human
Complete the below
def listen(target, identifier, fn, *args, **kw):

    ''''Register a listener function for the given target.
    e.g.::
    from sqlalchemy import event
    from sqlalchemy.schema import UniqueConstraint
    def unique_constraint_name(const, table):
    const.name = "uq_%s_%s" % (
    table.name,
    list(const.columns)[0].name
    event.listen(
    UniqueConstraint,
    "after_parent_attach",
    unique_constraint_name)
    A given function can also be invoked for only the first invocation
    of the event using the ``once`` argument::
    def on_config():
    do_config()
    event.listen(Mapper, "before_configure", on_config, once=True)
    .. versionadded:: 0.9.3 Added ``once=True`` to :func:`.event.listen`
    and :func:`.event.listens_for`.'
    '''
 What is a the Python code you requested?
assistant
#Here is the Python code you requested.
    _event_key(target, identifier, fn).listen(*args, **kw)