Add support for querying netCDF4 file for groups

[fergu]

Basically as the title says - it would be nice to have support to query NetCDF4 files for the groups that they contain.

I can do something similar using the netCDF4 library directly:

import netCDF4 as nc

myDset = nc.Dataset("/path/to/dataset.nc","r")
# From here you can use myDset.groups to get all groups in the file, or use a dict-like interface to get the groups of subgroups, etc

Assuming implementing this is really this simple, I can take a swing at this and submit a PR.

[shoyer]

This has come up a couple of times before -- please see #2916 and #1092.

The basic issue is that xarray.Dataset doesn't have any notion of a group in it, so it isn't clear how we could work in xarray.

If it's just a matter of listing groups that can be found in a file, it seems like netCDF4 works fine for that.

[lamorton]

I hacked a quick solution for exploring HDF5 files that might be of interest.

import h5py
def explore_file(filepath,show="arrays"):
    """
    View the internal structure of an HDF5 file
    Returns a dictionary of the entity names & representations of their values
    Arguments:
        filepath: string
        show: one of ('groups','arrays','all')
            groups: display the number of direct array-type members of each group/subgroup
            arrays: display the shape & dtype of each array (if not a scalar)
            all: display the shape & dtype of every array
    """
    with h5py.File(filepath,mode='r') as f:
        descriptions = {}
        if show=="groups":
            def visitor(k,v):
                if isinstance(v, h5py.Group):
                    arrays = [k for k in v.keys() if isinstance(v[k],h5py.Dataset)]
                    if len(arrays) >0:
                        descriptions[k] = len(arrays)
        elif show == "arrays":
            def visitor(k,v):
                if isinstance(v,h5py.Dataset) and len(v.shape)>0:
                    descriptions[k] = "{},{}".format(v.shape,v.dtype)
        elif show =="all":
            def visitor(k,v):
                if isinstance(v,h5py.Dataset):
                    descriptions[k] = "{},{}".format(v.shape,v.dtype)
        f.visititems(visitor)#Apply names.append to each name in the file
    return descriptions

[stale]

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[dcherian]

Closing as duplicate of #2916. Eventually this will be fixed by datatree