GH-50429: [Python] Convert maps to dicts without per-element Scalars in to_pylist

python/pyarrow/array.pxi

@@ -1865,23 +1865,16 @@ cdef class Array(_PandasConvertible):
         """
         self._assert_cpu()
         cdef int64_t i, n = self.length()
-        if maps_as_pydicts is not None:
-            # Converting maps to dicts has per-entry semantics (duplicate-key
-            # detection); use the Scalar-based conversion for exact behavior.
-            # TODO(GH-50429): this falls back to the Scalar path for the whole
-            # array even when the type contains no maps; threading
-            # maps_as_pydicts through _getitem_py keeps the fast paths instead.
-            return [x.as_py(maps_as_pydicts=maps_as_pydicts) for x in self]
         # TODO(GH-50448): convert per range instead of per element to cut
         # the per-element call overhead further.
-        return [self._getitem_py(i) for i in range(n)]
+        return [self._getitem_py(i, maps_as_pydicts) for i in range(n)]
 
-    cdef object _getitem_py(self, int64_t i):
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts):
         # Return self[i] as a Python object, without creating a Python Scalar
         # (nor, for nested types, per-row Array wrappers) where a subclass
         # provides a specialization; this base implementation goes through
         # Scalar.as_py and thus preserves its semantics exactly (see GH-50326).
-        return self.getitem(i).as_py()
+        return self.getitem(i).as_py(maps_as_pydicts=maps_as_pydicts)
 
     def tolist(self):
         """
@@ -2458,7 +2451,7 @@ cdef class BooleanArray(Array):
     Concrete class for Arrow arrays of boolean data type.
     """
 
-    cdef object _getitem_py(self, int64_t i):
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts):
         if self.ap.IsNull(i):
             return None
         return (<CBooleanArray*> self.ap).Value(i)
@@ -2477,7 +2470,7 @@ cdef class NumericArray(Array):
     A base class for Arrow numeric arrays.
     """
 
-    cdef object _getitem_py(self, int64_t i):
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts):
         cdef Type tid = self.ap.type_id()
         if self.ap.IsNull(i):
             return None
@@ -2503,7 +2496,7 @@ cdef class NumericArray(Array):
             return (<CDoubleArray*> self.ap).Value(i)
         # Subclasses whose as_py returns non-primitive objects (dates, times,
         # timestamps, durations, half floats, ...) use the exact Scalar path.
-        return Array._getitem_py(self, i)
+        return Array._getitem_py(self, i, maps_as_pydicts)
 
 
 cdef class IntegerArray(NumericArray):
@@ -2823,15 +2816,15 @@ cdef class ListArray(BaseListArray):
     Concrete class for Arrow arrays of a list data type.
     """
 
-    cdef object _getitem_py(self, int64_t i):
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts):
         cdef CListArray* arr = <CListArray*> self.ap
         if arr.IsNull(i):
             return None
         if self._children_cache is None:
             self._children_cache = pyarrow_wrap_array(arr.values())
         cdef Array values = <Array> self._children_cache
         cdef int64_t j, start = arr.value_offset(i), end = arr.value_offset(i + 1)
-        return [values._getitem_py(j) for j in range(start, end)]
+        return [values._getitem_py(j, maps_as_pydicts) for j in range(start, end)]
 
     @staticmethod
     def from_arrays(offsets, values, DataType type=None, MemoryPool pool=None, mask=None):
@@ -3018,15 +3011,15 @@ cdef class LargeListArray(BaseListArray):
     Identical to ListArray, but 64-bit offsets.
     """
 
-    cdef object _getitem_py(self, int64_t i):
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts):
         cdef CLargeListArray* arr = <CLargeListArray*> self.ap
         if arr.IsNull(i):
             return None
         if self._children_cache is None:
             self._children_cache = pyarrow_wrap_array(arr.values())
         cdef Array values = <Array> self._children_cache
         cdef int64_t j, start = arr.value_offset(i), end = arr.value_offset(i + 1)
-        return [values._getitem_py(j) for j in range(start, end)]
+        return [values._getitem_py(j, maps_as_pydicts) for j in range(start, end)]
 
     @staticmethod
     def from_arrays(offsets, values, DataType type=None, MemoryPool pool=None, mask=None):
@@ -3618,18 +3611,40 @@ cdef class MapArray(ListArray):
     Concrete class for Arrow arrays of a map data type.
     """
 
-    cdef object _getitem_py(self, int64_t i):
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts):
         cdef CListArray* arr = <CListArray*> self.ap
+        # Matches MapScalar.as_py, which validates before the null check.
+        _check_maps_as_pydicts(maps_as_pydicts)
         if arr.IsNull(i):
             return None
         if self._children_cache is None:
             self._children_cache = (self.keys, self.items)
         cdef Array keys = <Array> (<tuple> self._children_cache)[0]
         cdef Array items = <Array> (<tuple> self._children_cache)[1]
         cdef int64_t j, start = arr.value_offset(i), end = arr.value_offset(i + 1)
-        # Matches MapScalar.as_py with the default maps_as_pydicts=None:
-        # an association list of (key, value) tuples.
-        return [(keys._getitem_py(j), items._getitem_py(j)) for j in range(start, end)]
+        if maps_as_pydicts is None:
+            # Matches MapScalar.as_py with the default maps_as_pydicts=None:
+            # an association list of (key, value) tuples.
+            return [
+                (keys._getitem_py(j, None), items._getitem_py(j, maps_as_pydicts))
+                for j in range(start, end)
+            ]
+        # Like MapScalar.as_py: each key is checked for duplicates before
+        # its corresponding value is converted. Building the dict directly
+        # avoids the per-entry tuple of the association-list form.
+        cdef dict result = {}
+        for j in range(start, end):
+            key = keys._getitem_py(j, None)
+            if key in result:
+                if maps_as_pydicts == "strict":
+                    raise KeyError(
+                        "Converting to Python dictionary is not supported in strict mode "
+                        f"when duplicate keys are present (duplicate key was '{key}')."
+                    )
+                warnings.warn(
+                    f"Encountered key '{key}' which was already encountered.")
+            result[key] = items._getitem_py(j, maps_as_pydicts)
+        return result
 
     @staticmethod
     def from_arrays(offsets, keys, items, DataType type=None, MemoryPool pool=None, mask=None):
@@ -3768,15 +3783,15 @@ cdef class FixedSizeListArray(BaseListArray):
     Concrete class for Arrow arrays of a fixed size list data type.
     """
 
-    cdef object _getitem_py(self, int64_t i):
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts):
         cdef CFixedSizeListArray* arr = <CFixedSizeListArray*> self.ap
         if arr.IsNull(i):
             return None
         if self._children_cache is None:
             self._children_cache = pyarrow_wrap_array(arr.values())
         cdef Array values = <Array> self._children_cache
         cdef int64_t j, start = arr.value_offset(i), end = arr.value_offset(i + 1)
-        return [values._getitem_py(j) for j in range(start, end)]
+        return [values._getitem_py(j, maps_as_pydicts) for j in range(start, end)]
 
     @staticmethod
     def from_arrays(values, list_size=None, DataType type=None, mask=None):
@@ -4064,7 +4079,7 @@ cdef class StringArray(Array):
     Concrete class for Arrow arrays of string (or utf8) data type.
     """
 
-    cdef object _getitem_py(self, int64_t i):
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts):
         if self.ap.IsNull(i):
             return None
         cdef cpp_string_view view = (<CBinaryArray*> self.ap).GetView(i)
@@ -4103,7 +4118,7 @@ cdef class LargeStringArray(Array):
     Concrete class for Arrow arrays of large string (or utf8) data type.
     """
 
-    cdef object _getitem_py(self, int64_t i):
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts):
         if self.ap.IsNull(i):
             return None
         cdef cpp_string_view view = (<CLargeBinaryArray*> self.ap).GetView(i)
@@ -4141,7 +4156,7 @@ cdef class StringViewArray(Array):
     Concrete class for Arrow arrays of string (or utf8) view data type.
     """
 
-    cdef object _getitem_py(self, int64_t i):
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts):
         if self.ap.IsNull(i):
             return None
         cdef cpp_string_view view = (<CBinaryViewArray*> self.ap).GetView(i)
@@ -4153,7 +4168,7 @@ cdef class BinaryArray(Array):
     Concrete class for Arrow arrays of variable-sized binary data type.
     """
 
-    cdef object _getitem_py(self, int64_t i):
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts):
         if self.ap.IsNull(i):
             return None
         cdef cpp_string_view view = (<CBinaryArray*> self.ap).GetView(i)
@@ -4173,7 +4188,7 @@ cdef class LargeBinaryArray(Array):
     Concrete class for Arrow arrays of large variable-sized binary data type.
     """
 
-    cdef object _getitem_py(self, int64_t i):
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts):
         if self.ap.IsNull(i):
             return None
         cdef cpp_string_view view = (<CLargeBinaryArray*> self.ap).GetView(i)
@@ -4193,7 +4208,7 @@ cdef class BinaryViewArray(Array):
     Concrete class for Arrow arrays of variable-sized binary view data type.
     """
 
-    cdef object _getitem_py(self, int64_t i):
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts):
         if self.ap.IsNull(i):
             return None
         cdef cpp_string_view view = (<CBinaryViewArray*> self.ap).GetView(i)
@@ -4358,7 +4373,7 @@ cdef class StructArray(Array):
     Concrete class for Arrow arrays of a struct data type.
     """
 
-    cdef object _getitem_py(self, int64_t i):
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts):
         if self.ap.IsNull(i):
             return None
         cdef int64_t k, num_fields = self.type.num_fields
@@ -4375,9 +4390,18 @@ cdef class StructArray(Array):
         fields = (<tuple> self._children_cache)[1]
         cdef Array field_arr
         result = {}
-        for k in range(num_fields):
-            field_arr = <Array> fields[k]
-            result[names[k]] = field_arr._getitem_py(i)
+        try:
+            for k in range(num_fields):
+                field_arr = <Array> fields[k]
+                result[names[k]] = field_arr._getitem_py(i, maps_as_pydicts)
+        except KeyError:
+            # StructScalar.as_py translates any KeyError raised while
+            # converting the field values (e.g. a nested map in 'strict'
+            # mode) into its duplicate-field-names ValueError; reproduce
+            # that translation exactly.
+            raise ValueError(
+                "Converting to Python dictionary is not supported when "
+                "duplicate field names are present")
         return result
 
     def field(self, index):

python/pyarrow/lib.pxd

@@ -296,7 +296,7 @@ cdef class Array(_PandasConvertible):
 
     cdef void init(self, const shared_ptr[CArray]& sp_array) except *
     cdef getitem(self, int64_t i)
-    cdef object _getitem_py(self, int64_t i)
+    cdef object _getitem_py(self, int64_t i, object maps_as_pydicts)
     cdef int64_t length(self)
     cdef void _assert_cpu(self) except *
 

python/pyarrow/scalar.pxi

@@ -1177,6 +1177,15 @@ cdef class StructScalar(Scalar, Mapping):
         return str(self._as_py_tuple())
 
 
+cdef inline _check_maps_as_pydicts(maps_as_pydicts):
+    if maps_as_pydicts not in (None, "lossy", "strict"):
+        raise ValueError(
+            "Invalid value for 'maps_as_pydicts': "
+            + "valid values are 'lossy', 'strict' or `None` (default). "
+            + f"Received {maps_as_pydicts!r}."
+        )
+
+
 cdef class MapScalar(ListScalar, Mapping):
     """
     Concrete class for map scalars.
@@ -1234,12 +1243,7 @@ cdef class MapScalar(ListScalar, Mapping):
             The last seen value of a duplicate key will be in the Python dictionary.
             If 'strict', this instead results in an exception being raised when detected.
         """
-        if maps_as_pydicts not in (None, "lossy", "strict"):
-            raise ValueError(
-                "Invalid value for 'maps_as_pydicts': "
-                + "valid values are 'lossy', 'strict' or `None` (default). "
-                + f"Received {maps_as_pydicts!r}."
-            )
+        _check_maps_as_pydicts(maps_as_pydicts)
         if not self.is_valid:
             return None
         if not maps_as_pydicts:

python/pyarrow/tests/test_array.py

@@ -523,6 +523,79 @@ def test_to_pylist_bulk_paths():
         dup.to_pylist()
 
 
+def test_to_pylist_maps_as_pydicts():
+    # GH-50429: maps_as_pydicts converts through the scalar-free path; the
+    # semantics must match MapScalar.as_py exactly.
+    map_type = pa.map_(pa.string(), pa.int32())
+    flat = pa.array(
+        [None, [("k1", 1), ("k2", None)], []], type=map_type)
+    # Expected values are written out literally so the reference stays
+    # independent of Array.to_pylist (ListScalar.as_py delegates to it).
+    cases = [
+        (flat, [None, {"k1": 1, "k2": None}, {}]),
+        (flat.slice(1), [{"k1": 1, "k2": None}, {}]),
+        (pa.array([[[('k', 1)], None], None], type=pa.list_(map_type)),
+         [[{"k": 1}, None], None]),
+        (pa.array([[[('k', 1)], None], None], type=pa.large_list(map_type)),
+         [[{"k": 1}, None], None]),
+        (pa.array([[[('k', 1)], None], None], type=pa.list_(map_type, 2)),
+         [[{"k": 1}, None], None]),
+        (pa.array([[("o", [("i", 5)])]],
+                  type=pa.map_(pa.string(), map_type)),
+         [{"o": {"i": 5}}]),
+        (pa.array([{"m": [("k", 1)]}, None],
+                  type=pa.struct([("m", map_type)])),
+         [{"m": {"k": 1}}, None]),
+    ]
+    for arr, expected in cases:
+        assert arr.to_pylist(maps_as_pydicts="strict") == expected
+
+    dup = pa.array([[("k", 1), ("k", 2)]], type=map_type)
+    with pytest.warns(UserWarning, match="already encountered"):
+        assert dup.to_pylist(maps_as_pydicts="lossy") == [{"k": 2}]
+    with pytest.raises(KeyError, match="strict mode"):
+        dup.to_pylist(maps_as_pydicts="strict")
+
+    # Duplicate keys must be detected before converting values: with a
+    # poison value *after* the duplicate, strict mode raises the outer
+    # duplicate-key error, and lossy mode warns before converting that value.
+    nested_map = pa.map_(pa.string(), map_type)
+    poison = pa.array(
+        [[("k1", [("a", 1)]), ("k1", [("d", 1), ("d", 2)])]], type=nested_map)
+    with pytest.raises(KeyError, match="duplicate key was 'k1'"):
+        poison.to_pylist(maps_as_pydicts="strict")
+    with pytest.warns(UserWarning) as caught:
+        assert poison.to_pylist(maps_as_pydicts="lossy") == [{"k1": {"d": 2}}]
+    assert [str(warning.message) for warning in caught] == [
+        "Encountered key 'k1' which was already encountered.",
+        "Encountered key 'd' which was already encountered.",
+    ]
+
+    # Preserve StructScalar.as_py's translation of nested KeyErrors.
+    nested_duplicate = pa.array(
+        [{"m": [("k", 1), ("k", 2)]}],
+        type=pa.struct([("m", map_type)]))
+    with pytest.raises(ValueError, match="duplicate field names"):
+        nested_duplicate.to_pylist(maps_as_pydicts="strict")
+
+    null_map = pa.array([None], type=map_type)
+    with pytest.raises(ValueError, match="Invalid value for 'maps_as_pydicts'"):
+        null_map.to_pylist(maps_as_pydicts="bogus")
+    # Invalid values are only rejected when a map value is converted.
+    assert pa.array([1, 2]).to_pylist(maps_as_pydicts="bogus") == [1, 2]
+
+    # The association-list mode does not hash unhashable map keys, while
+    # either dictionary mode raises at the first membership test.
+    struct_keyed = pa.MapArray.from_arrays(
+        [0, 2],
+        pa.array([{"a": 1}, {"a": 2}], type=pa.struct([("a", pa.int32())])),
+        pa.array([10, 20], type=pa.int32()))
+    assert struct_keyed.to_pylist() == [x.as_py() for x in struct_keyed]
+    for mode in ("lossy", "strict"):
+        with pytest.raises(TypeError, match="unhashable"):
+            struct_keyed.to_pylist(maps_as_pydicts=mode)
+
+
 def test_array_slice():
     arr = pa.array(range(10))