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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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fix(avro): read doubles at full precision in the Cython decoder
m0g3r Aug 18, 2026
eee6ee5
Merge remote-tracking branch 'origin/main' into fix/cython-decoder-do…
m0g3r Aug 23, 2026
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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.

@m0g3r m0g3r Aug 23, 2026

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Thanks for checking. The test does fail on the unfixed tree, but only if the Cython extension is actually rebuilt — and setup.py makes that easy to miss.

The cythonize call is wrapped in try/except Exception with allowed_to_fail = os.environ.get("CIBUILDWHEEL", "0") != "1", so if the interpreter running it can't import Cython, ext_modules stays [] and build_ext --inplace prints running build_ext, exits 0, and builds nothing. The previously built decoder_fast.*.so stays on disk with the fix compiled into it, and the test passes against that stale artifact.

Reverting the source and forcing a real rebuild:

$ git checkout HEAD~1 -- pyiceberg/avro/decoder_fast.pyx   # back to cpdef float
$ .venv/bin/python setup.py build_ext --inplace
Compiling pyiceberg/avro/decoder_fast.pyx because it changed.
building 'pyiceberg.avro.decoder_fast' extension

$ .venv/bin/python -m pytest tests/avro/test_decoder.py -k full_precision -q
6 failed, 6 passed, 44 deselected

All six failures are the CythonBinaryDecoder parameters. The six StreamingBinaryDecoder ones pass, which is expected — the pure-Python decoder was always correct. Decoded values on the unfixed build:

3.141592653589793 -> 3.1415927410125732
1e+308            -> inf
5e-324            -> 0.0

With the fix restored and rebuilt, tests/avro/test_decoder.py is 56 passed.

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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