OK, how do I make this program allow the user to modify the price of items and save and load the modified catalog? i've spent ages trying and failing. pls help!

'''maintain a catalog'''

def main():
    global catalog

    loadCatalog()
    while True:
        pick = showMenu()
        if pick == 0:
            break
        elif pick == 1:
            sellItem()
        elif pick == 2:
            buyItem()
        else:
            print "Invalid selection"
    saveCatalog()

def showMenu():
    global catalog

    while True:
        print "Catalog:\n\t%6s %12s %8s %8s" % ( "ItemID", "Item", "Price", "Stock" )
        for k in catalog.keys():
            print "\t%6d %12s %8s %8d" % (k,catalog[k][0], str("$%0.2f" % catalog[k][1]), catalog[k][2])

        try:
            pick = int(raw_input('''
Enter selection:
0 - Quit
1 - sell Item
2 - buy Item

Selection : ''' ))
        except:
            print "Pick must be an integer\n"
            continue
        return pick

def sellItem():
    global catalog
    print "sellItem"

    try:
        itemID = int(raw_input("Which item? "))
        if itemID in catalog.keys():
            howMany = int(raw_input("How many? "))
            if 0 < howMany <= catalog[itemID][2]:
                catalog[itemID][2] -= howMany
                print "Price for %d %s is $%0.2f" % ( howMany, catalog[itemID][0],catalog[itemID][1]*howMany )
            else:
                print "Can't sell %d %s" % ( howMany, catalog[itemID][0] ) 
        else:
            print "Invalid itemID"
    except:
        print "Bad value entered"
    print

def buyItem():
    global catalog
    print "buyItem"
    try:
        itemID = int(raw_input("Which item? "))
        if itemID in catalog.keys():
            howMany = int(raw_input("How many? "))
            if 0 < howMany:
                catalog[itemID][2] += howMany
            else:
                print "Can't buy %d %s" % ( howMany, catalog[itemID][0] ) 
        else:
            print "Invalid itemID"
    except:
        print "Bad value entered"
    print

def loadCatalog():
    global catalog
    catalog = { 1:["Bread", 1.50, 10 ], 2:["Cheese", 5.00, 5], 3:["Apples", 2.50,12] }

def saveCatalog():
    global catalog
    pass

if __name__ == "__main__":
    main()

You don't need that many global statements.
Actually, the use of global is discouraged.
Since catalog variable is constant, just declare it in the beginning of your file.

this is the program that i was given to work with, i tried using a def modPrice() but couldn't get it to work :(

try this:

def modPrice():
    global catalog
    print "modifyPrice"
    try:
        itemID = int(raw_input("Which item? "))
        if itemID in catalog.keys():
            price = float(raw_input("How many? "))
            if 0 < price:
                catalog[itemID][1] = price
                print "Price has been changed"
            else:
                print "Price needs to be greater than 0"
         else:
            print "Invalid itemID"
    except:
        print "Bad value entered"
    print
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