[issue38929] Float // Integer doesn't give best result.

[Steven D'Aprano]

Steven D'Aprano <steve+python at pearwood.info> added the comment:

Although it often gets called "integer division", that's not actually what // does, it is actually *floor* division, as documented here:

https://docs.python.org/3/library/stdtypes.html#numeric-types-int-float-complex

So the behaviour as given is correct: it returns the floor of the quotient. Arguments are coerced to a common type in the normal fashion, so float//int performs the floor  division in floating point, which may not give the same result as integer floor division due to floating point rounding:

py> 10**17 // 7
14285714285714285
py> 1e17 // 7
1.4285714285714284e+16

If you don't want floating point rounding, don't use floats.

If you want "true division", use / not the floor division operator.

This isn't a bug, it is working as designed.

Even if the behaviour you are asking for was possible, practical and desirable, it would break backwards compatibility. There's no way to do a "floor division that returns an int" for all float arguments, since there are no int NANs.

py> float('inf')//7
nan

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nosy: +steven.daprano
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38929>
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