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Safari | Python Developer's Handbook -> Creating User-defined Exceptions





< BACKMake Note | BookmarkCONTINUE >
152015024128143245168232148039199167010047123209178152124239215162147038000014099175001108

Creating User-defined Exceptions

Python allows you to create your own exceptions by subclassing any standard Python exception.

Note

Take a look at Chapter 5, "Object-Oriented Programming," for more details about working with classes.



					
>>> import exceptions
>>> class ConfigError (exceptions.Exception):
       def __init__(self, arg=None):
           self.args = arg

>>> try:
       raise ConfigError("Bad hostname")
   except ConfigError, e:
       print e.args

Bad hostname

				

The import statement from the previous example isn't really necessary because the exceptions module contents are automatically imported by the interpreter. Remember that you can't use the prefix "exceptions" because the exceptions module is not available in the __main__ namespace until you import it.

The next example uses the class created in the previous example as a base class to create a new class.

					
>>> class TimeoutError(ConfigError):

       def printargs(self):
           print self.args

>>> try:
       raise TimeoutError, "Timeout"
   except TimeoutError, e:

       e.printargs()

Timeout

				

As you could see, just by overriding the __init__ method, you are able to create your own exception classes.

You can also change the output of a traceback message by overwriting the __str__ method.

					
>>> class ConfigError(Exception):
       def __init__(self, args=None):
           self.args = args

         def __str__(self):
           return "\ nError in the module configuration\ n" + 
}
                  `self.args` + "\ n"
>>> raise ConfigError, "bad hostname"
Traceback (innermost last):
  File "<stdin>", line 1, in ?
__main__.ConfigError
Error in the module configuration
bad hostname

				


Last updated on 1/30/2002
Python Developer's Handbook, © 2002 Sams Publishing

< BACKMake Note | BookmarkCONTINUE >

Index terms contained in this section

creating
      user-defined exceptions
exceptions
      subclassing 2nd
      user-defined, creating
import statement
statements
      import
subclassing
      exceptions 2nd
user-defined exceptions
      creating

2002, O'Reilly & Associates, Inc.

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