Propagating Exceptions with raise
A raise statement that does not include an exception name or extra data value simply reraises the current exception. This form is typically used if you need to catch and handle an exception but don’t want the exception to die in your code:
>>> try:
... raise IndexError('spam') # Exceptions remember arguments
... except IndexError:
... print('propagating')
... raise # Reraise most recent exception
...
propagating
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
IndexError: spam
Running a raise this way reraises the exception and propagates it to a higher handler (or the default handler at the top, which stops the program with a standard error message). Notice how the argument we passed to the exception class shows up in the error messages; you’ll learn why this happens in the next chapter.
