Author Dusty Phillips Reviewers

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Of course, if we do want to customize the initializer, we are free to do so. Here's 
an exception whose initializer accepts the current balance and the amount the user 
wanted to withdraw. In addition, it adds a method to calculate how overdrawn the 
request was:
class InvalidWithdrawal(Exception):
def __init__(self, balance, amount):
super().__init__("account doesn't have ${}".format(
amount))
self.amount = amount
self.balance = balance
def overage(self):
return self.amount - self.balance
raise InvalidWithdrawal(25, 50)
The 
raise
statement at the end illustrates how to construct this exception. As you 
can see, we can do anything with an exception that we would do with other objects. 
We could catch an exception and pass it around as a working object, although it 
is more common to include a reference to the working object as an attribute on an 
exception and pass that around instead.
Here's how we would handle an 
InvalidWithdrawal
exception if one was raised:
try:
raise InvalidWithdrawal(25, 50)
except InvalidWithdrawal as e:
print("I'm sorry, but your withdrawal is "
"more than your balance by "
"${}".format(e.overage()))
Here we see a valid use of the 
as
keyword. By convention, most Python coders
name the exception variable 
e
, although, as usual, you are free to call it 
ex
, 
exception
, or 
aunt_sally
if you prefer.
There are many reasons for defining our own exceptions. It is often useful to add 
information to the exception or log it in some way. But the utility of custom exceptions 
truly comes to light when creating a framework, library, or API that is intended for 
access by other programmers. In that case, be careful to ensure your code is raising 
exceptions that make sense to the client programmer. They should be easy to handle 
and clearly describe what went on. The client programmer should easily see how to
fix the error (if it reflects a bug in their code) or handle the exception (if it's a situation 
they need to be made aware of).
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Chapter 4