[GitHub] [arrow] AlenkaF commented on issue #34739: [Python] Cannot append nullable string columns to table

["AlenkaF (via GitHub)" <gi...@apache.org>]

AlenkaF commented on issue #34739:
URL: https://github.com/apache/arrow/issues/34739#issuecomment-1485064943

   The issue in the example that doesn't work is that a `NullArray` is created (in [table.add_column](https://github.com/apache/arrow/blob/main/python/pyarrow/table.pxi#L4562-L4565)) as the only element in the column being appended is `None`. `NullArray` is of type `pa.null()` and not `pa.string()` and so we get an `ArrowInvalid` error:
   
   ```python
   >>> pa.chunked_array([["x"]])
   <pyarrow.lib.ChunkedArray object at 0x11672b600>
   [
     [
       "x"
     ]
   ]
   >>> pa.chunked_array([["x"]]).chunk(0)
   <pyarrow.lib.StringArray object at 0x11671ae60>
   [
     "x"
   ]
   >>> pa.chunked_array([[None]])
   <pyarrow.lib.ChunkedArray object at 0x11672b740>
   [
   1 nulls
   ]
   >>> pa.chunked_array([[None]]).chunk(0)
   <pyarrow.lib.NullArray object at 0x11671ae60>
   1 nulls
   ```
   
   That will not happen if you have examples with more than one row and not all elements of a column missing:
   
   ```python
   >>> pa.chunked_array([[None, "x"]]).chunk(0)
   <pyarrow.lib.StringArray object at 0x11671af80>
   [
     null,
     "x"
   ]
   ```
   
   ```python
   import pyarrow as pa
   table = pa.Table.from_pylist([{"a": None}, {"a": "first"}], pa.schema([pa.field("a", pa.string(), nullable=True)]))
   table = table.append_column(pa.field("b", pa.string(), nullable=True), [["x", "y"]])
   table = table.append_column(pa.field("n", pa.string(), nullable=True), [[None, "second"]])
   table
   # pyarrow.Table
   # a: string
   # b: string
   # n: string
   # ----
   # a: [[null,"first"]]
   # b: [["x","y"]]
   # n: [[null,"second"]]
   table.schema.field("n")
   # pyarrow.Field<n: string>
   table.schema.field("n").nullable
   # True
   ```


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