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AVRO-3834: [python] Rescale decimals to the schema scale when encoding #4020

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AVRO-3834: [python] Rescale decimals to the schema scale when encoding #4020

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2 changes: 2 additions & 0 deletions lang/py/avro/io.py
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Original file line number Diff line number Diff line change
Expand Up @@ -558,6 +558,7 @@ def write_decimal_bytes(self, datum: decimal.Decimal, scale: int) -> None:
for digit in digits:
unscaled_datum = (unscaled_datum * 10) + digit

unscaled_datum *= 10 ** (scale + int(exp))
bits_req = unscaled_datum.bit_length() + 1
if sign:
unscaled_datum = (1 << bits_req) - unscaled_datum
Expand All @@ -584,6 +585,7 @@ def write_decimal_fixed(self, datum: decimal.Decimal, scale: int, size: int) ->
for digit in digits:
unscaled_datum = (unscaled_datum * 10) + digit

unscaled_datum *= 10 ** (scale + int(exp))
bits_req = unscaled_datum.bit_length() + 1
size_in_bits = size * 8
offset_bits = size_in_bits - bits_req
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31 changes: 31 additions & 0 deletions lang/py/avro/test/test_io.py
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Original file line number Diff line number Diff line change
Expand Up @@ -756,6 +756,37 @@ def test_recursive_record_min_bytes_is_non_zero(self) -> None:


class TestMisc(unittest.TestCase):
def test_decimal_encoding_preserves_value_at_schema_scale(self) -> None:
cases = (
("3.1", 2, 310),
("-3.1", 2, -310),
("3", 2, 300),
("-3", 2, -300),
("3E+1", 2, 3000),
("-3E+1", 2, -3000),
("3E+1", 0, 30),
("3.1", 4, 31000),
("3.14", 2, 314),
("0", 2, 0),
("12345.6", 2, 1234560),
)
for storage_type, (value, scale, expected_unscaled) in itertools.product(("bytes", "fixed"), cases):
with self.subTest(storage_type=storage_type, value=value, scale=scale):
schema = avro.schema.parse(
json.dumps({"type": storage_type, "logicalType": "decimal", "precision": 8, "scale": scale, "name": "Test", "size": 4})
)
datum = decimal.Decimal(value)
# Scaling must not round according to the caller's decimal context.
with decimal.localcontext() as context:
context.prec = 2
writer, _, _ = write_datum(datum, schema)
encoded = writer.getvalue()
decoder = avro.io.BinaryDecoder(io.BytesIO(encoded))
payload = decoder.read_bytes() if storage_type == "bytes" else decoder.read(4)
self.assertEqual(int.from_bytes(payload, byteorder="big", signed=True), expected_unscaled)
decoded = avro.io.DatumReader(schema).read(avro.io.BinaryDecoder(io.BytesIO(encoded)))
self.assertEqual(decoded, datum)

def test_decimal_bytes_small_scale(self) -> None:
"""Avro should raise an AvroTypeException when attempting to write a decimal with a larger exponent than the schema's scale."""
datum = decimal.Decimal("3.1415")
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