Infer `time_unit` in `pl.duration` when nanoseconds is specified

[douglas-raillard-arm]

Infer `time_unit` in `pl.duration` when nanoseconds is specified

[stinodego]

I think this can be handled purely on the Python side. Make time_unit optional and set it based on the value of nanoseconds.

[douglas-raillard-arm]

It can also be seen with just pl.duration():

>>> pl.LazyFrame().with_columns(pl.duration(nanoseconds=1)).collect()
shape: (1, 1)
┌──────────────┐
│ literal      │
│ ---          │
│ duration[μs] │
╞══════════════╡
│ 0µs          │
└──────────────┘

>>> pl.LazyFrame().with_columns(pl.duration(nanoseconds=1).cast(pl.Int64)).collect()
shape: (1, 1)
┌─────────┐
│ literal │
│ ---     │
│ i64     │
╞═════════╡
│ 0       │
└─────────┘

>>> pl.LazyFrame().with_columns(pl.duration(nanoseconds=1).cast(pl.Duration('ns'))).collect()
shape: (1, 1)
┌──────────────┐
│ literal      │
│ ---          │
│ duration[ns] │
╞══════════════╡
│ 0ns          │
└──────────────┘

>>> pl.LazyFrame().with_columns(pl.duration(nanoseconds=1).cast(pl.Duration('ns')).cast(pl.Int64)).collect()
shape: (1, 1)
┌─────────┐
│ literal │
│ ---     │
│ i64     │
╞═════════╡
│ 0       │
└─────────┘

[douglas-raillard-arm]

Note that all seems to fall back in order if pl.duration(..., time_unit='ns') is specified, so maybe all this needs is a an automatic setting of time_unit that is consistent with the values provided, or an exception if a sub-Xs component is provided when time_unit='Xs' is specified.

[stinodego]

This is not a bug, but we can improve the API here. For now, you can specify time_unit='ns' as you have found.

The default time unit for duration is microseconds. So pl.duration(nanoseconds=999) results in a Duration(us).
We can possibly improve here by inferring the time unit from the inputs, e.g. if time_unit is None andnanoseconds is specified, create a Duration(ns).

Adding two durations of different precisions will use the lowest precision to avoid overflows. So the result here is expected.

[cojmeister]

Can I assign this to me?
Any places I should start my work at?

[cojmeister]

Opened PR: #14987