15_exceptions.md

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. Press [ctrl-d] and see what happens.

>>> s = raw_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.

You should almost never need to use a bare except: or except Exception: to silence an error. There is typically always a better way to handle it (either being more specific with the exception type you're looking for, or logging extra information about the error, or re-raising the error).

Refer to Python's exception hierarchy to get a sense of what exceptions are typically used and how they relate to each other.

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.

Summary

We have discussed the usage of the try, except, and raise. We have seen how to create our own exception types and how to raise exceptions as well. We also briefly saw how the with statement can be used to handle errors when accessing file objects.