ncdata

Generic NetCDF data in Python.

Represents raw netCDF data in Python, with no structural assumptions or interpretations, and provides facilities to inspect and manipulate it with complete freedom.

Also

Provides fast data exchange between analysis packages, and full control of storage formatting.

Especially : Ncdata exchanges data between Xarray and Iris as efficiently as possible

"lossless, copy-free and lazy-preserving".

This enables the user to freely mix+match operations from both projects, getting the "best of both worlds".

import xarray
import ncdata.iris_xarray as nci
import iris.quickplot as qplt

ds = xarray.open_dataset(filepath)
ds_resample = ds.rolling(time=3).mean()
cubes = nci.cubes_from_xarray(ds_resample)
temp_cube = cubes.extract_cube("air_temperature")
qplt.contourf(temp_cube[0])

Purposes

• represent netcdf data as structures of Python objects
• easy manipulation of netcdf data with pythonic syntax
• Fast and efficient translation of data between Xarray and Iris objects.
  • This allows the user to mix+match features from either package in code.

See : https://ncdata.readthedocs.io/en/latest/userdocs/user_guide/design_principles.html

Documentation

On ReadTheDocs. Please see:

• stable
• latest

Demonstration code examples:

• Apply Iris regrid to xarray data
• Use Zarr data in Iris
• Correct a mis-coded attribute in Iris input
• Rename a dimension in xarray output
• Copy selected data to a new file

Apply Iris regrid to xarray data

from ncdata.iris_xarray import cubes_to_xarray, cubes_from_xarray
dataset = xarray.open_dataset("file1.nc", chunks="auto")
(cube,) = cubes_from_xarray(dataset)
cube2 = cube.regrid(grid_cube, iris.analysis.PointInCell)
dataset2 = cubes_to_xarray(cube2)

Use Zarr data in Iris

from ncdata.threadlock_sharing import enable_lockshare
enable_lockshare(iris=True, xarray=True)
import xarray as xr
dataset = xr.open_dataset(input_zarr_path, engine="zarr", chunks="auto")
input_cubes = cubes_from_xarray(dataset)
output_cubes = my_process(input_cubes)
dataset2 = cubes_to_xarray(output_cubes)
dataset2.to_zarr(output_zarr_path)

Correct a miscoded attribute in Iris input

from ncdata.iris import to_iris
enable_lockshare(iris=True)
ncdata = from_nc4(input_path)
for var in ncdata.variables.values():
    if "coords" in var.attributes:
        var.attributes.rename("coords", "coordinates")
cubes = to_iris(ncdata)

Rename a dimension in xarray output

enable_lockshare(xarray=True)
dataset = xr.open_dataset("file1.nc")
xr_ncdata = from_xarray(dataset)
from ncdata.utils import rename_dimension
rename_dimension(xr_ncdata, "dim0", "newdim")
to_nc4(ncdata, "file_2a.nc")

Copy selected data to a new file

from ncdata.netcdf4 import from_nc4, to_nc4
ncdata = from_nc4("file1.nc")

# Make a list of partial names to select the wanted variables
keys = ["air_", "surface"]

# Explicitly add dimension names, to include all the dimension variables
keys += list(ncdata.dimensions)

# Identify the wanted variables
select_vars = [
    var
    for var in ncdata.variables.values()
    if any(var.name.startswith(key) for key in keys)
]

# Add any referenced coordinate variables
for var in select_vars:
    coordnames = var.avals.get("coordinates")
    if coordnames:
        for coordname in coordnames.split(" "):
            select_vars.append(ncdata.variables[coordname])

# Replace variables with only the wanted ones
ncdata.variables.clear()
ncdata.variables.addall(select_vars)

# Save
to_nc4(ncdata, "pruned.nc")