Retrieve the attributes for a given set of mixin classes.

    The attributes of each mixin class are being merged into a single
    dictionary.

def _get_mixin_attributes(mixins):
    """Retrieve the attributes for a given set of mixin classes.

    The attributes of each mixin class are being merged into a single
    dictionary.
    """
    return {attribute: mixin.__dict__[attribute]
            for mixin in mixins
            for attribute in _MIXIN_ATTRIBUTES[mixin]}