Exceptions and Error Handling

Whenever an error is encountered in your program, Python will raise an exception stating the line number and what went wrong:

>>> x = [1,2,3]
>>> x[10]
Traceback (most recent call last):
File "
", line 1, in
x[10]
IndexError: list index out of range

If we want the code to continue running, we can use a try/except block:

>>> x = [1,2,3]
>>> try:
>>>     x[10]
>>> except IndexError:
>>>     print "What are you trying to pull?"
>>> print "Continuing program..."

Another variation is:

>>> x = [1,2,3]
>>> try:
>>>     x[10]
>>> except IndexError:
>>>     print "What are you trying to pull?"
>>> else:                   # Will only run if no exception was raised
>>>     print "No exception was encountered"
>>> print "Continuing program..."

You can raise your own exception if you want to prevent the script from running:

>>> if func() == "bad return value":
>>>     raise RuntimeError("Something bad happened")