Many languages have the concept of the “Try-Catch” block. Python uses four keywords:
finally. Code that can possibly throw an exception goes in the
except gets the code that runs if an exception is raised.
else is an optional block that runs if no exception was raise 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
except for this chapter.
A basic example looks like this:
>>> try: ... x = int(input("Enter a number: ")) ... except ValueError: ... print("That number was invalid")
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 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.
except clause my 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
try statement can also have more than one
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, we have
finally is an optional block that runs after
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.