Expr.is_between() not implemented for str

[2-5]

Problem description

Expr.is_between() is not implemented for str, only < and >:

import polars as pl

df = pl.DataFrame({
    "text": ["a", "b", "c", "d"],
})
print(df)

df2 = df.filter(
    pl.col("text") < "c",
)
print(df2)

df3 = df.filter(
    pl.col("text").is_between("b", "d"),
)
print(df3)

Output:

Traceback (most recent call last):
  File "test.py", line 13, in <module>
    df3 = df.filter(
  File ".venv\lib\site-packages\polars\internals\dataframe\frame.py", line 2702, in filter
    self.lazy()
  File ".venv\lib\site-packages\polars\internals\lazyframe\frame.py", line 1143, in collect
    return pli.wrap_df(ldf.collect())
exceptions.NotFoundError: b

[stinodego]

It is implemented, but you should write this like:

df3 = df.filter(
    pl.col("text").is_between(pl.lit("b"), pl.lit("d")),
)

It interprets "b" as pl.col("b") (which does not exist). That happens in many places throughout Polars for ease of use, e.g. df.select(["a", "b"]).

So this is by design, although it may be poor design. It can definitely be confusing.

[2-5]

Interesting, sounds like a useful feature.

Maybe an example could be added in the docs:

https://pola-rs.github.io/polars/py-polars/html/reference/expressions/api/polars.Expr.is_between.html

The error message could be improved, by mentioning that a column was not found, something like this:

exceptions.NotFoundError: b -> exceptions.ColumnNotFoundError: b

Or better exceptions.NotFoundError: No "b" column. Maybe you mean pl.lit("b")?

[alexander-beedie]

Maybe an example could be added in the docs:

Good idea; done. Will also (separately) see if we can slightly enrich the NotFoundError to better-indicate what has not been found 👍

[stinodego]

Maybe an example could be added in the docs:

While this is nice, it's a broader thing that affects many Polars functions. Every function should clearly state how it handles string inputs. We should do that, but it's quite cumbersome 😞

Better error messaging is really the best solution here.

[alexander-beedie]

@2-5, @stinodego: FYI ,have just done a sweep through the NotFound exception classes, improving error context as suggested... (#6670)