Rounding decimals with new Python format function

Question

How do I round a decimal to a particular number of decimal places using the Python 3.0 format function?

Answer 1

Here's a typical, useful example...:

>>> n = 4 >>> p = math.pi >>> '{0:.{1}f}'.format(p, n) '3.1416' 

the nested {1} takes the second argument, the current value of n, and applies it as specified (here, to the "precision" part of the format -- number of digits after the decimal point), and the outer resulting {0:.4f} then applies. Of course, you can hardcode the 4 (or whatever number of digits) if you wish, but the key point is, you don't have to!

Even better...:

>>> '{number:.{digits}f}'.format(number=p, digits=n) '3.1416' 

...instead of the murky "argument numbers" such as 0 and 1 above, you can choose to use shiny-clear argument names, and pass the corresponding values as keyword (aka "named") arguments to format -- that can be so much more readable, as you see!!!

Answer 2

An updated answer based on [Alex Martelli]'s solution but using Python 3.6.2 and it's updated format syntax I would suggest:

>>> n=4 >>> p=math.pi >>> f'{p:.{n}f}' '3.1416' 

But by choosing your variables wisely your code becomes self documenting

>>> precision = 4 >>> pi = math.pi >>> f'{pi:.{precision}f}' '3.1416'