How to extract mapping between raw and cf-python's coordinate names

[robertjwilson]

cf-python does an excellent job of giving proper names to coordinates (time, lon, lat etc.) after using cf.read. However, is it possibly to extract how the new coordinate names map to those in the raw netCDF file?

Is that something that is purely internal or is it preserved somewhere?

[davidhassell]

Hi @robertjwilson,

Thanks for the question. cf.read preserves all netCDF names and makes them available and editable via the various nc_* methods that exist on many cf-python objects. e.g. https://ncas-cms.github.io/cf-python/class/cf.DimensionCoordinate.html#netcdf, https://ncas-cms.github.io/cf-python/class/cf.DomainAxis.html#netcdf, etc.

In this example f wasn't read from a file, but the "example fields" come with netCDF information as if they had been:

>>> import cf
>>> f = cf.example_field(0)
>>> x = f.coordinate('X')
>>> x.nc_get_variable()    # will give the name from the file (if read from one)
'lon'
>>> x.nc_set_variable('newlon')   # set a new name
>>> x.nc_get_variable()
'newlon'
>>> cf.write(f, 'test_file.nc')   # writes the X coordinate with netCDF name given by x.nc_get_variable() 

If no netCDF variable exists (either because you created the Field ab initio and didn't set one, or you deleted it with nc_del_variable) then cf.write will make up as nice a name as it can.

Hope that helps,
David

[robertjwilson]

Thanks for the quick response @davidhassell. That has solved my problem succicently.

Much appreciated

Robert