pythondecimal

Python float to Decimal conversion


Python Decimal doesn't support being constructed from float; it expects that you have to convert float to a string first.

This is very inconvenient since standard string formatters for float require that you specify number of decimal places rather than significant places. So if you have a number that could have as many as 15 decimal places you need to format as Decimal("%.15f" % my_float), which will give you garbage at the 15th decimal place if you also have any significant digits before decimal (Decimal("%.15f" % 100000.3) == Decimal('100000.300000000002910')).

Can someone suggest a good way to convert from float to Decimal preserving value as the user has entered, perhaps limiting number of significant digits that can be supported?


Solution

  • Python <2.7

    "%.15g" % f
    

    Or in Python 3.0:

    format(f, ".15g")
    

    Python 2.7+, 3.2+

    Just pass the float to Decimal constructor directly, like this:

    from decimal import Decimal
    Decimal(f)