Scope problem, can access object attributes but can't access the object instance

Question

I am creating a text adventure engine, the engine itself is working great but I am trying to implement custom events for game creators with callbacks. I have a main.py file that implements all of the game objects and builds the game. The problem is that I seem to be having trouble accessing the objects after I have instantiated them. have a look at this pseudo example code,

import engine

def build():
    # create new item objects
    key = engine.Item()
    door = engine.Item(a = 0)

    # set state of door
    door.locked(requirement = key)

    # CALLBACK FUNCTION
    def on_door_unlock():
        # ACCESSING THE ATTRIBUTE `a` IN `door` WORKS FINE
        door.a = 1
        # ACCESSING THE `key` OBJECT THROWS UnboundLocalError
        del key

    # assign callback to unlock event
    door.on_unlock(on_door_unlock)

build()
engine.run()

My file is obviously much larger than this but it is just as simple and this code isolates my problem. I am able to access the attributes of any object, but when I try to use del keyword on an object itself I get UnboundLocalError: local variable 'key' referenced before assignment

my callback function is written after the creation of the key object
my callback is assigned to an event after the function is written
I can access object attributes but can't access object itself.

everything seems to be placed in order. So what is the problem? How can i write callback functions that can access instances of the objects I create?

key is not local (and del is not that idiomatic in Python because most people would ratter use a shared object or collection as a container instead of a function's namespace). — Paulo Scardine
– Paulo Scardine, Commented Jan 11, 2019 at 16:59
Then why am I able to access the attributes of door because it is not local either. — Riley Hughes
– Riley Hughes, Commented Jan 11, 2019 at 17:01
You can access it because scope in Python resolves according to the LEGB Rule (local, enclosing, global, builtin) but del is concerned only with the local scope. — Paulo Scardine
– Paulo Scardine, Commented Jan 11, 2019 at 17:03
del key means no more or less than "remove the name key from the local scope". But this symbol has never been brought into the local scope of on_door_unlock and even if it had, removing it from there would not do anything to the scope of build. — jez
– jez, Commented Jan 11, 2019 at 17:09
If you're attempting to say "this key has been used, destroy it" you may need to this more in terms of a property of player, or the player's inventory, and reduce the "key count" to 0. — Joe P
– Joe P, Commented Jan 11, 2019 at 17:11

jez · Accepted Answer · 2019-01-11 17:12:22Z

del key means no more or less than "remove the name key from the local scope". But this symbol has never been brought into the local scope of on_door_unlock and even if it had, removing it from there would not do anything to the scope of build.

One of many better approaches would be to create an explicitly-named registry of objects, for example as a dict called all_objects. Create the key inside it. Remove key from it by referring to it by name in your function.

ALL_OBJECTS = {}

def build():
    ALL_OBJECTS[ 'copperKey' ] = engine.Item()

    ...

    def on_door_unlock():

        del ALL_OBJECTS[ 'copperKey' ]
        ...

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