def _get_columns(var_type, var_type_columns_dico):
    """ function to get the columns 
    
    Parameters
    ----------
    var_type : tuple or str
        the type of variable we want (one type : string, several : tuple)
    
    var_type_columns_dico : dict
        dictionnary with keys = type of variable and values = name of columns
        
    Returns
    -------
    list of columns corresponding to that type(s)
    
    """
    if not isinstance(var_type, tuple):
        var_type = (var_type,)()
    res = []
    for v in var_type:
        res += var_type_columns_dico[v]
    return res
