Field.subspace fails to work properly for cyclic field

[sws98slg]

On upgrading from a very old version of cfpython (3.7.0) to the latest version (3.18.0) field.subspace no longer seems to work properly for a cyclic field. As an example using the datasets provided with cfplot

f=cf.read('cfplot_data/tas_A1.nc')[0]
f.subspace(X=cf.wi(175,-175))
Gives for the old version:
Out[139]: <CF Field: air_temperature(time(1680), latitude(73), longitude(3)) K> (correct answer)
But for the new version:
ValueError: No indices found from: X=<CF Query: (wi [175, -175])>

f.subspace(X=cf.wi(-5,5))
Gives for both versions
Out[140]: <CF Field: air_temperature(time(1680), latitude(73), longitude(3)) K> (correct answer)

f.subspace(X=cf.wi(355,5))
Gives for the old version:
Out[142]: <CF Field: air_temperature(time(1680), latitude(73), longitude(3)) K>
But for the new version:
ValueError: No indices found from: X=<CF Query: (wi [355, 5])>

I've tried explicitly setting the field to be cyclic

f.cyclic('X',iscyclic=True)
Before doing the subspacing but this doesn't help.

I hope I've explained this fully. I can see there was a previous issue reported (#828) that seems related, but this is marked as closed. For further info my environments are below.

My old environment is
Platform: Linux-3.10.0-1160.119.1.el7.x86_64-x86_64-with-centos-7.9.2009-Core
HDF5 library: 1.10.4
netcdf library: 4.6.3
udunits2 library: libudunits2.so.0
python: 3.7.7
netCDF4: 1.5.3
cftime: 1.2.1
numpy: 1.19.2
psutil: 5.6.3
scipy: 1.4.1
matplotlib: 3.3.2
ESMF: 7.1.0r
cfdm: 1.8.7.0
cfunits: 3.3.0
cfplot: 3.0.38
cf: 3.7.0

And my new one is
Platform: Linux-4.18.0-553.34.1.el8_10.x86_64-x86_64-with-glibc2.28
Python: 3.12.11
packaging: 25.0
numpy: 2.3.2
cfdm.core: 1.12.2.0
udunits2 library: /home/users/sws98slg/.conda/envs/myenv3_RACC2/lib/libudunits2.so.0
HDF5 library: 1.14.6
netcdf library: 4.9.2
netCDF4: 1.7.2
h5netcdf: 1.6.4
h5py: 3.14.0
zarr: 3.1.1
s3fs: 2025.7.0
scipy: 1.16.0
dask: 2025.7.0
cftime: 1.6.4
cfunits: 3.3.7
cfdm: 1.12.2.0
esmpy/ESMF: 8.8.1
psutil: 7.0.0
matplotlib: 3.10.5
cfplot: 3.4.0
cf: 3.18.0

[davidhassell]

Hi, thanks for the report.

The answer is to order the arguments cf.wi so that they are increasing:

>>> import cf
>>> f = cf.read('tas_A1.nc')[0]
>>> f.subspace(X=cf.wi(-175, 175))
<CF Field: air_temperature(time(1680), latitude(73), longitude(93)) K>
>>> f.subspace(X=cf.wi(-5, 5))
<CF Field: air_temperature(time(1680), latitude(73), longitude(3)) K>
>>> f.subspace(X=cf.wi(355, 365))
<CF Field: air_temperature(time(1680), latitude(73), longitude(3)) K>

OK - that works, but I agree that it should be clever enough, in the cyclic case, to map cf.wi(355, 5) to cf.wi(-5, 5)) for you. I don't know why it stopped working sometime after v3.7.0 (2020) - that's far too long ago for me to remember or want to work out why :) - but I'll put in a fix so your original code works as you expected, for the next release (not the one that's coming later this week, but the one due on early October).

[alaniwi]

I have another test case (based on a report on the JASMIN helpdesk), which seems to be somewhat related to this issue, but the exact solution/workaround proposed above does not seem to hold in this case because the longitudes used in this case (-21, -19) are already in increasing order.

import cf
field_data = cf.read("test3.nc")[0]
field_data.dim('dimensioncoordinate0').set_property('standard_name', 'latitude')
field_data.dim('dimensioncoordinate1').set_property('standard_name', 'longitude')
field_data.cyclic('X', period=360)
sub = field_data.subspace(latitude=cf.wi(34,36), longitude=cf.wi(-21,-19))

This uses the input file that is wrapped in this zip file (because GitHub doesn't like .nc attachments). In fact it is subsetted from an original 4d file, as you will see in the history attribute, for sake of a more minimal example.

It fails with cf-python 3.18.0 but succeeds with cf-python 3.16.2. (These are as installed in different versions of Jaspy.)

In the failure case, it will nonetheless work for the longitude range (360-21, 360-19).

[alaniwi]

For convenience, here is a dump of the test file (with edits to the list of coordinate values for conciseness):

$ ncdump -c test3.nc 
netcdf test3 {
dimensions:
	latitude = 180 ;
	longitude = 360 ;
variables:
	float latitude(latitude) ;
		latitude:units = "degrees_north" ;
		latitude:long_name = "latitude" ;
	float longitude(longitude) ;
		longitude:units = "degrees_east" ;
		longitude:long_name = "longitude" ;
	double msl(latitude, longitude) ;
		msl:_FillValue = -32767. ;
		msl:missing_value = -32767s ;
		msl:units = "Pa" ;
		msl:long_name = "Mean sea level pressure" ;
		msl:standard_name = "air_pressure_at_mean_sea_level" ;
		msl:cell_methods = "time, number: mean" ;

// global attributes:
		:Conventions = "CF-1.6" ;
		:history = "Fri Aug 15 12:11:26 2025: ncks -x -v number,time /tmp/test2.nc /tmp/test3.nc\n",
			"Fri Aug 15 12:10:55 2025: ncwa -O -a time,number /tmp/test.nc /tmp/test2.nc\n",
			"Fri Aug 15 12:10:14 2025: ncks -O -d time,0,0 -d number,0,0 /gws/nopw/j04/ncas_climate_vol1/users/lbaker/c3s_data/reconverted_data/ukmo_mslp_members_oct_sys602_hindcast.nc /tmp/test.nc\n",
			"2025-07-07 13:26:49 GMT by grib_to_netcdf-2.36.0: grib_to_netcdf ukmo_mslp_members_oct_sys602_hindcast.grib -o reconverted_data/ukmo_mslp_members_oct_sys602_hindcast.nc" ;
		:NCO = "netCDF Operators version 5.3.3 (Homepage = http://nco.sf.net, Code = http://github.com/nco/nco, Citation = 10.1016/j.envsoft.2008.03.004)" ;
data:

 latitude = 89.5, 88.5, 87.5, [...etc...] -88.5, -89.5 ;

 longitude = 0.5, 1.5, 2.5, [...etc...], 357.5, 358.5, 359.5 ;
}