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)
``````