What is float('123.987') in Python?

It's 开发者_高级运维123.98699999999999 !

Why is that?

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See Why can't decimal numbers be represented exactly in binary

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The Python FAQ and tutorial address this issue pretty well, I think. More generally, both are excellent resources, well worth your time to browse if you have any interest in Python!-)

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This has changed in/since Python 3.1.

See also: issue 1580

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To your (already deleted) addition: Use the decimal package.