Raising Exceptions

So far, we’ve been letting Python raise exceptions for us by making mistakes (on purpose this time!), but our scripts can raise exceptions too—that is, exceptions can be raised by Python or by your program, and can be caught or not. To trigger an exception manually, simply run a raise statement. User-triggered exceptions are caught the same way as those Python raises. The following may not be the most useful Python code ever penned, but it makes the point:

>>> try:
...     raise IndexError                  # Trigger exception manually
... except IndexError:
...     print('got exception')
...
got exception

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As usual, if they’re not caught, user-triggered exceptions are propagated up to the top-level default exception handler and terminate the program with a standard error message:

>>> raise IndexError
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError

As we’ll see in the next chapter, the assert statement can be used to trigger exceptions, too—it’s a conditional raise, used mostly for debugging purposes during development:

>>> assert False, 'Nobody expects the Spanish Inquisition!'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError: Nobody expects the Spanish Inquisition!