Allow null values in conversion to datetime object

[pawel-wolff]

Description

This PR addresses the following issue:

import earthkit.data as ekd

fl = ekd.from_source("sample", "pl.grib").to_fieldlist()
fl2 = fl.sel({"parameter.variable": "t", "vertical.level": 500})[:3]

# len(fl2) == 3
# fl2.unique(["time.base_datetime", "time.step"]) == {
#     'time.base_datetime': (datetime.datetime(2024, 6, 3, 0, 0), datetime.datetime(2024, 6, 3, 12, 0)),
#     'time.step': (datetime.timedelta(0), datetime.timedelta(seconds=21600))
# }
# so a field for base_datetime=2024-6-3T12 and step=6h is missing

ds = fl2.to_xarray(allow_holes=True, add_valid_time_coord=True)

The above script crashes in

earthkit-data/src/earthkit/data/utils/dates.py

Line 72 in 03ca2ad

return to_datetime(dt.to_datetime())

with
AttributeError: 'FloatData' object has no attribute 'to_datetime'. The expected behaviour is to have an xarray Dataset in ds with valid_time auxiliary coordinate variable having 3 values + one 'NaT'.

To this end, the current PR extends the conversion to datetime object implemented in

earthkit-data/src/earthkit/data/utils/dates.py

Line 47 in 03ca2ad

def to_datetime(dt):

allowing for null values.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.48%. Comparing base (03ca2ad) to head (d3eecba).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #999      +/-   ##
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+ Coverage    81.46%   81.48%   +0.01%     
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  Files          236      236              
  Lines        15432    15444      +12     
  Branches       754      754              
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+ Hits         12572    12584      +12     
  Misses        2634     2634              
  Partials       226      226              

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[sandorkertesz]

@pawel-wolff, thanks for this fix. As safety measure, I suggest you add tests for these new to_datatime() cases to tests/utils/test_dates.py in a separate PR.

[pawel-wolff]

Will do!