fix: support conversion to arrow and back with non-option Unknown type

src/awkward/_connect/pyarrow.py

@@ -89,13 +89,15 @@ def __init__(
             node_parameters,
             record_is_tuple,
             record_is_scalar,
+            is_nonnullable_nulltype=False,
         ):
             self._mask_type = mask_type
             self._node_type = node_type
             self._mask_parameters = mask_parameters
             self._node_parameters = node_parameters
             self._record_is_tuple = record_is_tuple
             self._record_is_scalar = record_is_scalar
+            self._is_nonnullable_nulltype = is_nonnullable_nulltype
             super().__init__(storage_type, "awkward")
 
         def __str__(self):
@@ -140,6 +142,7 @@ def __arrow_ext_serialize__(self):
                     "node_parameters": self._node_parameters,
                     "record_is_tuple": self._record_is_tuple,
                     "record_is_scalar": self._record_is_scalar,
+                    "is_nonnullable_nulltype": self._is_nonnullable_nulltype,
                 }
             ).encode(errors="surrogatescape")
 
@@ -154,6 +157,7 @@ def __arrow_ext_deserialize__(cls, storage_type, serialized):
                 metadata["node_parameters"],
                 metadata["record_is_tuple"],
                 metadata["record_is_scalar"],
+                is_nonnullable_nulltype=metadata.get("is_nonnullable_nulltype", False),
             )
 
         @property
@@ -610,10 +614,18 @@ def popbuffers(paarray, awkwardarrow_type, storage_type, buffers, generate_bitma
         validbits = buffers.pop(0)
         assert storage_type.num_fields == 0
 
+        empty_array = ak.contents.EmptyArray(
+            parameters=node_parameters(awkwardarrow_type)
+        )
+        if awkwardarrow_type is not None and awkwardarrow_type._is_nonnullable_nulltype:
+            # Special case: pyarrow does not support a non-option null type,
+            # So we short-cut the Option wrapper when _is_nonnullable_nulltype is True.
+            return revertable(empty_array, empty_array)
+
         # This is already an option-type and offsets-corrected, so no popbuffers_finalize.
         return ak.contents.IndexedOptionArray(
             ak.index.Index64(numpy.full(len(paarray), -1, dtype=np.int64)),
-            ak.contents.EmptyArray(parameters=node_parameters(awkwardarrow_type)),
+            empty_array,
             parameters=mask_parameters(awkwardarrow_type),
         )
 
@@ -857,17 +869,25 @@ def form_popbuffers(awkwardarrow_type, storage_type):
 
 
 def to_awkwardarrow_type(
-    storage_type, use_extensionarray, record_is_scalar, mask, node
+    storage_type,
+    use_extensionarray,
+    record_is_scalar,
+    mask,
+    node,
+    is_nonnullable_nulltype=False,
 ):
     if use_extensionarray:
         return AwkwardArrowType(
-            storage_type,
-            direct_Content_subclass_name(mask),
-            direct_Content_subclass_name(node),
-            None if mask is None else mask.parameters,
-            None if node is None else node.parameters,
-            node.is_tuple if isinstance(node, ak.contents.RecordArray) else None,
-            record_is_scalar,
+            storage_type=storage_type,
+            mask_type=direct_Content_subclass_name(mask),
+            node_type=direct_Content_subclass_name(node),
+            mask_parameters=None if mask is None else mask.parameters,
+            node_parameters=None if node is None else node.parameters,
+            record_is_tuple=node.is_tuple
+            if isinstance(node, ak.contents.RecordArray)
+            else None,
+            record_is_scalar=record_is_scalar,
+            is_nonnullable_nulltype=is_nonnullable_nulltype,
         )
     else:
         return storage_type

src/awkward/contents/content.py

@@ -1264,6 +1264,10 @@ def _fill_none(self, value: Content) -> Content:
     def copy(self, *, parameters: JSONMapping | None = UNSET) -> Self:
         raise NotImplementedError
 
+    @classmethod
+    def _arrow_needs_option_type(cls):
+        return cls.is_option  # is_option is a class property of Meta
+
 
 @register_backend_lookup_factory
 def find_content_backend(obj: type):

src/awkward/contents/emptyarray.py

@@ -364,11 +364,12 @@ def _to_arrow(
         if options["emptyarray_to"] is None:
             return pyarrow.Array.from_buffers(
                 ak._connect.pyarrow.to_awkwardarrow_type(
-                    pyarrow.null(),
-                    options["extensionarray"],
-                    options["record_is_scalar"],
-                    mask_node,
-                    self,
+                    storage_type=pyarrow.null(),
+                    use_extensionarray=options["extensionarray"],
+                    record_is_scalar=options["record_is_scalar"],
+                    mask=mask_node,
+                    node=self,
+                    is_nonnullable_nulltype=mask_node is None,
                 ),
                 length,
                 [
@@ -386,6 +387,10 @@ def _to_arrow(
             )
             return next._to_arrow(pyarrow, mask_node, validbytes, length, options)
 
+    @classmethod
+    def _arrow_needs_option_type(cls):
+        return True  # This overrides Content._arrow_needs_option_type
+
     def _to_backend_array(self, allow_missing, backend):
         return backend.nplike.empty(0, dtype=np.float64)
 

src/awkward/contents/listoffsetarray.py

@@ -1974,7 +1974,7 @@ def _to_arrow(
             )
 
             content_type = pyarrow.list_(paarray.type).value_field.with_nullable(
-                akcontent.is_option
+                akcontent._arrow_needs_option_type()
             )
 
             if issubclass(npoffsets.dtype.type, np.int32):

src/awkward/contents/recordarray.py

@@ -1082,7 +1082,7 @@ def _to_arrow(
         types = pyarrow.struct(
             [
                 pyarrow.field(self.index_to_field(i), values[i].type).with_nullable(
-                    x.is_option
+                    x._arrow_needs_option_type()
                 )
                 for i, x in enumerate(self._contents)
             ]

src/awkward/contents/regulararray.py

@@ -1336,7 +1336,7 @@ def _to_arrow(
             )
 
             content_type = pyarrow.list_(paarray.type).value_field.with_nullable(
-                akcontent.is_option
+                akcontent._arrow_needs_option_type()
             )
 
             return pyarrow.Array.from_buffers(

src/awkward/contents/unionarray.py

@@ -1481,7 +1481,8 @@ def _to_arrow(
         types = pyarrow.union(
             [
                 pyarrow.field(str(i), values[i].type).with_nullable(
-                    mask_node is not None or self._contents[i].is_option
+                    mask_node is not None
+                    or self._contents[i]._arrow_needs_option_type()
                 )
                 for i in range(len(values))
             ],

src/awkward/operations/ak_to_arrow_table.py

@@ -136,7 +136,7 @@ def _impl(
             # accounting for options above the record layout.
             arrow_fields.append(
                 pyarrow.field(name, arrow_arrays[-1].type).with_nullable(
-                    outer_field_content.is_option
+                    outer_field_content._arrow_needs_option_type()
                 )
             )
 

tests/test_2340_unknown_type_to_arrow_and_back.py

@@ -0,0 +1,164 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward/blob/main/LICENSE
+
+# from __future__ import annotations
+from __future__ import annotations
+
+import io
+
+import numpy as np
+import pytest
+
+import awkward as ak
+from awkward.contents import (
+    EmptyArray,
+    NumpyArray,
+    RecordArray,
+    RegularArray,
+    UnionArray,
+)
+from awkward.operations import to_list
+from awkward.types import ListType, OptionType, UnknownType
+
+pyarrow = pytest.importorskip("pyarrow")
+pq = pytest.importorskip("pyarrow.parquet")
+
+
+def test_bare_unknown():
+    unk_array = ak.Array([[], []])  # sub-arrays are type unknown
+    assert unk_array[0].type.content == UnknownType()
+
+    unk_array_arrow = ak.to_arrow(unk_array)
+    # This array has one field, where we can get at pyarrow storage info
+    if hasattr(unk_array_arrow.type.storage_type, "field"):
+        # We support older versions of pyarrow that lack a field method
+        field = unk_array_arrow.type.storage_type.field(0)
+        assert field.type.storage_type == pyarrow.null()
+        assert field.nullable  # Must be nullable to be valid in Arrow
+    array_is_valid_within_parquet(unk_array_arrow)
+
+    orig_array = ak.from_arrow(unk_array_arrow)
+    assert to_list(orig_array) == [[], []]
+    assert orig_array.type == unk_array.type
+
+
+def test_option_unknown():
+    unk_option_array = ak.Array([[None, None], []])  # type is ?unknown
+    assert unk_option_array.type.content == ListType(OptionType(UnknownType()))
+
+    unk_opt_array_arrow = ak.to_arrow(unk_option_array)
+    if hasattr(unk_opt_array_arrow.type.storage_type, "field"):
+        field = unk_opt_array_arrow.type.storage_type.field(0)
+        assert field.type.storage_type == pyarrow.null()
+        assert field.nullable  # Nullable but this time it's because we're ?unknown
+    array_is_valid_within_parquet(unk_opt_array_arrow)
+
+    orig_array = ak.from_arrow(unk_opt_array_arrow)
+    assert orig_array.type == unk_option_array.type
+    assert to_list(orig_array) == [[None, None], []]
+
+    # This is different for ... reasons:
+    ua1 = ak.Array([[], [], [], [None]])[0:3]
+    assert ua1.type.content == ListType(OptionType(UnknownType()))
+    ua1a = ak.to_arrow(ua1)
+    if hasattr(ua1a.type.storage_type, "field"):
+        field = ua1a.type.storage_type.field(0)
+        assert field.type.storage_type == pyarrow.null()
+        assert field.nullable  # Like above, still nullable
+    ua1aa = ak.from_arrow(ua1a)
+    assert ua1aa.type.content == ListType(OptionType(UnknownType()))
+    assert len(ua1aa) == 3
+
+
+def test_toplevel_unknown():
+    unk_array = ak.Array([])
+    assert unk_array.type.content == UnknownType()
+
+    unk_array_arrow = ak.to_arrow(unk_array)
+    assert len(unk_array_arrow) == 0
+    # Note: we cannot test the nullability of this arrow array, since it has zero fields
+    # field = unk_array_arrow.type.storage_type.field(0)
+    assert unk_array_arrow.type.num_fields == 0  # (just demonstrating the above)
+    # But we can still give Parquet conversion a shot
+    array_is_valid_within_parquet(unk_array_arrow)
+
+    orig_array = ak.from_arrow(unk_array_arrow)
+    assert orig_array.type == unk_array.type
+    assert to_list(orig_array) == []
+
+    ua2 = ak.Array([None])[0:0]  # This is a top-level Option<EmptyArray>
+    assert ua2.type.content == OptionType(UnknownType())
+    ua2a = ak.to_arrow(ua2)
+    ua2aa = ak.from_arrow(ua2a)
+    assert ua2aa.type.content == OptionType(UnknownType())
+
+
+def test_recordarray_with_unknowns():
+    a = RecordArray([EmptyArray(), NumpyArray([])], ["x", "y"], length=0)
+    arw = ak.to_arrow(a)
+    if hasattr(arw.type.storage_type, "field"):
+        assert arw.type.storage_type.field(0).nullable
+    array_is_valid_within_parquet(arw)
+    # This is a strange, laboratory kind of object.
+    # It seems unlikely to be found in the wild.
+    # I'm not sure what other tests here would be meaningful.
+
+
+def test_table_with_unknowns():
+    a = RecordArray([EmptyArray(), NumpyArray([1, 2])], ["x", "y"])
+    # Again this is a strange one!
+    table = ak.to_arrow_table(a)
+    assert table.field(0).nullable
+    temp = io.BytesIO()
+    pq.write_table(table, temp)
+
+
+def test_regulararray_with_unknown():
+    a = RegularArray(EmptyArray(), 0)
+    # RegularArray is helpful.
+    # But when it's given an EmptyArray to recycle, it becomes absolutely unhelpful.
+    arw = ak.to_arrow(a)
+    if hasattr(arw.type.storage_type, "field"):
+        assert arw.type.storage_type.field(0).nullable
+    assert to_list(arw) == []
+    array_is_valid_within_parquet(arw)
+
+
+def test_unionarray_with_unknown():
+    # Although a UnionArray with an EmptyArray content type has no application,
+    # we can still exercise a code path this way.
+    a = UnionArray(
+        tags=ak.index.Index8(np.array([1, 1, 1], dtype=np.int8)),
+        # tags: none of the elements are, or could be, taken from the EmptyArray.
+        index=ak.index.Index64(np.array([0, 1, 2], dtype=np.int64)),
+        contents=[
+            EmptyArray(),
+            NumpyArray([10, 20, 30]),
+        ],
+    )
+    assert to_list(a) == [10, 20, 30]
+    arw = ak.to_arrow(a)
+    if hasattr(arw.type.storage_type, "field"):
+        assert arw.type.storage_type.field(0).nullable
+        assert not arw.type.storage_type.field(1).nullable
+    # array_is_valid_within_parquet(arw)  # This fails for unrelated reasons.
+    art = ak.from_arrow(arw)
+    # round-trip is okay but the UnionArray is lost. Separate issue?
+    assert to_list(art) == [10, 20, 30]
+
+
+#### Helper method(s)
+
+
+def array_is_valid_within_parquet(arrow_array):
+    """
+    Helper function that writes the given array to a Parquet table.
+    Prior to 2340, this would raise:
+    pyarrow.lib.ArrowInvalid: NullType Arrow field must be nullable
+    """
+    table = pyarrow.Table.from_arrays([arrow_array], names=["col1"])
+    table.validate(
+        full=True
+    )  # Frustratingly, this does *not* produce the anticipated exception, but..
+    temp = io.BytesIO()
+    pq.write_table(table, temp)  # *this does* perform the validation we need.
+    assert len(temp.getbuffer()) > 0