Specify string format for numeric during conversion to pl.Utf8 - python-polars

Question

Is there any way to specify a format specifier if, for example, casting a pl.Float32, without resorting to complex searches for the period character? As in something like:
s = pl.Series([1.2345, 2.3456, 3.4567])
s.cast(pl.Utf8, fmt="%0.2f") # fmt obviously isn't an argument
My current method is the following:
n = 2 # number of decimals desired
expr = pl.concat_str((
c.floor().cast(pl.Int32).cast(pl.Utf8),
pl.lit('.'),
((c%1)*(10**n)).round(0).cast(pl.Int32).cast(pl.Utf8)
)).str.ljust(width)
i.e separate the pre-decimal and post-decimal, format individually as strings, and concat together. Is there an easier way to do this?
Expected output:
shape: (3,)
Series: '' [str]
[
"1.23"
"2.34"
"3.45"
]

Answer 1

I'm not aware of a direct way to specify a format when casting, but here's two easy ways to obtain a specific number of decimal points.
Use write_csv
We can write a DataFrame as a csv file (to a StringIO buffer), which allows us to set a float_precision parameter. We can then use read_csv to parse the StringIO buffer to obtain our result. (This is much faster than you might think.) Note: we must use infer_schema_length=0 in the read_csv to prevent parsing the string back to a float.
from io import StringIO
s = pl.Series([1.2345, 2.3456, 3.4567])
n = 2
(
pl.read_csv(
StringIO(
pl.select(s)
.write_csv(float_precision=n)
),
infer_schema_length=0
)
.to_series()
)
shape: (3,)
Series: '1.23' [str]
[
"1.23"
"2.35"
"3.46"
]
Pad with zeros and then use a single regex
Another approach is to cast to a string and then append zeroes. From this, we can use a single regex expression to extract our result.
n = 2
zfill = '0' * n
regex = r"^([^\.]*\..{" + str(n) + r"})"
(
pl.select(s)
.with_column(
pl.concat_str([
pl.col(pl.Float64).cast(pl.Utf8),
pl.lit(zfill)
])
.str.extract(regex)
)
.to_series()
)
shape: (3,)
Series: '' [str]
[
"1.23"
"2.34"
"3.45"
]