`Gaze.split()` drops gaze if there are no events

[saeub]

Current Behavior

Calling gaze.split() when gaze.events is empty results in an empty list being returned.

The problem is the use of zip here:

pymovements/src/pymovements/gaze/gaze.py

Line 355 in e61b668

for samples, events in zip(samples_list, events_list)

Also, if only a subset of trials have no events, the Gaze and Event objects in the returned list will be misaligned.

Expected Behavior

Gaze should be split and returned even if there are no events. Samples and events should be aligned even if some trials have no events.

Steps to Reproduce

Example file: gaze.csv

To reproduce empty return value:

import pymovements as pm

gaze = pm.gaze.from_csv(
    "gaze.csv",
    pixel_columns=["pixel_x", "pixel_y"],
    trial_columns=["trial"],
)
gaze.split()  # empty

To reproduce misaligned events:

import pymovements as pm
import polars as pl

gaze = pm.gaze.from_csv(
    "gaze.csv",
    pixel_columns=["pixel_x", "pixel_y"],
    trial_columns=["trial"],
)
# Add event for trial2 only
gaze.events.frame.extend(
    pl.DataFrame(
        {
            "trial": ["trial2"],
            "name": ["fixation"],
            "onset": [0],
            "offset": [1000],
            "duration": [1000],
        }
    )
)
gaze.split()  # gaze for trial1 and event for trial2; gaze for trial2 missing

Checklist

• I am running the latest version
• I checked the documentation and found no answer
• I checked to make sure that this issue has not already been filed
• I have provided sufficient information for the team

[dkrako]

Thank you very much for the bug report!

What would be the best fix? As you correctly identified, the main problem is that events_list and samples_list may be unaligned.

Maybe polars.DataFrame.partition_by() should be called with include_key=True?

I'll try to create I PR today to fix this issue.