python-tutorials/15-exceptions.md

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.

Similarly, 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 function call. What if we misspelt print as Print? Note the capitalization. In this case, Python raises a syntax error.

>>> Print('Hello World')
Traceback (most recent call last):
  File "<pyshell#0>", line 1, in <module>
    Print('Hello World')
NameError: name 'Print' is not defined
>>> print('Hello World')
Hello World

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

Exceptions

We will try to read input from the user. Press ctrl-d and see what happens.

>>> s = input('Enter something --> ')
Enter something --> 
Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    s = input('Enter something --> ')
EOFError: EOF when reading a line

Python raises an error called EOFError which basically means it found an end of file symbol (which is represented by ctrl-d) when it did not expect to see it.

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 (save as try_except.py):

try:
    text = input('Enter something --> ')
except EOFError:
    print('Why did you do an EOF on me?')
except KeyboardInterrupt:
    print('You cancelled the operation.')
else:
    print('You entered {0}'.format(text))

Output:

$ python3 try_except.py
Enter something -->     # Press ctrl-d
Why did you do an EOF on me?

$ python3 try_except.py
Enter something -->     # Press ctrl-c
You cancelled the operation.

$ python3 try_except.py
Enter something --> no exceptions
You entered no exceptions

How It Works:

We put all the statements that might raise exceptions/errors inside the try block and then put handlers for the appropriate errors/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.

Note that there has to be at least one except clause associated with every try clause. Otherwise, what's the point of having a try block?

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 an error message. We have already seen this in action above.

You can also have an else clause associated with a try..except block. The else clause is executed if no exception occurs.

In the next example, we will also see how to get the exception object so that we can retrieve additional information.

Raising Exceptions

You can raise exceptions using the raise statement by providing the name of the error/exception and the exception object that is to be thrown.

The error or exception that you can raise should be a class which directly or indirectly must be a derived class of the Exception class.

Example (save as 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:
    text = input('Enter something --> ')
    if len(text) < 3:
        raise ShortInputException(len(text), 3)
    # Other work can continue as usual here
except EOFError:
    print('Why did you do an EOF on me?')
except ShortInputException as ex:
    print('ShortInputException: The input was {0} long, expected at least {1}'\
          .format(ex.length, ex.atleast))
else:
    print('No exception was raised.')

Output:

$ python3 raising.py
Enter something --> a
ShortInputException: The input was 1 long, expected at least 3

$ python3 raising.py
Enter something --> abc
No exception was raised.

How It Works:

Here, we are creating our own exception type. This new exception type is called ShortInputException. 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 which will be stored as the variable name 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 thelength and atleast fields of the exception object to print an appropriate message to the user.

Try .. Finally

Suppose you are reading a file in your program. How do you ensure that the file object is closed properly 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.

Save as finally.py:

import time

try:
    f = open('poem.txt')
    while True: # our usual file-reading idiom
        line = f.readline()
        if len(line) == 0:
            break
        print(line, end='')
        time.sleep(2) # To make sure it runs for a while
except KeyboardInterrupt:
    print('!! You cancelled the reading from the file.')
finally:
    f.close()
    print('(Cleaning up: Closed the file)')

Output:

$ python3 finally.py
Programming is fun
When the work is done
if you wanna make your work also fun:
!! You cancelled the reading from the file.
(Cleaning up: Closed the file)

How It Works:

We do the usual file-reading stuff, but we have arbitrarily introduced sleeping for 2 seconds after printing each line using the time.sleep function 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 the KeyboardInterrupt exception is thrown and the program quits. However, before the program exits, the finally clause is executed and the file object is always closed.

The with statement

Acquiring a resource in the try block and subsequently releasing the resource in the finally block is a common pattern. Hence, there is also a with statement that enables this to be done in a clean manner:

Save as using_with.py:

with open("poem.txt") as f:
    for line in f:
        print(line, end='')

How It Works:

The output should be same as the previous example. The difference here is that we are using the open function with the with statement - we leave the closing of the file to be done automatically by with open.

What happens behind the scenes is that there is a protocol used by the with statement. It fetches the object returned by the open statement, let's call it "thefile" in this case.

It always calls the thefile.__enter__ function before starting the block of code under it and always calls thefile.__exit__ after finishing the block of code.

So the code that we would have written in a finally block should be taken care of automatically by the __exit__method. This is what helps us to avoid having to use explicit try..finally statements repeatedly.

More discussion on this topic is beyond scope of this book, so please refer PEP 343 for a comprehensive explanation.

Summary

We have discussed the usage of the try..except and try..finally statements. We have seen how to create our own exception types and how to raise exceptions as well.

Next, we will explore the Python Standard Library.