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 try/except clause. The code which can cause an exception to occur is put in the try block and the handling of the exception is implemented in the except block. 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 exception in a separate except block. 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 is passed on to the second block. Now the last method involves trapping ALL exceptions:

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

This can be helpful when you have no idea about the exception which can be thrown by your program.

Finally clause

We wrap our main code in the try clause. After that we wrap some code in except clause which gets executed if an exception occurs in the code wrapped in try clause. But 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 even if no exception occurs. It might be used for cleaning 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 even if no exception occurs!")

# Output: An IOError occurred. No such file or directory
# This would be printed even if no exception occurs!

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. 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:
    print('This would only run if no exception occurs.')
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.
# 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.