wjones127 commented on code in PR #3276:
URL: https://github.com/apache/arrow-rs/pull/3276#discussion_r1040121228
##########
arrow-pyarrow-integration-testing/tests/test_sql.py:
##########
@@ -193,6 +200,34 @@ def test_time32_python():
del b
del expected
+
+def test_empty_array_python():
+ """
+ Python -> Rust -> Python
+ """
+ a = pa.array([], pa.int32())
Review Comment:
For the dictionary array, note that `The external buffer at position 0 is
null.` is [excluding the null
buffer](https://github.com/apache/arrow-rs/blob/b155461f770eb2ab8cc5d3296f6123582cf5073d/arrow/src/ffi.rs#L655-L656).
So it appears that PyArrow creates empty dictionary arrays with a null buffer
for indices, but Rust FFI is expected that to be non-null.
<details>
<summary>PyArrow code check</summary>
```python
import pyarrow as pa
from pyarrow.cffi import ffi as arrow_c
# array = pa.array([], pa.null())
array = pa.array([], pa.dictionary(pa.int8(), pa.string()))
# array = pa.array([], pa.list_(pa.int8()))
c_array = arrow_c.new("struct ArrowArray*")
c_schema = arrow_c.new("struct ArrowSchema*")
c_array_ptr = int(arrow_c.cast("uintptr_t", c_array))
c_schema_ptr = int(arrow_c.cast("uintptr_t", c_schema))
array._export_to_c(c_array_ptr)
array.type._export_to_c(c_schema_ptr)
print(c_array.n_buffers) # 2
print(c_array.buffers) # <cdata 'void * *' 0x20000041280>
for i in range(c_array.n_buffers):
print(c_array.buffers[i])
# <cdata 'void *' NULL>
# <cdata 'void *' NULL>
```
</details>
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