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Fix false timedelta decoding SerializationWarning and improve warning message #10072

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Fix false timedelta decoding SerializationWarning and improve warning message #10072

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As reported in #10071, false warnings for resolution changes can be emitted when decoding timedeltas encoded with floating point values. In addition, the warning message could be clearer when resolution changes do occur. This PR fixes both issues.

The warning message in the event of a resolution change now reads:

>>> import xarray as xr
>>> var = xr.Variable(["time"], [0, 0.1], attrs={"units": "seconds"})
>>> xr.conventions.decode_cf_variable("foo", var, decode_timedelta=xr.coders.CFTimedeltaCoder(time_unit="s")).compute()
<stdin>:1: SerializationWarning: Can't decode floating point timedeltas to 's' without precision loss; decoding to 'ms' instead. To silence this warning pass decode_timedelta=CFTimedeltaCoder(time_unit='ms') to your opening function.
<xarray.Variable (time: 2)> Size: 16B
array([  0, 100], dtype='timedelta64[ms]')

cc: @kmuehlbauer

mathause
mathause approved these changes Feb 24, 2025
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Looks good!

kmuehlbauer
kmuehlbauer approved these changes Feb 24, 2025
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I'm fully on-board with this change! This is much more informative now. Thanks @spencerkclark!

xarray/coding/times.py
Comment on lines +618 to +624
formatted_kwarg = f"{kwarg}={coder}(time_unit={new_time_unit!r})"
message = (
f"Can't decode floating point {datatype} to {time_unit!r} "
f"without precision loss; decoding to {new_time_unit!r} "
f"instead. To silence this warning pass {formatted_kwarg} "
f"to your opening function."
)
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👍

@spencerkclark spencerkclark enabled auto-merge (squash) March 1, 2025 13:18
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@kmuehlbauer kmuehlbauer kmuehlbauer approved these changes

@mathause mathause mathause approved these changes

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Invalid conversion of generic time coordinates
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@spencerkclark @kmuehlbauer @mathause