skeleton

def requires_nodes(func):
    """Return `None` if neuron has no nodes."""
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        self = args[0]
        # Return 0
        if isinstance(self.nodes, str) and self.nodes == 'NA':
            return 'NA'
        if not isinstance(self.nodes, pd.DataFrame):
            return None
        return func(*args, **kwargs)
    return wrapper