Allow pl.Duration in write_csv

[gab23r]

Description

Today it is not possible to write csv files with pl.Duration

pl.Series([1, 2, 3]).cast(pl.Duration('us')).to_frame().write_csv("a.csv")

# ComputeError: datatype duration[μs] cannot be written to CSV
# Consider using JSON or a binary format.

[gab23r]

@alexander-beedie, pinging you as maybe It was on your roadmap after the great work you've done here #19697

[Julian-J-S]

Imo this is difficult.
What is your expected result?

date/datetime and duration are physically stored as int.
date/datetime have very common and standardized "text representation" which is YYYY-MM-DD

duration does not really have it (yes, there is an iso format but you almost never see it and almost no tools have support for parsing this).
Most of this times you want to cast the Duration to your desired int value (total_seconds, total_minutes, total_hours, total_days) or keep the underlying physical representation.

[alexander-beedie]

The best thing might be to improve the error message, so the caller sees that there are now some more specific options (such as doing a cs.duration().to_string() before export to return ISO format) 🤔

I haven't looked at much of the wider tooling around this, so not sure what the degree of support is for parsing ISO durations is like (we don't yet, for example). I don't think the degree of support is near-zero though, as I've definitely seen a few libraries that handle it quite comfortably.