human
Complete this python program:
def listen(target, identifier, fn, *args, **kw):
assistant
#Here is the Python program you requested.

    """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:: 0.9.4 Added ``once=True`` to :func:`.event.listen`
    and :func:`.event.listens_for`.
    .. note::
    The :func:`.listen` function cannot be called at the same time
    that the target event is being run.   This has implications
    for thread safety, and also means an event cannot be added
    from inside the listener function for itself.  The list of
    events to be run are present inside of a mutable collection
    that can\t be changed during iteration.
    Event registration and removal is not intended to be a "high
    velocity" operation; it is a configurational operation.  For
    systems that need to quickly associate and deassociate with
    events at high scale, use a mutable structure that is handled
    from inside of a single listener.
    .. versionchanged:: 1.0.0 - a ``collections.deque()`` object is now
    used as the container for the list of events, which explicitly
    disallows collection mutation while the collection is being
    iterated.
    .. seealso::
    :func:`.listens_for`
    :func:`.remove`
    """
    _event_key(target, identifier, fn).listen(*args, **kw)