CS105: Introduction to Python

TRY EXCEPT

Many languages have the concept of the "Try-Catch" block. Python uses four keywords: try, except,  else, and finally. Code that can possibly throw an exception goes in the  try block.  except gets the code that runs if an exception is raised. else is an optional block that runs if no exception was raised in the try block, and finally is an optional block of code that will run last, regardless of if an exception was raised. We'll focus on try and except for this chapter.

A basic example looks like this:

>>> try:
... x = int(input("Enter a number: "))
... except ValueError:
... print("That number was invalid")

First, the try clause is executed. If no exception occurs, the except  clause is skipped and execution of the try statement is finished. If an exception occurs in the try  clause, the rest of the clause is skipped. If the exception's type matches the exception named after the except keyword, then the  except clause is executed. If the exception doesn't match, then the exception is unhandled and execution stops.

The except Clause

An except clause may have multiple exceptions, given as a parenthesized tuple:

try:
 # Code to try
except (RuntimeError, TypeError, NameError):
 # Code to run if one of these exceptions is hit

A try statement can also have more than one except  clause:

try:
 # Code to try
except RuntimeError:
 # Code to run if there's a RuntimeError
except TypeError:
 # Code to run if there's a TypeError
except NameError:
 # Code to run if there's a NameError

Finally

Finally, we have finally. finally is an optional block that runs after  try, except, and else, regardless of if an exception is thrown or not. This is good for doing any cleanup that you want to happen, whether or not an exception is thrown.

>>> try:
... raise KeyboardInterrupt
... finally:
... print("Goodbye!")
...
Goodbye!
Traceback (most recent call last):
 File "<stdin>", line 2, in <module>
KeyboardInterrupt

As you can see, our Goodbye! gets printed just before the unhandled KeyboardInterrupt gets propagated up and triggers the traceback.