Author Dusty Phillips Reviewers

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 98 
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Raising exceptions
In principle, an exception is just an object. There are many different exception classes 
available, and we can easily define more of our own. The one thing they all have 
in common is that they inherit from a built-in class called 
BaseException
. These 
exception objects become special when they are handled inside the program's flow of 
control. When an exception occurs, everything that was supposed to happen doesn't 
happen, unless it was supposed to happen when an exception occurred. Make sense? 
Don't worry, it will!
The easiest way to cause an exception to occur is to do something silly! Chances 
are you've done this already and seen the exception output. For example, any time 
Python encounters a line in your program that it can't understand, it bails with 
SyntaxError
, which is a type of exception. Here's a common one:
>>> print "hello world"
File "", line 1
print "hello world"
^
SyntaxError: invalid syntax
This 
print
statement was a valid command in Python 2 and previous versions,
but in Python 3, because 
print
is now a function, we have to enclose the arguments 
in parenthesis. So, if we type the preceding command into a Python 3 interpreter,
we get the 
SyntaxError
.
In addition to 
SyntaxError
, some other common exceptions, which we can handle, 
are shown in the following example:
>>> x = 5 / 0
Traceback (most recent call last):
File "", line 1, in
ZeroDivisionError: int division or modulo by zero
>>> lst = [1,2,3]
>>> print(lst[3])
Traceback (most recent call last):
File "", line 1, in
IndexError: list index out of range