Return the WRAPPER_CLASS for a given backend.

    :rtype: pyvisa.highlevel.VisaLibraryBase

def get_wrapper_class(backend_name):
    """Return the WRAPPER_CLASS for a given backend.

    :rtype: pyvisa.highlevel.VisaLibraryBase
    """
    try:
        return _WRAPPERS[backend_name]
    except KeyError:
        if backend_name == 'ni':
            from .ctwrapper import NIVisaLibrary
            _WRAPPERS['ni'] = NIVisaLibrary
            return NIVisaLibrary

    try:
        pkg = __import__('pyvisa-' + backend_name)
        _WRAPPERS[backend_name] = cls = pkg.WRAPPER_CLASS
        return cls
    except ImportError:
        raise ValueError('Wrapper not found: No package named pyvisa-%s' % backend_name)