Python float to string, precision as variable

Question

I want to convert a float like a = 1.1234567 to a string, giving the precision as a second variable (which is why this is no duplicate of "Fixed digits after decimal with f-strings"):

def float2str(val, precision):
    ...

float2str(1.1234567, 3)  # '1.123'
float2str(1.1234567, 5)  # '1.12346' (mind correct rounding)

I know that f-strings can do the correct rounding using f'{a:.5f}', but the precision has to be part of the string.

I came up with this horribly ugly solutions and hope someone can point me to a more elegant way:

f'%.{precision}f' % a

Possible duplicate of Fixed digits after decimal with f-strings — Marius Mucenicu
– Marius Mucenicu, Commented Sep 25, 2019 at 7:57
@Aryerez you're doing two string operations in that case, one to convert the precision to a string, then another that parses the precision out of a string and uses it to format the float... — Sam Mason
– Sam Mason, Commented Sep 25, 2019 at 8:02
@Aryerez a) It mixes two string formatting concepts, b) as Sam said you perform two formatting operations. — Niklas Mertsch
– Niklas Mertsch, Commented Sep 25, 2019 at 8:03
@MariusMucenicu No, this is no duplicate, please at least read the questions' first sentence, not only the heading. — Niklas Mertsch
– Niklas Mertsch, Commented Sep 25, 2019 at 8:05

Sam Mason · Accepted Answer · 2019-09-25 07:56:59Z

3

you have a couple of options, given:

a = 1.1234567
b = 3

we can use either:

using format strings: f'{a:.{b}f}'
using old-style percent formatting: '%.*f' % (b, a)

answered Sep 25, 2019 at 7:56

Sam Mason

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2 Comments

Niklas Mertsch Over a year ago

Thanks, didn't think it would be possible to do nesting in f-strings.

Sam Mason Over a year ago

pyformat.info contains lots of useful recipes like this, especially if you know one variant it helps for searching so you can translate between idioms

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