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fix(avro): read doubles at full precision in the Cython decoder #3808

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fix(avro): read doubles at full precision in the Cython decoder #3808

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2 changes: 1 addition & 1 deletion pyiceberg/avro/decoder_fast.pyx
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Original file line number Diff line number Diff line change
Expand Up @@ -138,7 +138,7 @@ cdef class CythonBinaryDecoder:
"""
return float(STRUCT_FLOAT.unpack(self.read(4))[0])

cpdef float read_double(self):
cpdef double read_double(self):
"""Reads a value from the stream as a double.

A double is written as 8 bytes.
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17 changes: 17 additions & 0 deletions tests/avro/test_decoder.py
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Expand Up @@ -160,6 +160,23 @@ def test_read_double(decoder_class: Callable[[bytes], BinaryDecoder]) -> None:
assert decoder.read_double() == 19.25


@pytest.mark.parametrize("decoder_class", AVAILABLE_DECODERS)
@pytest.mark.parametrize(
"value",
[
3.141592653589793,
429496729622.314,
0.1,
1.0000000000000002, # smallest double above 1.0
1e308, # overflows to inf in single precision
5e-324, # underflows to 0.0 in single precision
],
)
def test_read_double_keeps_full_precision(decoder_class: Callable[[bytes], BinaryDecoder], value: float) -> None:

@ebyhr ebyhr Aug 18, 2026

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I don't think this is a valid regression test. It passes even if we revert decoder_fast.pyx's change.

decoder = decoder_class(struct.pack("<d", value))
assert decoder.read_double() == value


@pytest.mark.parametrize("decoder_class", AVAILABLE_DECODERS)
def test_skip_double(decoder_class: Callable[[bytes], BinaryDecoder]) -> None:
decoder = decoder_class(b"\x00\x00\x00\x00\x00\x40\x33\x40")
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