Allow kwargs constraints in pl.when and expr.filter

[bkos]

Description

As mentioned on discord with @stinodego, similar to what has recently been introduced for df.filter, it would be nice to have kwargs also for pl.when and expr.filter, e.g.:

# for pl.when
pl.when(a=1).then...

# for expr.filter
df.select(pl.col('a').filter(b=1))

[alexander-beedie]

Makes sense; I'll see if I can knock this out on the flight, since I implemented the recent filter update - just hit 11,500m altitude, have a few hours to kill, and seem to have got some free (but absolutely terrible) WiFi access ;)

[stinodego]

Thanks @alexander-beedie ! Feel like picking up Expr/Series.filter as well?

[alexander-beedie]

Thanks @alexander-beedie ! Feel like picking up Expr/Series.filter as well?

@stinodego Ah! Somehow looked right past the Expr request - I blame the altitude in the cabin, lol... Will upgrade Expr to match, though I don't think Series makes sense with **kwargs (as it can only filter itself), but it should also support *predicates 👌