input {val} with the unit '{unit}'"
-                        )
+                    iresult[i] = _wrapped_cast_from_unit(val, unit)
 
             elif isinstance(val, str):
                 if len(val) == 0 or val in nat_strings:
@@ -319,23 +317,7 @@ def array_with_unit_to_datetime(
                             f"non convertible value {val} with the unit '{unit}'"
                         )
 
-                    if is_ym and not fval.is_integer():
-                        # Analogous to GH#47266 for Timestamp
-                        raise ValueError(
-                            f"Conversion of non-round float with unit={unit} "
-                            "is ambiguous"
-                        )
-
-                    try:
-                        iresult[i] = cast_from_unit(fval, unit)
-                    except ValueError:
-                        raise ValueError(
-                            f"non convertible value {val} with the unit '{unit}'"
-                        )
-                    except OverflowError:
-                        raise OutOfBoundsDatetime(
-                            f"cannot convert input {val} with the unit '{unit}'"
-                        )
+                    iresult[i] = _wrapped_cast_from_unit(fval, unit)
 
             else:
                 # TODO: makes more sense as TypeError, but that would be an

pandas/_libs/tslibs/conversion.pyx

@@ -108,6 +108,15 @@ cdef int64_t cast_from_unit(object ts, str unit) except? -1:
     if ts is None:
         return m
 
+    if unit in ["Y", "M"] and  is_float_object(ts) and not ts.is_integer():
+        # 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.
+        raise ValueError(
+            f"Conversion of non-round float with unit={unit} "
+            "is ambiguous"
+        )
+
     # cast the unit, multiply base/frace separately
     # to avoid precision issues from float -> int
     base = <int64_t>ts
@@ -287,13 +296,6 @@ cdef _TSObject convert_to_tsobject(object ts, tzinfo tz, str unit,
                     # GH#47266 Avoid cast_from_unit, which would give weird results
                     #  e.g. with "Y" and 150.0 we'd get 2120-01-01 09:00:00
                     return convert_to_tsobject(int(ts), tz, unit, False, False)
-                else:
-                    # 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.
-                    raise ValueError(
-                        f"Conversion of non-round float with unit={unit} is ambiguous."
-                    )
 
             ts = cast_from_unit(ts, unit)
             obj.value = ts