exceptions.rst

Exceptions

Exception handling is an art which once you master grants you immense powers. I am going to show you some of the ways in which we can handle exceptions.

In basic terminology we are aware of the try/except structure. The code that can cause an exception to occur is put in the try block and the handling of the exception is implemented in the except block. The code in the except block will only execute if the try block runs into an exception. Here is a simple example:

try:
    file = open('test.txt', 'rb')
except IOError as e:
    print('An IOError occurred. {}'.format(e.args[-1]))

In the above example we are handling only the IOError exception. What most beginners do not know is that we can handle multiple exceptions.

Handling multiple exceptions:

We can use three methods to handle multiple exceptions. The first one involves putting all the exceptions which are likely to occur in a tuple. Like so:

try:
    file = open('test.txt', 'rb')
except (IOError, EOFError) as e:
    print("An error occurred. {}".format(e.args[-1]))

Another method is to handle individual exceptions in separate except blocks. We can have as many except blocks as we want. Here is an example:

try:
    file = open('test.txt', 'rb')
except EOFError as e:
    print("An EOF error occurred.")
    raise e
except IOError as e:
    print("An error occurred.")
    raise e

This way if the exception is not handled by the first except block then it may be handled by a following block, or none at all. Now the last method involves trapping ALL exceptions:

try:
    file = open('test.txt', 'rb')
except Exception as e:
    # Some logging if you want
    raise e

This can be helpful when you have no idea about the exceptions that may be thrown by your program. If you are just looking to catch all execptions, but don't actually care about what they are, you can even exclude the Exception as e part.

Note:: catching all exceptions may have unintended consequences because catching all exceptions may also catch the ones you want to occur; for example, in many command-line based programs, pressing control+c will terminate the program, but if you catch all excepts, the KeyboardInterrupt will be caught as an exception, so pressing control+c will NOT terminate the program.

finally clause

We wrap our main code in the try clause. After that we wrap some code in an except clause which gets executed if an exception occurs in the code wrapped in the try clause. In this example we will use a third clause as well which is the finally clause. The code which is wrapped in the finally clause will run whether or not an exception occurred. It might be used to perform clean-up after a script. Here is a simple example:

try:
    file = open('test.txt', 'rb')
except IOError as e:
    print('An IOError occurred. {}'.format(e.args[-1]))
finally:
    print("This would be printed whether or not an exception occurred!")

# Output: An IOError occurred. No such file or directory
# This would be printed whether or not an exception occurred!

try/else clause

Often times we might want some code to run if no exception occurs. This can easily be achieved by using an else clause. One might ask: why, if you only want some code to run if no exception occurs, wouldn't you simply put that code inside the try? The answer is that then any exceptions in that code will be caught by the try, and you might not want that. Most people don't use it and honestly I have myself not used it widely. Here is an example:

try:
    print('I am sure no exception is going to occur!')
except Exception:
    print('exception')
else:
    # any code that should only run if no exception occurs in the try,
    # but for which exceptions should NOT be caught
    print('This would only run if no exception occurs. And an error here '
          'would NOT be caught.')
finally:
    print('This would be printed in every case.')

# Output: I am sure no exception is going to occur!
# This would only run if no exception occurs. And an error here would NOT be caught
# This would be printed in every case.

The else clause would only run if no exception occurs and it would run before the finally clause.