ENH: improved dtype inference for Index.map

doc/source/whatsnew/v1.4.0.rst

@@ -218,6 +218,8 @@ Other enhancements
   ``USFederalHolidayCalendar``.  See also `Other API changes`_.
 - :meth:`.Rolling.var`, :meth:`.Expanding.var`, :meth:`.Rolling.std`, :meth:`.Expanding.std` now support `Numba <http://numba.pydata.org/>`_ execution with the ``engine`` keyword (:issue:`44461`)
 - :meth:`Series.info` has been added, for compatibility with :meth:`DataFrame.info` (:issue:`5167`)
+- :meth:`UInt64Index.map` now retains ``dtype`` where possible (:issue:`44609`)
+-
 
 
 .. ---------------------------------------------------------------------------

pandas/core/indexes/base.py

@@ -68,6 +68,7 @@
     can_hold_element,
     find_common_type,
     infer_dtype_from,
+    maybe_cast_pointwise_result,
     validate_numeric_casting,
 )
 from pandas.core.dtypes.common import (
@@ -5977,6 +5978,15 @@ def map(self, mapper, na_action=None):
             # empty
             dtype = self.dtype
 
+        # e.g. if we are floating and new_values is all ints, then we
+        #  don't want to cast back to floating.  But if we are UInt64
+        #  and new_values is all ints, we want to try.
+        same_dtype = lib.infer_dtype(new_values, skipna=False) == self.inferred_type
+        if same_dtype:
+            new_values = maybe_cast_pointwise_result(
+                new_values, self.dtype, same_dtype=same_dtype
+            )
+
         if self._is_backward_compat_public_numeric_index and is_numeric_dtype(
             new_values.dtype
         ):

pandas/tests/indexes/common.py

@@ -10,9 +10,7 @@
 
 from pandas.core.dtypes.common import (
     is_datetime64tz_dtype,
-    is_float_dtype,
     is_integer_dtype,
-    is_unsigned_integer_dtype,
 )
 from pandas.core.dtypes.dtypes import CategoricalDtype
 
@@ -557,20 +555,9 @@ def test_map(self, simple_index):
         # callable
         idx = simple_index
 
-        # we don't infer UInt64
-        if is_integer_dtype(idx.dtype):
-            expected = idx.astype("int64")
-        elif is_float_dtype(idx.dtype):
-            expected = idx.astype("float64")
-            if idx._is_backward_compat_public_numeric_index:
-                # We get a NumericIndex back, not Float64Index
-                expected = type(idx)(expected)
-        else:
-            expected = idx
-
         result = idx.map(lambda x: x)
         # For RangeIndex we convert to Int64Index
-        tm.assert_index_equal(result, expected, exact="equiv")
+        tm.assert_index_equal(result, idx, exact="equiv")
 
     @pytest.mark.parametrize(
         "mapper",
@@ -583,27 +570,26 @@ def test_map_dictlike(self, mapper, simple_index):
 
         idx = simple_index
         if isinstance(idx, CategoricalIndex):
+            # TODO(2.0): see if we can avoid skipping once
+            #  CategoricalIndex.reindex is removed.
             pytest.skip(f"skipping tests for {type(idx)}")
 
         identity = mapper(idx.values, idx)
 
-        # we don't infer to UInt64 for a dict
-        if is_unsigned_integer_dtype(idx.dtype) and isinstance(identity, dict):
-            expected = idx.astype("int64")
-        else:
-            expected = idx
-
         result = idx.map(identity)
         # For RangeIndex we convert to Int64Index
-        tm.assert_index_equal(result, expected, exact="equiv")
+        tm.assert_index_equal(result, idx, exact="equiv")
 
         # empty mappable
+        dtype = None
         if idx._is_backward_compat_public_numeric_index:
             new_index_cls = NumericIndex
+            if idx.dtype.kind == "f":
+                dtype = idx.dtype
         else:
             new_index_cls = Float64Index
 
-        expected = new_index_cls([np.nan] * len(idx))
+        expected = new_index_cls([np.nan] * len(idx), dtype=dtype)
         result = idx.map(mapper(expected, idx))
         tm.assert_index_equal(result, expected)
 

pandas/tests/indexes/multi/test_analytics.py

@@ -174,14 +174,8 @@ def test_map(idx):
     # callable
     index = idx
 
-    # we don't infer UInt64
-    if isinstance(index, UInt64Index):
-        expected = index.astype("int64")
-    else:
-        expected = index
-
     result = index.map(lambda x: x)
-    tm.assert_index_equal(result, expected)
+    tm.assert_index_equal(result, index)
 
 
 @pytest.mark.parametrize(

pandas/tests/indexes/numeric/test_numeric.py

@@ -670,3 +670,20 @@ def test_float64_index_equals():
 
     result = string_index.equals(float_index)
     assert result is False
+
+
+def test_map_dtype_inference_unsigned_to_signed():
+    # GH#44609 cases where we don't retain dtype
+    idx = UInt64Index([1, 2, 3])
+    result = idx.map(lambda x: -x)
+    expected = Int64Index([-1, -2, -3])
+    tm.assert_index_equal(result, expected)
+
+
+def test_map_dtype_inference_overflows():
+    # GH#44609 case where we have to upcast
+    idx = NumericIndex(np.array([1, 2, 3], dtype=np.int8))
+    result = idx.map(lambda x: x * 1000)
+    # TODO: we could plausibly try to infer down to int16 here
+    expected = NumericIndex([1000, 2000, 3000], dtype=np.int64)
+    tm.assert_index_equal(result, expected)

pandas/tests/indexes/test_any_index.py

@@ -3,11 +3,8 @@
 """
 import re
 
-import numpy as np
 import pytest
 
-from pandas.core.dtypes.common import is_float_dtype
-
 import pandas._testing as tm
 
 
@@ -49,16 +46,7 @@ def test_mutability(index):
 def test_map_identity_mapping(index):
     # GH#12766
     result = index.map(lambda x: x)
-    if index._is_backward_compat_public_numeric_index:
-        if is_float_dtype(index.dtype):
-            expected = index.astype(np.float64)
-        elif index.dtype == np.uint64:
-            expected = index.astype(np.uint64)
-        else:
-            expected = index.astype(np.int64)
-    else:
-        expected = index
-    tm.assert_index_equal(result, expected, exact="equiv")
+    tm.assert_index_equal(result, index, exact="equiv")
 
 
 def test_wrong_number_names(index):

pandas/tests/indexes/test_base.py

@@ -25,7 +25,6 @@
     period_range,
 )
 import pandas._testing as tm
-from pandas.api.types import is_float_dtype
 from pandas.core.api import (
     Float64Index,
     Int64Index,
@@ -535,11 +534,15 @@ def test_map_dictlike(self, index, mapper):
             # to match proper result coercion for uints
             expected = Index([])
         elif index._is_backward_compat_public_numeric_index:
-            if is_float_dtype(index.dtype):
-                exp_dtype = np.float64
-            else:
-                exp_dtype = np.int64
-            expected = index._constructor(np.arange(len(index), 0, -1), dtype=exp_dtype)
+            expected = index._constructor(
+                np.arange(len(index), 0, -1), dtype=index.dtype
+            )
+        elif type(index) is Index and index.dtype != object:
+            # i.e. EA-backed, for now just Nullable
+            expected = Index(np.arange(len(index), 0, -1), dtype=index.dtype)
+        elif index.dtype.kind == "u":
+            # TODO: case where e.g. we cannot hold result in UInt8?
+            expected = Index(np.arange(len(index), 0, -1), dtype=index.dtype)
         else:
             expected = Index(np.arange(len(index), 0, -1))