Let's modify our code to catch the exception before it propagates up to the top of the call stack (thereby causing our program to stop) using the try … except construct:

def main():
    print(sqrt(9))
    print(sqrt(2))
    try:
        print(sqrt(-1))
    except ZeroDivisionError:
        print("Cannot compute square root of a negative number.")

    print("Program execution continues normally here.")

Now when we run the script we see that we're handling the exception cleanly:

$ python sqrt.py
3.0
1.41421356237
Cannot compute square root of a negative number.
Program execution continues normally here.

We should be careful to avoid a beginners mistake of having too-tight scopes for exception handling blocks; we can easily use one try … except block for all of our calls to sqrt(). We also add a third print statement to show how execution of the enclosed block is terminated:

def main():
    try:
        print(sqrt(9))
        print(sqrt(2))
        print(sqrt(-1))
        print("This is never printed.")
    except ZeroDivisionError:
        print("Cannot compute square root of a negative number.")

    print("Program execution continues normally here.")