python netcdf: making a copy of all variables and attributes but one

Question

I need to process a single variable in a netcdf file that actually contains many attributes and variable. I think it is not possible to update a netcdf file (see question How to delete a variable in a Scientific.IO.NetCDF.NetCDFFile?)

My approach is the following:

1. get the variable to process from the original file
2. process the variable
3. copy all data from the original netcdf BUT the processed variable to the final file
4. copy the processed variable to the final file

My problem is to code step 3. I started with the following:

def  processing(infile, variable, outfile):
        data = fileH.variables[variable][:]

        # do processing on data...

        # and now save the result
        fileH = NetCDFFile(infile, mode="r")
        outfile = NetCDFFile(outfile, mode='w')
        # build a list of variables without the processed variable
        listOfVariables = list( itertools.ifilter( lamdba x:x!=variable , fileH.variables.keys() ) )
        for ivar in listOfVariables:
             # here I need to write each variable and each attribute

How can I save all data and attribute in a handfull of code without having to rebuild a whole structure of data?

Solution 1

Here's what I just used and worked. @arne's answer updated for Python 3 and also to include copying variable attributes:

import netCDF4 as nc
toexclude = ['ExcludeVar1', 'ExcludeVar2']

with netCDF4.Dataset("in.nc") as src, netCDF4.Dataset("out.nc", "w") as dst:
    # copy global attributes all at once via dictionary
    dst.setncatts(src.__dict__)
    # copy dimensions
    for name, dimension in src.dimensions.items():
        dst.createDimension(
            name, (len(dimension) if not dimension.isunlimited() else None))
    # copy all file data except for the excluded
    for name, variable in src.variables.items():
        if name not in toexclude:
            x = dst.createVariable(name, variable.datatype, variable.dimensions)
            dst[name][:] = src[name][:]
            # copy variable attributes all at once via dictionary
            dst[name].setncatts(src[name].__dict__)

Solution 2

If you just want to copy the file picking out variables, nccopy is a great tool as submitted by @rewfuss.

Here's a Pythonic (and more flexible) solution with python-netcdf4. This allows you to open it for processing and other calculations before writing to file.

with netCDF4.Dataset(file1) as src, netCDF4.Dataset(file2) as dst:

  for name, dimension in src.dimensions.iteritems():
    dst.createDimension(name, len(dimension) if not dimension.isunlimited() else None)

  for name, variable in src.variables.iteritems():

    # take out the variable you don't want
    if name == 'some_variable': 
      continue

    x = dst.createVariable(name, variable.datatype, variable.dimensions)
    dst.variables[x][:] = src.variables[x][:]

This does not take into account of variable attributes, such as fill_values. You can do that easily following the documentation.

Do be careful, netCDF4 files once written/created this way cannot be undone. The moment you modify the variable, it is written to file at the end of with statement, or if you call .close() on the Dataset.

Of course, if you wish to process the variables before writing them, you have to be careful about which dimensions to create. In a new file, Never write to variables without creating them. Also, never create variables without having defined dimensions, as noted in python-netcdf4's documentation.

Solution 3

This answer builds on the one from Xavier Ho (https://stackoverflow.com/a/32002401/7666), but with the fixes I needed to complete it:

import netCDF4 as nc
import numpy as np
toexclude = ["TO_REMOVE"]
with nc.Dataset("orig.nc") as src, nc.Dataset("filtered.nc", "w") as dst:
    # copy attributes
    for name in src.ncattrs():
        dst.setncattr(name, src.getncattr(name))
    # copy dimensions
    for name, dimension in src.dimensions.iteritems():
        dst.createDimension(
            name, (len(dimension) if not dimension.isunlimited else None))
    # copy all file data except for the excluded
    for name, variable in src.variables.iteritems():
        if name not in toexclude:
            x = dst.createVariable(name, variable.datatype, variable.dimensions)
            dst.variables[name][:] = src.variables[name][:]

Solution 4

The nccopy utility in C netCDF versions 4.3.0 and later includes an option to list which variables are to be copied (along with their attributes). Unfortunately, it doesn't include an option for which variables to exclude, which is what you need.

However, if the list of (comma-delimited) variables to be included doesn't cause the nccopy command-line to exceed system limits, this would work. There are two variants for this option:

nccopy -v var1,var2,...,varn input.nc output.nc
nccopy -V var1,var2,...,varn input.nc output.nc

The first (-v) includes all the variable definitions, but only data for the named variables. The second (-V) doesn't include definitions or data for unnamed variables.

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