Textual formatting of floating point numbers

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1 
assert f"{385.0:.2}" == "3.9e+02" # FAILS
assert f"{385.1:.2}" == "3.9e+02" # passes
assert f"{385.01:.2}" == "3.9e+02" # passes
assert f"{385.000000004:.2}" == "3.9e+02" # passes

Why is this the current python behavior? Is this related to IEEE floating point bit representations? Should this not be a bug?

The problem is 385.0 being rounded to two decimals equalling 380 rather than 39

2

This is pretty standard rounding for IEEE floating point. “Banker’s rounding” rounds to the nearest even number. This method reduces the cumulative error when adding rounded numbers together.

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3

It is intentional, as explained here: Built-in Functions — Python 3.13.5 documentation and links following. If you want more control, there’s the decimal module.

4

Thanks! The documentation explicitly calls this out