Why does this function change my dictionary to None?

Question

I'm writing a practice program to add items from a list to a existing dictionary

def addToInventory(inventory, addedItems):
    for i in addedItems:
        inventory[i] = inventory.get(i,0) + 1

stuff = {'gold coin': 42, 'rope': 1}
loot = ['gold coin', 'dagger', 'gold coin', 'gold coin', 'ruby']
stuff = addToInventory(stuff, loot)

Why is stuff changed to None after running this?

So just change the last line to `addToInventory(stuff, loot)`, without assigning the returned None to `stuff`. — RemcoGerlich, Jul 25 '17 at 14:02
Also see https://stackoverflow.com/questions/15300550/python-return-return-none-and-no-return-at-all — Błotosmętek, Jul 25 '17 at 14:06

RomainL. · Accepted Answer · 2017-07-25T15:12:58.257

In fact your code is almost good you have just one mistake. Dictionary in python are mutable then when you modify it in your fonction your modifying the one you pass as argument. But because your function do not return anything when you write:

stuff = addToInventory(stuff, loot)

you assign stuff to None.

you have two choice: return the dictionary at the end of the function:

def addToInventory(inventory, addedItems):
    for i in addedItems:
        inventory[i] = inventory.get(i,0) + 1
    return inventory

or do not reassign stuff:

 addToInventory(stuff, loot)

score 2 · Answer 2 · answered Jul 25 '17 at 14:05

There's no return statement in your function so it returns nothing - None.

Solution? Add return statement in your function

def addToInventory(inventory, addedItems):
    for i in addedItems:
         inventory[i] = inventory.get(i,0) + 1
    return inventory

Why does this function change my dictionary to None?

2 Answers2