How can I format a float to variable precision?

In 2019 with Python >= 3.6

From Python 3.6 with PEP 498, you can use "f-strings" to format like this

>>> x = 123456
>>> n = 3
>>> f"{x:.{n}f}"
'123456.000'

Reference here for more detail:

Your solution is fine.

However, as a personal matter of style, I tend to use either only %, or only str.format().

So in this case I would define your formatting function as:

def my_precision(x, n):
    return '{:.{}f}'.format(x, n)

(thanks to @MarkDickinson for suggesting an shorter alternative to '{{:.{:d}f}}'.format(n).format(x))

Incidentally, you can simply do:

my_precision = '{:.{}f}'.format

and it works:

>>> my_precision(3.14159, 2)
'3.14'

:-)