Exceptions

Exceptions occur when 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 "<stdin>", line 1, in <module>
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. Enter the first line below and hit the Enter key. When your computer prompts you for input, instead press [ctrl-d] on a Mac or [ctrl-z] with Windows and see what happens. (If you’re using Windows and neither option works, you can try [ctrl-c] in the Command Prompt to generate a KeyboardInterrupt error instead).

>>> s = input('Enter something --> ')
Enter something --> Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
EOFError

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 exceptions_handle.py):

Example:

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', text)

Output:


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

# Press ctrl + c
$ python exceptions_handle.py
Enter something --> ^CYou cancelled the operation.

$ python exceptions_handle.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.

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:

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

How It Works

We are using here 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.