Python bindings for NCO. A fork from Ralf Mueller's cdo-bindings.

pynco - Use Python to access the power of NCO

This package contains the module python nco, which implements a python style access to the NetCDF Operators (NCO). NCO is a command line tool for processing netCDF data. Its main focus is climate data, but it can by used for other purposes too.


This will install all required and optional dependencies and is the quickest and easiest way to a working pynco installation.

conda install -c conda-forge pynco

PyPI installation

Please see the requirements before installing

pip install nco

Python Installation

Please see the requirements before installing

python install



  • Platform: Unix or Mac OS (Windows has not bee tested)
  • NetCDF Operators (NCO) - Version 4.6.9 or later. We don't test against every NCO version.
  • Python 3.6 or 3.7


These will allow pynco operations to return numpy arrays


Importing the Nco class

from nco import Nco

Run operators

For python an instance has to be created first

from nco import Nco
nco = Nco()

Now any NCO command (i.e. ncks, ncra, ...) can be called as a method of nco.

  • Required arguments

    • input - Input netcdf file name, str
  • Optional arguments

    • output - str or list of strings representing input netCDF filenames. If not provided and operator returns a file (not an array or stdout text), the method will return a temporary file.
    • debug - bool or int, if less than 0 or True, debug statements will be turned on for NCO and NCOpy (default: False)
    • returnCdf - bool, return a netCDF file handle (default: False)
    • returnArray - str. return a numpy array of variable name (default: '')
    • returnMaArray - str. return a numpy masked array of variable name (default: '')
    • use_shell - bool. use shell to execute commands, useful if you need to pass wildcards or other characters in arguments that can be expanded by shell interpretor (default: False)
    • options - list, NCO input options, for example options=['-7', '-L 1'] (default: [])
    • Atted - a wrapper object to be used for ncatted. Atted objects can be included in the options list
    • Limit - a wrapper object for the hyperslab (-d) command line option
    • Rename - a wrapper object for the -d, -a, -v, and -g command line options in ncrename
    • **kwargs - any kwarg will be passed as a key, value pair to the nco command --{key}={value}. This allows the user to pass any number of long name commands list in the nco help pages.


  • File information:

    python ncdump_string = nco.ncdump(input=ifile)

  • Operators with user defined regular output files:

    python nco.ncra(input=ifile, output=ofile)

  • Use temporary output files:

    python temp_ofile = nco.ncrcat(input=ifile)

  • Set global NCO options:

    python nco.ncks(input=ifile, output=ofile, options="--netcdf4")

  • Return multi-dimension arrays:

    python temperatures = nco.ncra(input=ifile, returnArray=True).variables['T'][:] temperatures = nco.ncra(input=ifile, returnCdf=True).variables['T'][:] temperatures = nco.ncra(input=ifile, returnArray='T')

  • Wrapper Objects

    The Atted opject is a convienent wrapper object to the -a command-line switch in ncatted. The Limit object is a wrapper to the -d command-line switch. The Rename is a wrapper for the -a, -v, -d , -g switches in ncrename.

    e.g the following are equivalent:

    ncatted -a _FillValue,three_dmn,o,d,-9.91e+33 nco.ncatted(input="",options=[c.atted("overwrite","_FillValue","three_dmn",-9.91e+33,'d')])

Tempfile helpers

pynco includes a simple tempfile wrapper, which makes life easier. In the absence of a specified output file, pynco will create a temporary file to allow the results of the task to be returned to the user. For example:

temperatures = nco.ncra(input=ifile, returnArray='T')

is equivalent to:

temperatures = nco.ncra(input=ifile, output=tempfile.mktemp(), returnArray='T')

Atted wrapper

It is sometimes more tidy to define the atted objects in a separate list then add that list the options in the nco call

opt = [
    c.Atted("o", "units", "temperature", "Kelvin"),
    c.Atted("c", "min", "temperature", 0.16, 'd'),
    c.Atted("m", "max", "temperature", 283.01, 'float64'),
    c.Atted("c", "bnds", "time", [0.5, 1.5], 'f')
nco.ncatted(input="", options=opt)

You can also use keyword arguments in the call so the above options become

opt = [
    c.Atted(mode="o", attName="units", varName="temperature", Value="Kelvin", sType="c"),
    c.Atted(mode="create", attName="min", varName="temperature", Value=0.16, sType='d' ),
    c.Atted(mode="modify", attName="max", varName="temperature", Value=283.01, sType='float64'),
    c.Atted(mode="create", attName="bnds", varName="time", Value=[0.5, 1.5], sType='float32')
nco.ncatted(input="", options=opt)


Can be a single value or a list (or any python iterable type or a numpy array).

  • If sType is not included then the type is inferred from the first value in the list
  • If sType is included then any values in the list are not of sType are converted to the sType


You can use the following: f, d, l/i, s, b, ub, us, u, ll, ull Or their numpy equivalents: float32, float64, int32, int16, byte, ubyte, uint16, uint32, int64, uint64

For a netCDF3 character string use c or char For netCDF4 string(s) use sng or string


For mode you can use the single character abbreviations as per ncatted: a, c, d, m, n, o or the following words: a)ppend, create, delete, modify, overwrite

Limit and LimitSingle wrapper

The following are equivalent:


ncks -d time,0,8,2 -d time,10 -d lat,-20.0,20.0 -d lon,50.0,350.0  -d lev,,,4


opt = [
    c.Limit("time", 0, 8, 2),
    c.LimitSingle("time", 10),
    c.Limit("lat", -20.0, 20.0),
    c.Limit(dmn_name="lon", srt=50.0, end=350.0),
    c.Limit(dmn_name="lev", srd=4)

nco.ncks(input="", output="", options=opt)

Rename wrapper

The following are equivalent:


ncrename -v p,pressure -v t,temperature


rDict = {
    'p': 'pressure',
    't': 'temperature'
nco.ncrename(input="", options=[ c.Rename("variable", rDict) ])

Also equivalent:


ncrename -d lon,longitude -d lat,latitude -v lon,longitude -v lat,latitude


rDict = {
    'lon': 'longitude',
    'lat': 'latitude'
nco.ncrename(input="", options=[ c.Rename("d", rDict), c.Rename("v", rDict) ])

Support, Issues, Bugs, ...

Please use the github page to report issues/bugs/features:

For usage questions, please use Stack Overflow.


pynco makes use of the MIT License, see LICENSE.txt.

Other stuff

  • Requires: NCO version 4.6.9 or newer, Python 3.6 or later
  • Optional: scipy or Python netCDF4