from functools import wraps

def singleton(cls):
    instances = {}
    
    @wraps(cls)
    def get_instance(*args, **kwargs):
        instance_id = kwargs.get('instance_id')
        
        # If instance_id provided, use localized mode (one instance per ID)
        # Otherwise, use global mode (single instance)
        key = (cls, instance_id) if instance_id else cls
        
        if key not in instances:
            print(f"Creating new instance with key: {key}")
            instances[key] = cls(*args, **kwargs)
        else:
            print(f"Reusing existing instance with key: {key}")

        # either way:
        return instances[key]
    
    return get_instance