Can not Compute the Difference Between Two Steps

[HitachiWangNJU]

What happened?

Hello Earthkit team,

I am trying to compute the difference between two time steps (e.g., total precipitation with step=12,24, from grib file "data.grib2") using Earthkit.
According to the JUPYTER medium-mslp-rain.ipynb for drawing past 12 hour rainfall in charts.ecmwf.int, I tried to do like:

IN [1]:
ds = ek.data.from_source("file", "data.grib2")
tp1 = ds.sel(shortName="tp", step=12)
tp2 = ds.sel(shortName="tp", step=24)
print(tp1)
print(tp2)
OUT [1]:
MaskIndex(GRIBReader(data.grib2),[0])
MaskIndex(GRIBReader(data.grib2),[1])

IN [2]:
tp = (tp2 -tp1)
print(tp)
OUT:
(Nothing)

**But there were no error messages.

It seems like this operation could not work.

Environment:

earthkit 0.11.1
earthkit-data 0.14.3
earthkit-geo 0.3.0
earthkit-hydro 0.1.3
earthkit-meteo 0.3.0
earthkit-plots 0.3.1
earthkit-plots-default-styles 0.1.0
earthkit-regrid 0.4.0
earthkit-time 0.1.7
earthkit-transforms 0.4.0
earthkit-utils 0.0.1

What are the steps to reproduce the bug?

ds = ek.data.from_source("file", "data.grib2")
tp1 = ds.sel(shortName="tp", step=12)
tp2 = ds.sel(shortName="tp", step=24)
tp = (tp2 - tp1)

Version

v0.11.1

Platform (OS and architecture)

ubuntu 22.04

Relevant log output

MaskIndex(GRIBReader(data.grib2),[0])
MaskIndex(GRIBReader(data.grib2),[1])

Accompanying data

No response

Organisation

No response

[sandorkertesz]

@HitachiWangNJU, thank you for reporting this issue. Earthkit is still under development and math operators on fields and fieldlists are not yet supported, but planned to be implemented.