Try..Finally

What if you were reading a file and you wanted to close the file whether or not an exception was raised?
This can be done using the finally block. Note that you can use an except clause along with a finally
block for the same corresponding try block. You will have to embed one within another if you
want to use both.

Using Finally

Example 13.3. Using Finally
#!/usr/bin/python
# Filename: finally.py
import time
try:
f = file('poem.txt')
while True: # our usual file-reading idiom
line = f.readline()
if len(line) == 0:
break
time.sleep(2)
print line,
finally:
f.close()
print 'Cleaning up...closed the file'


Output
$ python finally.py
Programming is fun
When the work is done
Cleaning up...closed the file
Traceback (most recent call last):
File "finally.py", line 12, in ?
time.sleep(2)
KeyboardInterrupt


How It Works
We do the usual file-reading stuff, but I've arbitrarily introduced a way of sleeping for 2 seconds before
printing each line using the time.sleep method. The only reason is so that the program runs slowly
(Python is very fast by nature). When the program is still running, press Ctrl-c to interrupt/cancel the
program.
Observe that a KeyboardInterrupt exception is thrown and the program exits, but before the program
exits, the finally clause is executed and the file is closed.

Raising Exceptions

You can raise exceptions using the raise statement. You also have to specify the name of the error/
exception and the exception object that is to be thrown along with the exception. The error or exception
that you can arise should be class which directly or indirectly is a derived class of the Error or Exception
class respectively.

How To Raise Exceptions

Example 13.2. How to Raise Exceptions
#!/usr/bin/python
# Filename: raising.py
class ShortInputException(Exception):
'''A user-defined exception class.'''
def __init__(self, length, atleast):
Exception.__init__(self)
self.length = length
self.atleast = atleast
try:
s = raw_input('Enter something --> ')
if len(s) < 3:
raise ShortInputException(len(s), 3)
# Other work can continue as usual here
except EOFError:
print '\nWhy did you do an EOF on me?'
except ShortInputException, x:
print 'ShortInputException: The input was of length %d, \
was expecting at least %d' % (x.length, x.atleast)
else:
print 'No exception was raised.'

Output
$ python raising.py
Enter something -->
Why did you do an EOF on me?
$ python raising.py
Enter something --> ab
ShortInputException: The input was of length 2, was expecting at least 3
$ python raising.py
Enter something --> abc
No exception was raised.

How It Works

Here, we are creating our own exception type although we could've used any predefined exception/error
for demonstration purposes. This new exception type is the ShortInputException class. It has two
fields - length which is the length of the given input, and atleast which is the minimum length that
the program was expecting.
In the except clause, we mention the class of error as well as the variable to hold the corresponding error/
exception object. This is analogous to parameters and arguments in a function call. Within this particular
except clause, we use the length and atleast fields of the exception object to print an appropriate
message to the user.

Exceptions

Exceptions occur when certain exceptional situations occur in your program. For example, what if you
are going to read a file and the file does not exist? Or what if you accidentally deleted it when the program
was running? Such situations are handled using exceptions.
What if your program had some invalid statements? This is handled by Python which raises its hands
and tells you there is an error.

Errors

Consider a simple print statement. What if we misspelt print as Print? Note the capitalization. In
this case, Python raises a syntax error.
>>> Print 'Hello World'
File "<stdin>", line 1
Print 'Hello World'
^
SyntaxError: invalid syntax
>>> print 'Hello World'
Hello World


Observe that a SyntaxError is raised and also the location where the error was detected is printed.
This is what an error handler for this error does.

Try..Except

We will try to read input from the user. Press Ctrl-d and see what happens.
>>> s = raw_input('Enter something --> ')
Enter something --> Traceback (most recent call last):
File "<stdin>", line 1, in ?
EOFError


Python raises an error called EOFError which basically means it found an end of file when it did not
expect to (which is represented by Ctrl-d)
Next, we will see how to handle such errors.
Handling Exceptions
We can handle exceptions using the try..except statement. We basically put our usual statements
within the try-block and put all our error handlers in the except-block.

Example 13.1. Handling Exceptions
#!/usr/bin/python
# Filename: try_except.py
import sys
try:
s = raw_input('Enter something --> ')
except EOFError:
print '\nWhy did you do an EOF on me?'
sys.exit() # exit the program
except:
print '\nSome error/exception occurred.'
# here, we are not exiting the program
print 'Done'


Output
$ python try_except.py
Enter something -->
Why did you do an EOF on me?
$ python try_except.py
Enter something --> Python is exceptional!
Done


How It Works
We put all the statements that might raise an error in the try block and then handle all the errors and
exceptions in the except clause/block. The except clause can handle a single specified error or exception,
or a parenthesized list of errors/exceptions. If no names of errors or exceptions are supplied, it
will handle all errors and exceptions. There has to be at least one except clause associated with every
try clause.
If any error or exception is not handled, then the default Python handler is called which just stops the execution
of the program and prints a message. We have already seen this in action.
You can also have an else clause associated with a try..catch block. The else clause is executed
if no exception occurs.
We can also get the exception object so that we can retrieve additional information about the exception
which has occurred. This is demonstrated in the next example.