def get_axis_num_from_str(axes_string):
    """
    u,v,w correspond to 0,1,2 in the trailing axis of adcpy velocity arrays.
    This method returns a list of 0,1, and 2s corresponding to an input 
    string composed u,v, and ws.
    Inputs:
        axes_string = string composed of u v or w only [str]
    Returns:
        ax_list = python list containing the integers 0,1, or 2 
    """    
    if type(axes_string) is not str:
        ValueError("axes_string argument must be a string")
        raise
    ax_list = []
    for char in axes_string:
        if char in 'UVW': char = char.lower()
        if char not in 'uvw':
            ValueError("axes_string letters must be u,v, or w only")
            raise
        if char == 'u':
            ax_list.append(0)
        elif char == 'v':
            ax_list.append(1)
        elif char == 'w':
            ax_list.append(2)
    return ax_list