Python classes - help calling a method inside another class to update an attribute in an instance

When you call

self.shopItems.catalogue(item).setStock(stock)

You're trying to call the methos catalogue of the object self.shopItems. The thing is, self.shopItems is a list. I think you should try with a code like this:

class catalogue(object):

    def __init__(self, catalogueitem):
        self.catalogueitem = catalogueitem
        self.colors = []
        self.stock = "No Stock"

    def setStock(self, stock):
        if self.stock == "No Stock":
            self.stock = stock

class shop(object):

    def __init__(self, items):
        self.shopItems = []
        for item in items:
            self.shopItems.append(catalogue(item))

    def setStock(self, stock, item="purse"):
        c = catalogue(item) #Instance the catalogue class
        c.setStock(stock) #Call the method setStock
        self.shopitems.append(c) #this adds the items into the instance, but # the original objects created by __init__ are still there, i.e. they've not # updated just added.

newshop = shop( ["purse","handbag","wallet", "clutchbag"] )

newshop.setStock(10, "handbag")

Changed only the function setStock from shop class.

EDIT 1:

When you do self.shopItems = catalogue(item) you're overriding what you have in the list. You should change that to:

self.shopItems.append(catalogue(item))

And call the method on the last element afterwars:

self.shopItems[-1].setStock(stock)

Result being:

    def setStock(self, stock, item="purse"):
        self.shopItems.append(catalogue(item))
        self.shopItems[-1].setStock(stock)

EDIT 2:

Ok, to update the existent items you should first check if they exist, iterating through the list:

    def setStock(self, stock, item="purse"):
        c = catalogue(item) #Instance the catalogue class
        c.setStock(stock) #Call the method setStock
        index = -1
        i = 0 #index
        for shItem in self.shopItems:
            if shItem.catalogueitem == item:
                index = i #Saves the index to update
                break #Breaks out of loop
            i += 1
        if index == -1:
            self.shopitems.append(c) #index wasn't updated, the item is new
        else:
            self.shopitems[index] = (c) #replaces the item info

Firstly, you want to be able to look up shop items by name. An appropriate data structure to use for this is a dictionary (rather than a list). So, this changes the constructor for shop to:

def __init__(self, items):
    self.shopItems = {}
    for item in items:
        self.shopItems[item] = catalogue(item)

Then, it is easy to update a shop item:

self.shopItems[item].setStock(stock)

Each shopItem is already a catalogue instance, so you can call setStock() on it directly. You should really check that there is such an item first, otherwise you will throw an exception. For example, if you call newshop.setStock(10, 'bag') -- there is no such item. You could protect against that as follows:

def setStock(self, stock, item="purse"):
    if item not in self.shopItems:
        self.shopItems[item] = catalogue(item)
    self.shopItems[item].setStock(stock)