DEPR: Index(ndarray[object]) inferring numeric dtype (pandas-dev#49458)

Author: jbrockmendel

doc/source/whatsnew/v2.0.0.rst

@@ -297,6 +297,7 @@ Removal of prior version deprecations/changes
 - Changed the behavior of :func:`to_datetime` with argument "now" with ``utc=False`` to match ``Timestamp("now")`` (:issue:`18705`)
 - Changed behavior of :meth:`SparseArray.astype` when given a dtype that is not explicitly ``SparseDtype``, cast to the exact requested dtype rather than silently using a ``SparseDtype`` instead (:issue:`34457`)
 - Changed behavior of :class:`DataFrame` constructor given floating-point ``data`` and an integer ``dtype``, when the data cannot be cast losslessly, the floating point dtype is retained, matching :class:`Series` behavior (:issue:`41170`)
+- Changed behavior of :class:`Index` constructor when given a ``np.ndarray`` with object-dtype containing numeric entries; this now retains object dtype rather than inferring a numeric dtype, consistent with :class:`Series` behavior (:issue:`42870`)
 - Changed behavior of :class:`DataFrame` constructor when passed a ``dtype`` (other than int) that the data cannot be cast to; it now raises instead of silently ignoring the dtype (:issue:`41733`)
 - Changed the behavior of :class:`Series` constructor, it will no longer infer a datetime64 or timedelta64 dtype from string entries (:issue:`41731`)
 - Changed behavior of :class:`Timestamp` constructor with a ``np.datetime64`` object and a ``tz`` passed to interpret the input as a wall-time as opposed to a UTC time (:issue:`42288`)

pandas/core/indexes/base.py

@@ -653,9 +653,7 @@ def _with_infer(cls, *args, **kwargs):
         Constructor that uses the 1.0.x behavior inferring numeric dtypes
         for ndarray[object] inputs.
         """
-        with warnings.catch_warnings():
-            warnings.filterwarnings("ignore", ".*the Index constructor", FutureWarning)
-            result = cls(*args, **kwargs)
+        result = cls(*args, **kwargs)
 
         if result.dtype == _dtype_obj and not result._is_multi:
             # error: Argument 1 to "maybe_convert_objects" has incompatible type
@@ -7206,13 +7204,8 @@ def _maybe_cast_data_without_dtype(
         if not cast_numeric_deprecated:
             # i.e. we started with a list, not an ndarray[object]
             return result
+        return subarr
 
-        warnings.warn(
-            "In a future version, the Index constructor will not infer numeric "
-            "dtypes when passed object-dtype sequences (matching Series behavior)",
-            FutureWarning,
-            stacklevel=find_stack_level(),
-        )
     result = ensure_wrapped_if_datetimelike(result)
     return result
 

pandas/tests/indexes/numeric/test_numeric.py

@@ -139,8 +139,8 @@ def test_constructor_coerce(self, mixed_index, float_index):
         self.check_coerce(mixed_index, Index([1.5, 2, 3, 4, 5]))
         self.check_coerce(float_index, Index(np.arange(5) * 2.5))
 
-        with tm.assert_produces_warning(FutureWarning, match="will not infer"):
-            result = Index(np.array(np.arange(5) * 2.5, dtype=object))
+        result = Index(np.array(np.arange(5) * 2.5, dtype=object))
+        assert result.dtype == object  # as of 2.0 to match Series
         self.check_coerce(float_index, result.astype("float64"))
 
     def test_constructor_explicit(self, mixed_index, float_index):
@@ -479,12 +479,13 @@ def test_constructor_corner(self, dtype):
         assert index.values.dtype == index.dtype
         if dtype == np.int64:
 
-            msg = "will not infer"
-            with tm.assert_produces_warning(FutureWarning, match=msg):
-                without_dtype = Index(arr)
+            without_dtype = Index(arr)
+            # as of 2.0 we do not infer a dtype when we get an object-dtype
+            #  ndarray of numbers, matching Series behavior
+            assert without_dtype.dtype == object
 
             exact = True if index_cls is Int64Index else "equiv"
-            tm.assert_index_equal(index, without_dtype, exact=exact)
+            tm.assert_index_equal(index, without_dtype.astype(np.int64), exact=exact)
 
         # preventing casting
         arr = np.array([1, "2", 3, "4"], dtype=object)

pandas/tests/indexes/ranges/test_constructors.py

@@ -148,8 +148,7 @@ def test_constructor_corner(self):
         arr = np.array([1, 2, 3, 4], dtype=object)
         index = RangeIndex(1, 5)
         assert index.values.dtype == np.int64
-        with tm.assert_produces_warning(FutureWarning, match="will not infer"):
-            expected = Index(arr).astype("int64")
+        expected = Index(arr).astype("int64")
 
         tm.assert_index_equal(index, expected, exact="equiv")