字典值在for循环分配中未更改

Question

So I am learning python using Learn python the hard way. I am trying to develop a inventory system. The goal is to build this into a class that will be pulled by the room class. I am going to use a dictionary for the inventory so that when I create the inventory for each room I can say where they are in the room.

So in the bedroom I can say that there is a knife on the bed rather than just saying you see knife, candle, oil.

Right now I am working on picking up the item when someone says take knife. I am able to do a weird search and set that value to None so that when someone looks in the room it does not show up but I seem to be running into a scope issue. I read multiple other questions on this site that said since its a mutable object i don't need to do Global dict and when i tried that it got an error. I am able to edit the object but when i go back out of my if statement and for loop it won't carry over.

Please help me :)

#This is the inventory in this room
inventory = {'desk': 'Miniature Fusion Warhead',  
         'bed':'knife', 'sink':None}        
def take(item):
    if item in inventory.itervalues():
        #add_item_to_inventory(item)

        for key, value in inventory.iteritems():

            if value == item:
                print ("Wow.\n"
                    "Look at you.\n"
                    "Picking shit up and everything.")
                value = None


    else:
        print ("What do you think this is?"
            "  The dollar store?  We don't "
            "got that shit here!")


# Prints out what items are in the room.  
# I am hoping to later come up with an idea for how
# To make different variations of this, and have it randomly
# pick between the different ways it says what it sees.
for key, value in inventory.iteritems():
    if value != None:
        print "You see a %s on the %s." % (value, key)
print "What do you want to pick up?"
ui = raw_input(">")
split_ui = ui.split()
print split_ui

if len(split_ui) > 1:
    if split_ui[0] == "take":
        print ("You reach over to grab the %s."
            "\n...") % split_ui[1]
    take(split_ui[1])

    else:
        print ("What you talking bout Willis?  "
            "Don't you know this is just about "
            "takin shit.")
else:
    print ("Who taught you how to talk?"
        "\n...\nLet me show you how its done.\n"
        "Use a verb, like take, then throw in an "
        "object like knife.")

print inventory

This is the output that I am given.

You see a knife on the bed.
You see a Miniature Fusion Warhead on the desk.
What do you want to pick up?
>take knife
['take', 'knife']
You reach over to grab the knife.
...
Wow.
Look at you.
Picking shit up and everything.
{'sink': None, 'bed': 'knife', 'desk': 'Miniature Fusion Warhead'}

Important note: This currently only works if you take the knife and not the Warhead. I need to figure out another solution for items with multiple words.

Thanks!

Answer 1

The value inside your loop is different than the real value of the dictionary. It is just a reference to that value, so when you do value = None you actually change the value of the reference to hold the new value None and not the value of the dictionary.

To demonstrate it better this is before the assignment inside the for key, value in inventory.iteritems():

-------            -------------------------
|value|   -------> |value of the dictionary|
-------            -------------------------

this is after value = None

                   -------------------------
                   |value of the dictionary|
                   -------------------------
-------            ------
|value|   -------> |None|
-------            ------

As you can see the dictionary value does not change. Only the variable value of the for loop changes. This variable belongs to the scope of the for loop and after that it is discarded.

An alternative would be instead of value = None to do:

inventory[key] = None

Answer 2

zbs is correct, you're only changing the value of the pointer to the dict value. However, you're making this way too hard:

#This is the inventory in this room
inventory = {'Miniature Fusion Warhead': 'desk',
             'knife':'bed'}
player_inventory = set()

def take(item):
  if item in inventory:
    print("You picked up the {}".format(item))
    player_inventory.add(item)
    del inventory[item]

  else:
    print("That item doesn't exist")

while True:
  print('')
  print("Inventory: " + ', '.join(player_inventory))
  for k,v in inventory.items():
    print("You see a {} on the {}".format(k, v))

  print("What do you want to pick up?")
  ui = raw_input("> ").split()
  verb = ui[0]
  item = ' '.join(ui[1:])
  if verb == 'take':
    if item:
      print("You take the {}".format(item))
      take(item)
    else:
      print("That item doesn't exist")
  else:
    print("That's not an action")