python 3 - instantiating class from user input

Thu Apr 7 00:04:44 EDT 2011

I dont understand why this is such a big deal. Nor do i understand why google can't find a reasonable answer. If one can't figure out from the title what I'm trying to do, then a look at code should firmly plant the intent. The general idea of the code is, in my opinion, very basic. 
I notice, that when you are coding a class instance, it's in the format of:
newInstance = someClass()
OK, so we are calling someClass as if it's a function. Is there a way to do something to the effect of:
 someClass(newInstance)
I've seen all kinds of hacks and workarounds that seem 10 times more complicated that this ought to be. 
class inventoryItem:
 itsDecrtiption = None
 itsVendor = None
 itsCost = None
 itsMarkup = None
 itsQuantity = None
newItem = input('enter the new item to add to inventory')
findItem = input('enter the item number to find')
def addItem(newItem):
 pass
def lookupItem(findItem):
 if findItem:
 doSomething()
 else:
 complain()
 addItem(findItem)