human
What is a the Python code you requested given the following:
Complete the below
def backref_listeners(attribute, key, uselist):

    ''''Apply listeners to synchronize a two-way relationship.'
    '''
assistant
#Here is the Python code you requested.
    parent_token = attribute.impl.parent_token
    parent_impl = attribute.impl
    def _acceptable_key_err(child_state, initiator, child_impl):
        raise ValueError(('Bidirectional    attribute    conflict    detected:    Passing    object    %s    to    attribute    "%s"    triggers    a    modify    event    on    attribute    "%s"    via    the    backref    "%s".' % (state_str(child_state), initiator.parent_token, child_impl.parent_token, attribute.impl.parent_token)))
    def emit_backref_from_scalar_set_event(state, child, oldchild, initiator):
        if (oldchild is child):
            return child
        if ((oldchild is not None) and (oldchild is not PASSIVE_NO_RESULT) and (oldchild is not NEVER_SET)):
            (old_state, old_dict) = (instance_state(oldchild), instance_dict(oldchild))
            impl = old_state.manager[key].impl
            if ((initiator.impl is not impl) or (initiator.op is OP_APPEND)):
                impl.pop(old_state, old_dict, state.obj(), (parent_impl._append_token or parent_impl._init_append_token()), passive=PASSIVE_NO_FETCH)
        if (child is not None):
            (child_state, child_dict) = (instance_state(child), instance_dict(child))
            child_impl = child_state.manager[key].impl
            if ((initiator.parent_token is not parent_token) and (initiator.parent_token is not child_impl.parent_token)):
                _acceptable_key_err(state, initiator, child_impl)
            elif ((initiator.impl is not child_impl) or (initiator.op is OP_REMOVE)):
                child_impl.append(child_state, child_dict, state.obj(), initiator, passive=PASSIVE_NO_FETCH)
        return child
    def emit_backref_from_collection_append_event(state, child, initiator):
        if (child is None):
            return
        (child_state, child_dict) = (instance_state(child), instance_dict(child))
        child_impl = child_state.manager[key].impl
        if ((initiator.parent_token is not parent_token) and (initiator.parent_token is not child_impl.parent_token)):
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