How to use the nexusformat.nexus.nxclasses function in nexusformat

def init_shell(self, filename):
        """Initialize imports in the shell."""
        global _shell
        _shell = self.window.user_ns
        s = ("import nexusformat.nexus as nx\n"
             "from nexusformat.nexus import NXgroup, NXfield, NXattr, NXlink\n"
             "from nexusformat.nexus import *\n"
             "import nexpy\n"
             "from nexpy.gui.plotview import NXPlotView")
        six.exec_(s, self.window.user_ns)

        s = ""
        for _class in nxclasses:
            s = "%s=nx.%s\n" % (_class,_class) + s
        six.exec_(s, self.window.user_ns)

        config_file = os.path.join(self.nexpy_dir, 'config.py')
        if not os.path.exists(config_file):
            s = ["import sys\n",
                 "import os\n",
                 "import h5py as h5\n",
                 "import numpy as np\n",
                 "import numpy.ma as ma\n",
                 "import scipy as sp\n",
                 "import matplotlib as mpl\n",
                 "from matplotlib import pylab, mlab, pyplot\n",
                 "plt = pyplot\n",
                 "os.chdir(os.path.expanduser('~'))\n"]
            with open(config_file, 'w') as f: