py/formatfloat: Fix formatting of whole-numbered floats. #8878
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This is to fix issue #4212
Many bugs occur when printing float values containing integer values. The successive-scaling approach in the original algorithm would quickly make these values depart from whole numbers.
This modification attempts to avoid scaling the original value for numbers in the range 1.0 to 1e10, and instead compares with other floats constructed only by multiplying whole numbers, so guaranteed to remain exact until the mantissa runs out.
I added a test that verifies consistency with CPython for a range of values. In fact, CPython also struggles with some values (see, e.g.
print(13 * .001)
), but at least we have agreement for the values in the test (on my machine).Signed-off-by: Dan Ellis dan.ellis@gmail.com