human
Solve the following python programming problem given the following code:
def __new__(cls, name, bases, attrs) : 
    column_dict = OrderedDict () 
    primary_keys = OrderedDict () 
    pk_name = None 
    inherited_columns = OrderedDict () 
    for base in bases : 
        for (k, v) in getattr (base, '_defined_columns', { 
            
}).items () : 
            inherited_columns.setdefault (k, v) 
is_abstract = attrs ['__abstract__'] = attrs.get ('__abstract__', False) 
    attrs ['__discriminator_value__'] = attrs.get ('__discriminator_value__') 
    options = (attrs.get ('__options__') or { 
        
}) 
    attrs ['__default_ttl__'] = options.get ('default_time_to_live') 
    def _transform_column(col_name, col_obj) : 
        column_dict [col_name] = col_obj 
        if col_obj.primary_key : 
            primary_keys [col_name] = col_obj 
col_obj.set_column_name (col_name) 
        attrs [col_name] = ColumnDescriptor (col_obj) 
column_definitions = [(k, v) for (k, v) in attrs.items () if isinstance (v, columns.Column)] 
    column_definitions = sorted (column_definitions, key = (lambda x : x [1].position)) 
    is_polymorphic_base = any ([c [1].discriminator_column for c in column_definitions]) 
    column_definitions = ([x for x in inherited_columns.items ()] + column_definitions) 
    discriminator_columns = [c for c in column_definitions if c [1].discriminator_column] 
    is_polymorphic = (len (discriminator_columns) > 0) 
    if (len (discriminator_columns) > 1) : 
        raise ModelDefinitionException ('only one discriminator_column can be defined in a model, {0} found'.format (len (discriminator_columns))) 
if (attrs ['__discriminator_value__'] and (not is_polymorphic)) : 
        raise ModelDefinitionException ('__discriminator_value__ specified, but no base columns defined with discriminator_column=True') 
(discriminator_column_name, discriminator_column) = (discriminator_columns [0] if <MASK> else (None, None)) 
    if isinstance (discriminator_column, (columns.BaseContainerColumn, columns.Counter)) : 
        raise ModelDefinitionException ('counter and...
    
    Given the code above, what is a proper replacement for <MASK>?
assistant
#Here is a proper replacement for <MASK>.
discriminator_columns