netCDF4 variable manipulation

[Sheldon]

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?
>
> There is no such thing as "the netCDF4 variable" in Python -- it is not a
> built-in part of the language, and therefore there is no built-in feature
> for manipulating it.
>
> You are going to have to be more specific about what you want than just
> "how do I modify a variable with for loops?".
>
> My wild guess is that you have some sort of config file which includes a
> field called "netCDF4" and you want to update it in place. We can't tell
> you how to do this without knowing what the config file is -- is it an
> INI file, XML, JSON, YAML, Unix-style rc file, a binary pickle, or
> something else?
>
> --
> Steven

Hi,

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.

/S