ENH: support downcasting of nullable EAs in pd.to_numeric

pandas/core/tools/numeric.py

@@ -7,6 +7,9 @@
     ensure_object,
     is_datetime_or_timedelta_dtype,
     is_decimal,
+    is_extension_array_dtype,
+    is_float_dtype,
+    is_integer_dtype,
     is_number,
     is_numeric_dtype,
     is_scalar,
@@ -15,6 +18,8 @@
 from pandas.core.dtypes.generic import ABCIndex, ABCSeries
 
 import pandas as pd
+from pandas.core.arrays.numeric import NumericArray
+from pandas.core.construction import extract_array
 
 
 def to_numeric(arg, errors="raise", downcast=None):
@@ -118,10 +123,14 @@ def to_numeric(arg, errors="raise", downcast=None):
     is_series = False
     is_index = False
     is_scalars = False
+    is_numeric_extension_dtype = False
 
     if isinstance(arg, ABCSeries):
         is_series = True
         values = arg.values
+        if is_extension_array_dtype(arg) and isinstance(values, NumericArray):
+            is_numeric_extension_dtype = True
+            values = extract_array(arg)
     elif isinstance(arg, ABCIndex):
         is_index = True
         if needs_i8_conversion(arg.dtype):
@@ -142,6 +151,12 @@ def to_numeric(arg, errors="raise", downcast=None):
     else:
         values = arg
 
+    if is_numeric_extension_dtype or (
+        is_extension_array_dtype(arg) and isinstance(values, NumericArray)
+    ):
+        is_numeric_extension_dtype = True
+        mask, values = values._mask, values._data
+
     values_dtype = getattr(values, "dtype", None)
     if is_numeric_dtype(values_dtype):
         pass
@@ -188,6 +203,16 @@ def to_numeric(arg, errors="raise", downcast=None):
                     if values.dtype == dtype:
                         break
 
+    if is_numeric_extension_dtype:
+        if is_integer_dtype(values):
+            from pandas.core.arrays import IntegerArray
+
+            values = IntegerArray(values, mask)
+        elif is_float_dtype(values):
+            from pandas.core.arrays import FloatingArray
+
+            values = FloatingArray(values, mask)
+
     if is_series:
         return arg._constructor(values, index=arg.index, name=arg.name)
     elif is_index:

pandas/tests/tools/test_to_numeric.py

@@ -725,3 +725,19 @@ def test_to_numeric_from_nullable_string(values, expected):
     s = Series(values, dtype="string")
     result = to_numeric(s)
     tm.assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "data, input_dtype, downcast, expected_dtype",
+    (
+        ([1, 1], "Int64", "integer", "Int8"),
+        ([450, 300], "Int64", "integer", "Int16"),
+        ([1, 1], "Float64", "float", "Float32"),
+        ([1, 1], "Float64", "integer", "Int8"),
+    ),
+)
+def test_downcast_nullable_numeric(data, input_dtype, downcast, expected_dtype):
+    arr = pd.array(data, dtype=input_dtype)
+    result = pd.to_numeric(arr, downcast=downcast)
+    expected = pd.array(data, dtype=expected_dtype)
+    tm.assert_extension_array_equal(result, expected)