def ignore_first_call(fn):
    called = False
    def wrapper(*args, **kwargs):
        nonlocal called
        if called:
            return fn(*args, **kwargs)
        else:
            called = True
            return None
    return wrapper

@ignore_first_call
def listener(event):
    print(event.event_type)  # can be 'put' or 'patch'
    print(event.path)  # relative to the reference, it seems
    print(event.data)  # new data at /reference/event.path. None if deleted

firebase_admin.db.reference('/').listen(listener)