There’s Something About Code

Programming Python is mostly straightforward. Not thinking, however, can lead to subtle bugs, and this one had me stumped for a while. If a program raises an exception that is not caught further up the call stack, I expect the interpreter to exit with a whimper. Consider the following code:

def throwing_function():
    try:
        print 1 / 0
    finally:
        print "Finally"
        return "Useful string"
 
print throwing_function()

Running this will print

Finally
Useful string

If you can spot the bug immediately, good for you, but I didn’t. I expected this code to fail with a ZeroDivisionError exception. Not so. It turns out that the return statement hides the exception.

The correct code should look like this:

def throwing_function():
    try:
        print 1 / 0
    finally:
        print "Finally"
    return "Useful string"
 
print throwing_function()

Now it behaves like expected:

Finally
Traceback (most recent call last):
  File "swallowed_exception_fixed.py", line 8, in 
    print throwing_function()
  File "swallowed_exception_fixed.py", line 3, in throwing_function
    print 1 / 0
ZeroDivisionError: integer division or modulo by zero

A good rule of thumb would be to never put a return statement inside a finally clause.