Author Dusty Phillips Reviewers

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'''If the item is not locked, raise an
exception. If the item_type does not exist,
raise an exception. If the item is currently
out of stock, raise an exception. If the item
is available, subtract one item and return
the number of items left.'''
pass
We could hand this object prototype to a developer and have them implement the 
methods to do exactly as they say while we work on the code that needs to make 
a purchase. We'll use Python's robust exception handling to consider different 
branches, depending on how the purchase was made:
item_type = 'widget'
inv = Inventory()
inv.lock(item_type)
try:
num_left = inv.purchase(item_type)
except InvalidItemType:
print("Sorry, we don't sell {}".format(item_type))
except OutOfStock:
print("Sorry, that item is out of stock.")
else:
print("Purchase complete. There are "
"{} {}s left".format(num_left, item_type))
finally:
inv.unlock(item_type)
Pay attention to how all the possible exception handling clauses are used to ensure 
the correct actions happen at the correct time. Even though 
OutOfStock
is not 
a terribly exceptional circumstance, we are able to use an exception to handle it 
suitably. This same code could be written with an 
if
...
elif
...
else
structure, but it 
wouldn't be as easy to read or maintain.
We can also use exceptions to pass messages between different methods. For 
example, if we wanted to inform the customer as to what date the item is expected to 
be in stock again, we could ensure our 
OutOfStock
object requires a 
back_in_stock
parameter when it is constructed. Then, when we handle the exception, we can check 
that value and provide additional information to the customer. The information 
attached to the object can be easily passed between two different parts of the 
program. The exception could even provide a method that instructs the inventory 
object to reorder or backorder an item.
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Expecting the Unexpected