Python float to Decimal conversion

Question

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?

Answer 1

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) 

Answer 2

You said in your question:

Can someone suggest a good way to convert from float to Decimal preserving value as the user has entered

But every time the user enters a value, it is entered as a string, not as a float. You are converting it to a float somewhere. Convert it to a Decimal directly instead and no precision will be lost.