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 Ho

Answer 1

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.