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CHAPTER 8 ■ EXCEPTIONS 161
■Note There are actually two other ways to use raise. The argument may be a string, or you can call
raise without any arguments. Using a string argument is considered obsolete; calling raise without arguments
is covered in the section “Look, Ma, No Arguments!” later in this chapter.
The first example (raise Exception) raises a generic exception with no information of
what went wrong. In the last two examples, I added the error message hyperdrive overload. As
you can see, the two forms raise class, message and raise class(message) are equivalent; both
raise an exception with the given error message.
There are many built-in classes available. You can find a description of all of them in the
Python Library Reference, in the section “Built-in Exceptions.” You can also explore them
yourself with the interactive interpreter; they are all found in the module exceptions, for your
convenience (as well as in the built-in namespace). To list the contents of a module, you can
use the dir function, which is described in Chapter 10:
>>> import exceptions
>>> dir(exceptions)
['ArithmeticError', 'AssertionError', 'AttributeError', ...]
In your interpreter, this list will be quite a lot longer—I’ve deleted most of the names in the
interest of legibility. All of these exceptions can be used in your raise statements:
>>> raise ArithmeticError
Traceback (most recent call last):
File "", line 1, in ?
ArithmeticError
Table 8-1 describes some of the most important built-in exceptions.
Table 8-1. Some Built-in Exceptions
Class Name
Exception
AttributeError
IOError
IndexError
KeyError
NameError
SyntaxError
TypeError
ValueError
ZeroDivisionError
Description
The root class for all exceptions
Raised when attribute reference or assignment fails
Raised when trying to open a nonexistent file (among other things)
Raised when using a nonexistent index on a sequence
Raised when using a nonexistent key on a mapping
Raised when a name (variable) is not found
Raised when the code is ill-formed
Raised when a built-in operation or function is applied to an object of
the wrong type
Raised when a built-in operation or function is applied to an object
with correct type, but with an inappropriate value
Raised when the second argument of a division or modulo operation
is zero