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https://issues.apache.org/jira/browse/SPARK-58487?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Josh Rosen updated SPARK-58487:
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Summary: Pandas UDFs and Arrow-enabled toPandas() silently corrupt nullable
BIGINT values of magnitude 2^53 or more by upcasting the column to float64
(was: "Pandas UDFs and Arrow-enabled toPandas() silently corrupt nullable
BIGINT values of magnitude 2^53 or more by upcasting the column to float64)
> Pandas UDFs and Arrow-enabled toPandas() silently corrupt nullable BIGINT
> values of magnitude 2^53 or more by upcasting the column to float64
> ---------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: SPARK-58487
> URL: https://issues.apache.org/jira/browse/SPARK-58487
> Project: Spark
> Issue Type: Bug
> Components: PySpark
> Affects Versions: 4.2.0
> Reporter: Josh Rosen
> Priority: Major
> Labels: correctness
>
> A nullable {{LongType}} column holding a value of magnitude at least {{2^53}}
> is silently changed to the nearest representable {{float64}} as soon as a
> NULL appears in the same Arrow batch.
> {code:python}
> from pyspark.sql.types import StructType, StructField, LongType
> from pyspark.sql.functions import pandas_udf, col
> import pandas as pd
> big = 9007199254740993 # 2**53 + 1, not exactly representable in
> float64
> schema = StructType([StructField("v", LongType())])
> df = spark.createDataFrame([(big,), (None,)], schema=schema)
> df.toPandas()['v'].iloc[0]
> # 9007199254740992.0 -- off by one, and now a float
> @pandas_udf(LongType())
> def identity(s: pd.Series) -> pd.Series:
> return s
> df.coalesce(1).withColumn("v2", identity(col("v"))).collect()
> # [Row(v=9007199254740993, v2=9007199254740992), ...] -- v2 != v
> {code}
> Remove the NULL row and both paths are exact. The NULL is what triggers the
> upcast.
> *This is a Spark self-inconsistency rather than a pandas limitation.* On the
> same DataFrame, {{collect()}} and {{toPandas()}} with Arrow disabled both
> return the exact integer; only the Arrow path differs. Two supported ways of
> reading the same column disagree.
> *Mechanism.* pyarrow's default {{Array.to_pandas()}} cannot represent an
> int64 column containing a null, because numpy has no nullable integer dtype,
> so it upcasts the whole column to {{float64}} with NaN as the null sentinel.
> {{float64}} has a 53-bit mantissa, so integers at or above {{2^53}} are not
> all representable and the upcast rounds.
> Neither {{pyspark.sql.pandas.conversion._convert_arrow_table_to_pandas}} nor
> the Arrow-column-to-Series conversion in {{pyspark.sql.pandas.serializers}}
> passes {{types_mapper=}} to opt into pandas' nullable extension dtypes, which
> would keep both the nulls and the precision.
> On the way back, {{{}create_arrow_array_from_pandas{}}} casts the
> already-corrupted float64 Series to the declared integer type. The corrupted
> value is a whole number, so the cast raises nothing and the wrong value is
> written out as if correct.
> *The trigger is batch-scoped, which matters for anyone trying to reproduce
> it.* The NULL and the large value must land in the same Arrow batch. A
> two-row, two-partition example does not reproduce, because each batch is
> either all-non-null or trivially the null row.
> *Measured scope:*
> ||path||result||
> |{{collect()}}|exact|
> |{{{}toPandas(){}}}, Arrow disabled|exact|
> |{{{}toPandas(){}}}, Arrow enabled|corrupted|
> |scalar and scalar-iter {{pandas_udf}}|corrupted|
> |grouped-agg {{{}pandas_udf{}}}, NULL and value in one batch|corrupted|
> |{{struct<a: bigint>}} through the Arrow paths above|corrupted|
> |{{{}mapInPandas{}}}, {{{}applyInPandas{}}}, {{applyInArrow}}|exact|
> |Arrow-optimized plain Python UDF|exact, by construction (per-row Python
> objects, no dtype decision)|
> |{{DecimalType(38, 18)}}|exact (pyarrow yields {{decimal.Decimal}} objects,
> no float64 fallback)|
> *Potential fixes.* Have the Arrow-to-pandas conversion sites opt in to
> {{{}integer_object_nulls=True{}}}, or pass a {{types_mapper}} that maps
> nullable Arrow integers to pandas nullable extension dtypes.
> {{ArrowArrayToPandasConversion.convert_legacy}} already uses the
> former for array, map and struct columns, so the mechanism is present and
> simply not applied to the scalar integral branches. Failing that, the
> return-path cast should not silently accept a float that lost precision.
> {*}Related bug{*}: {{createDataFrame()}} on a pandas {{Int64}}
> extension-dtype column containing a null infers DOUBLE and corrupts the same
> magnitudes.
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