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Custom Print Displays

As we saw in the preceding section, by default, instances of class-based exceptions display whatever you passed to the class constructor when they are caught and printed:

>>> class MyBad(Exception): pass
...
>>> try:
...     raise MyBad('Sorry--my mistake!')
... except MyBad as X:
...     print(X)
...
Sorry--my mistake!

This inherited default display model is also used if the exception is displayed as part of an error message when the exception is not caught:

>>> raise MyBad('Sorry--my mistake!')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.MyBad: Sorry--my mistake!

For many roles, this is sufficient. To provide a more custom display, though, you can define one of two string-representation overloading methods in your class (__repr__ or __str__) to return the string you want to display for your exception. The string the method returns will be displayed if the exception either is caught and printed or reaches the default handler:

>>> class MyBad(Exception):
...     def __str__(self):
...         return 'Always look on the bright side of life...'
...
>>> try:
...     raise MyBad()
... except MyBad as X:
...     print(X)
...
Always look on the bright side of life...

>>> raise MyBad()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.MyBad: Always look on the bright side of life...

A subtle point to note here is that you generally must redefine __str__ for this purpose, because the built-in superclasses already have a __str__ method, and __str__ is preferred to __repr__ in most contexts (including printing). If you define a __repr__, printing will happily call the superclass’s __str__ instead! See Chapter 29 for more details on these special methods.

Whatever your method returns is included in error messages for uncaught exceptions and used when exceptions are printed explicitly. The method returns a hardcoded string here to illustrate, but it can also perform arbitrary text processing, possibly using state information attached to the instance object. The next section looks at state information options.