netCDF4 variable manipulation

[Chris Rebert]

On Tue, Feb 21, 2012 at 12:29 AM, Sheldon <shejo284 at gmail.com> wrote:
> On Feb 21, 12:53 am, Steven D'Aprano <steve
> +comp.lang.pyt... at pearwood.info> wrote:
>> On Mon, 20 Feb 2012 12:37:22 -0800, Sheldon wrote:
>> > Hi,
>>
>> > I'm trying to read a netCDF4 variable from a file (no problem) and then
>> > scale it before writing over the original variable in the file.
>>
>> > I'm using python 2.7 and the latest netCDF4 module. It's not that I keep
>> > getting an error message but I want to do this without using for loops
>> > and all the manuals seems to be skipping this task as if it is never
>> > done. I'm new to python and coming over from matlab. Does anyone know
>> > how to modify the netCDF4 variable in python and then write it back
>> > without creating a new variable, or using for loops?
<snip>
> My apologies. I didn't realize this module was not popular. I'm using
> the python module "netCDF4"
> that I installed using PIP. This module gives me the ability to read
> and write netcdf4 files.
> The module is an interface to the netcdf4.1.3 and HDF51.8.8 libraries.
> What I'm trying to do is open an existing netcdf file, extract an
> variable, modify the variable, and then
> write it again to the file - effectively replacing the old variable.
>
> rgrp = netCDF4.Dataset('ODIN_NWP_2001_01_01_00.NC','r+')
> pv = rgrp.groups['Data_3D'].variables['PV']
>
> print pv
> <type 'netCDF4.Variable'>
> float32 PV(u'level', u'lat', u'lon')
>    longname: Potential Vorticity
>    _FillValue: -999.0
>    units: K m**2 kg**-1 s**-1
>    code: 60
>    scale_factor: 1.0
>    add_offset: 0.0
> path = /Data_3D
> unlimited dimensions = ()
> current size = (60, 181, 360)
>
> As you can see, pv is described here as a netCDF4.Variable. I'm trying
> to modify this pv and then write it back to the file
> but the manual is vague on this part. I tried this:
>
> pv = pv*2
> ---------------------------------------------------------------------------
> TypeError                                 Traceback (most recent call
> last)
> ----> 1 pv = pv*2
> TypeError: unsupported operand type(s) for *: 'netCDF4.Variable' and
> 'int'
>
> I'm unsure how this is suppose to work.

The API docs suggest that Variables don't support any arithmetic
operations, just slicing and attribute manipulation. They seem to
interoperate with numpy arrays though, so presumably you could avoid
explicit looping by loading the Variable's contents into a numpy
array, doing your calculations on that, and then assigning the
resulting numpy array back into a slice of the Variable.

Cheers,
Chris