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fix(schema): reject spec-forbidden decimal scale changes in update_column #14

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fix(schema): reject spec-forbidden decimal scale changes in update_column #14

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ff84fb3
Fix decimal type-promotion scale check (self-comparison tautology)
Jun 23, 2026
a88dbb5
Add test for decimal scale-change promotion rejection
Jun 24, 2026
d97dd51
Fix same scale tautology in test helper and cover decimal scale mismatch
Jun 24, 2026
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2 changes: 1 addition & 1 deletion pyiceberg/schema.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1697,7 +1697,7 @@ def _(file_type: BinaryType, read_type: IcebergType) -> IcebergType:
@promote.register(DecimalType)
def _(file_type: DecimalType, read_type: IcebergType) -> IcebergType:
if isinstance(read_type, DecimalType):
if file_type.precision <= read_type.precision and file_type.scale == file_type.scale:
if file_type.precision <= read_type.precision and file_type.scale == read_type.scale:
return read_type
else:
raise ResolveError(f"Cannot reduce precision from {file_type} to {read_type}")
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12 changes: 11 additions & 1 deletion tests/test_schema.py
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Original file line number Diff line number Diff line change
Expand Up @@ -68,6 +68,7 @@
DoubleType(),
DecimalType(10, 2),
DecimalType(100, 2),
DecimalType(10, 3),
StringType(),
DateType(),
TimeType(),
Expand Down Expand Up @@ -878,7 +879,7 @@ def should_promote(file_type: IcebergType, read_type: IcebergType) -> bool:
if isinstance(file_type, BinaryType) and isinstance(read_type, StringType):
return True
if isinstance(file_type, DecimalType) and isinstance(read_type, DecimalType):
return file_type.precision <= read_type.precision and file_type.scale == file_type.scale
return file_type.precision <= read_type.precision and file_type.scale == read_type.scale
if isinstance(file_type, FixedType) and isinstance(read_type, UUIDType) and len(file_type) == 16:
return True
return False
Expand Down Expand Up @@ -1316,6 +1317,15 @@ def test_type_promote_decimal_to_fixed_scale_with_wider_precision(table_v2: Tabl
assert decimal_type.scale == 1


def test_detect_invalid_promotion_decimal_scale_change(table_v2: Table) -> None:
# Per the Iceberg spec, decimal scale is fixed; only precision may grow.
current_schema = Schema(NestedField(field_id=1, name="aCol", field_type=DecimalType(precision=20, scale=1), required=False))
new_schema = Schema(NestedField(field_id=1, name="aCol", field_type=DecimalType(precision=22, scale=2), required=False))

with pytest.raises(ValidationError, match="Cannot change column type: aCol: decimal.20, 1. -> decimal.22, 2."):
_ = UpdateSchema(transaction=Transaction(table_v2), schema=current_schema).union_by_name(new_schema)._apply()


def test_add_nested_structs(primitive_fields: NestedField, table_v2: Table) -> None:
schema = Schema(
NestedField(
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