Beginning Python - From Novice to Professional

160 CHAPTER 8 ■ EXCEPTIONS
>>> import regex
__main__:1: DeprecationWarning: the regex module is deprecated;
please use the re module
>>> regex
It’s obvious that the interpreter didn’t like this; the regex module is old, and you should use the re module
instead. (You learn more about the re module in Chapter 10.) However, because a lot of code already uses the
regex module, it would be unreasonable to demand that re be used; that would simply break all the older
code. So instead, a warning is issued.
If, for some reason, you are stuck with the regex module, you can happily ignore the warning (although
you probably should rewrite your code). You can even filter it out (with the function filterwarnings), so it
isn’t printed:
>>> from warnings import filterwarnings
>>> filterwarnings('ignore')
>>> import regex
If you want to learn more about warnings, you can check out the warnings module in the standard
library documentation at http://www.python.org/doc/lib.
Making Things Go Wrong . . . Your Way
As you’ve seen, exceptions are raised automatically when something is wrong. Before looking
at how to deal with those exceptions, let’s take a look at how you can raise exceptions yourself—
and even create your own kinds of exceptions.
The raise Statement
To raise an exception, you use the raise statement with an argument that is either a class or an
instance. When using a class, an instance is created automatically; you can optionally provide
a string argument after the class, separated by a comma. Here are some simple examples, using
the built-in exception class Exception:
>>> raise Exception
Traceback (most recent call last):
File "", line 1, in ?
Exception
>>> raise Exception, 'hyperdrive overload'
Traceback (most recent call last):
File "", line 1, in ?
Exception: hyperdrive overload
>>> raise Exception('hyperdrive overload')
Traceback (most recent call last):
File "", line 1, in ?
Exception: hyperdrive overload