BUG: Index.get_value implementation for ExtensionArray (pandas-dev#29926)

Author: jorisvandenbossche

pandas/core/indexes/base.py

@@ -69,6 +69,7 @@
 from pandas.core.arrays import ExtensionArray
 from pandas.core.base import IndexOpsMixin, PandasObject
 import pandas.core.common as com
+from pandas.core.construction import extract_array
 from pandas.core.indexers import maybe_convert_indices
 from pandas.core.indexes.frozen import FrozenList
 import pandas.core.missing as missing
@@ -4489,22 +4490,26 @@ def get_value(self, series, key):
         # if we have something that is Index-like, then
         # use this, e.g. DatetimeIndex
         # Things like `Series._get_value` (via .at) pass the EA directly here.
-        s = getattr(series, "_values", series)
-        if isinstance(s, (ExtensionArray, Index)) and is_scalar(key):
-            # GH 20882, 21257
-            # Unify Index and ExtensionArray treatment
-            # First try to convert the key to a location
-            # If that fails, raise a KeyError if an integer
-            # index, otherwise, see if key is an integer, and
-            # try that
-            try:
-                iloc = self.get_loc(key)
-                return s[iloc]
-            except KeyError:
-                if len(self) > 0 and (self.holds_integer() or self.is_boolean()):
-                    raise
-                elif is_integer(key):
-                    return s[key]
+        s = extract_array(series, extract_numpy=True)
+        if isinstance(s, ExtensionArray):
+            if is_scalar(key):
+                # GH 20882, 21257
+                # First try to convert the key to a location
+                # If that fails, raise a KeyError if an integer
+                # index, otherwise, see if key is an integer, and
+                # try that
+                try:
+                    iloc = self.get_loc(key)
+                    return s[iloc]
+                except KeyError:
+                    if len(self) > 0 and (self.holds_integer() or self.is_boolean()):
+                        raise
+                    elif is_integer(key):
+                        return s[key]
+            else:
+                # if key is not a scalar, directly raise an error (the code below
+                # would convert to numpy arrays and raise later any way) - GH29926
+                raise InvalidIndexError(key)
 
         s = com.values_from_object(series)
         k = com.values_from_object(key)

pandas/tests/extension/decimal/test_decimal.py

@@ -478,3 +478,24 @@ def DecimalArray__my_sum(self):
     s = pd.Series(DecimalArray(data))
     result = s.groupby(np.array([0, 0, 0, 1, 1])).agg(lambda x: x.values.my_sum())
     tm.assert_series_equal(result, expected, check_names=False)
+
+
+def test_indexing_no_materialize(monkeypatch):
+    # See https://github.com/pandas-dev/pandas/issues/29708
+    # Ensure that indexing operations do not materialize (convert to a numpy
+    # array) the ExtensionArray unnecessary
+
+    def DecimalArray__array__(self, dtype=None):
+        raise Exception("tried to convert a DecimalArray to a numpy array")
+
+    monkeypatch.setattr(DecimalArray, "__array__", DecimalArray__array__, raising=False)
+
+    data = make_data()
+    s = pd.Series(DecimalArray(data))
+    df = pd.DataFrame({"a": s, "b": range(len(s))})
+
+    # ensure the following operations do not raise an error
+    s[s > 0.5]
+    df[s > 0.5]
+    s.at[0]
+    df.at[0, "a"]