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Comment by teh | Hacker News Reader

Python's print method is actually fairly clever picking the "nicest" representation that's still correct.

https://docs.python.org/2/tutorial/floatingpoint.html#tut-fp...

Quote:

"In current versions, Python displays a value based on the shortest decimal fraction that rounds correctly back to the true binary value, resulting simply in ‘0.1’."

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This doesn't seem true for me (Python 3.5.0 (default, Sep 20 2015, 11:28:25) [GCC 5.2.0] on linux).

    >>> print(.1 + .2)
    0.30000000000000004

Additionally, for exact calculations you might want to use the decimal module. https://docs.python.org/3.5/library/decimal.html#module-deci...

But be wary so you don't pass in floats when creating your decimals

    In [10]: from decimal import Decimal
    In [11]: Decimal(0.1) + Decimal(0.2) - Decimal(0.3)
    Out[11]: Decimal('2.775557561565156540423631668E-17')   

    In [12]: Decimal("0.1") + Decimal("0.2") - Decimal("0.3")
    Out[12]: Decimal('0.0')

Try the usual decimally-unrepresentable numbers (like 1/3).

As illumen pointed out, Python has also got fractions. For things like Pi however, there is no exact numerical representation and you'd probably want sympy or something similar.

Sure. But the inability to represent 1/3 in decimal is just the same as inability to represent 1/10 in binary. The binary case just seems different because we are using a foreign base (decimal) literal "0.1" to express the binary number.

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