viirya commented on code in PR #57099:
URL: https://github.com/apache/spark/pull/57099#discussion_r3540595159
##########
python/pyspark/sql/conversion.py:
##########
@@ -980,6 +980,60 @@ class ArrowTableToRowsConversion:
Conversion from Arrow Table to Rows.
"""
+ @staticmethod
+ def _to_pylist(column: Union["pa.Array", "pa.ChunkedArray"]) -> List[Any]:
+ """
+ Equivalent to ``column.to_pylist()``, but converts (nested) list
columns in bulk
+ instead of one scalar at a time.
+
+ ``Array.to_pylist()`` materializes one Scalar per element; for list
types each row
+ additionally allocates a C++ scalar, a Python Scalar wrapper and a
Python Array
+ wrapper for the row's values before converting elements one by one,
which is
+ several times slower than converting the flattened child values in a
single pass
+ and slicing the resulting Python list per row (see
apache/arrow#50326). The values
+ themselves are still converted by Arrow's own ``to_pylist``, so
results are exactly
+ identical: ``None`` stays ``None`` and values inside numeric lists
stay Python ints,
+ unlike a pandas round trip which would coerce them to floats/NaN.
NumPy is used
+ only for the offsets (non-null integers) and the validity bitmap
(booleans), so no
+ value coercion can occur.
+
+ This can be removed once the minimum supported PyArrow version
includes the fix
+ for apache/arrow#50326.
+ """
+ import pyarrow as pa
+ import pyarrow.types as pa_types
+
+ try:
+ import numpy # noqa: F401
+ except ImportError:
+ return column.to_pylist()
+
+ if isinstance(column, pa.ChunkedArray):
+ result: List[Any] = []
+ for chunk in column.chunks:
+ result.extend(ArrowTableToRowsConversion._to_pylist(chunk))
+ return result
+
+ column_type = column.type
+ if (pa_types.is_list(column_type) or
pa_types.is_large_list(column_type)) and len(
+ column
+ ) > 0:
+ n = len(column)
+ offsets = column.offsets.to_numpy(zero_copy_only=True).tolist()
Review Comment:
Good point — added the comment in eebbc4fd059. `zero_copy_only=True` also
doubles as an assertion: if a future Arrow list variant ever produced offsets
with a validity bitmap, this would fail loudly instead of silently copying.
##########
python/pyspark/sql/conversion.py:
##########
@@ -980,6 +980,60 @@ class ArrowTableToRowsConversion:
Conversion from Arrow Table to Rows.
"""
+ @staticmethod
+ def _to_pylist(column: Union["pa.Array", "pa.ChunkedArray"]) -> List[Any]:
+ """
+ Equivalent to ``column.to_pylist()``, but converts (nested) list
columns in bulk
+ instead of one scalar at a time.
+
+ ``Array.to_pylist()`` materializes one Scalar per element; for list
types each row
+ additionally allocates a C++ scalar, a Python Scalar wrapper and a
Python Array
+ wrapper for the row's values before converting elements one by one,
which is
+ several times slower than converting the flattened child values in a
single pass
+ and slicing the resulting Python list per row (see
apache/arrow#50326). The values
+ themselves are still converted by Arrow's own ``to_pylist``, so
results are exactly
+ identical: ``None`` stays ``None`` and values inside numeric lists
stay Python ints,
+ unlike a pandas round trip which would coerce them to floats/NaN.
NumPy is used
+ only for the offsets (non-null integers) and the validity bitmap
(booleans), so no
+ value coercion can occur.
+
+ This can be removed once the minimum supported PyArrow version
includes the fix
+ for apache/arrow#50326.
+ """
+ import pyarrow as pa
+ import pyarrow.types as pa_types
+
+ try:
+ import numpy # noqa: F401
+ except ImportError:
+ return column.to_pylist()
+
+ if isinstance(column, pa.ChunkedArray):
+ result: List[Any] = []
+ for chunk in column.chunks:
+ result.extend(ArrowTableToRowsConversion._to_pylist(chunk))
+ return result
+
+ column_type = column.type
+ if (pa_types.is_list(column_type) or
pa_types.is_large_list(column_type)) and len(
+ column
+ ) > 0:
+ n = len(column)
+ offsets = column.offsets.to_numpy(zero_copy_only=True).tolist()
+ start = offsets[0]
+ flat = ArrowTableToRowsConversion._to_pylist(
+ column.values.slice(start, offsets[-1] - start)
+ )
+ if column.null_count == 0:
+ return [flat[offsets[i] - start : offsets[i + 1] - start] for
i in range(n)]
+ valid = column.is_valid().to_numpy(zero_copy_only=False).tolist()
+ return [
Review Comment:
Right — row assembly is still an O(rows) Python loop; the win is that each
row costs one list slice instead of a C++ scalar + Python Scalar wrapper +
Python Array wrapper + generator. "Bulk" refers to the child-values conversion
being a single call, not to vectorized row assembly. The fully vectorized fix
is the upstream one (apache/arrow#50327); this stays intentionally simple as
the interim.
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