How to print float to n decimal places including trailing 0s?

Question

I need to print or convert a float number to 15 decimal place string even if the result has many trailing 0s eg:

1.6 becomes 1.6000000000000000

I tried round(6.2,15) but it returns 6.2000000000000002 adding a rounding error

I also saw various people online who put the float into a string and then added trailing 0's manually but that seems bad...

What is the best way to do this?

Answer 1

For Python versions in 2.6+ and 3.x

You can use the str.format method. Examples:

>>> print('{0:.16f}'.format(1.6)) 1.6000000000000001  >>> print('{0:.15f}'.format(1.6)) 1.600000000000000 

Note the 1 at the end of the first example is rounding error; it happens because exact representation of the decimal number 1.6 requires an infinite number binary digits. Since floating-point numbers have a finite number of bits, the number is rounded to a nearby, but not equal, value.

For Python versions prior to 2.6 (at least back to 2.0)

You can use the "modulo-formatting" syntax (this works for Python 2.6 and 2.7 too):

>>> print '%.16f' % 1.6 1.6000000000000001  >>> print '%.15f' % 1.6 1.600000000000000 

Answer 2

The cleanest way in modern Python >=3.6, is to use an f-string with string formatting:

>>> var = 1.6 >>> f"{var:.15f}" '1.600000000000000'