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With try/except/else, we’ve seen how to run certain code when an exception occurs and other code when it does not. There is also a way to execute code regardless of whether an exception occurs - the finally clause.

Here is our final flow chart demonstrating try/except/else/finally:

Try/Except/Else/Finally

Let’s return to our fictional login program from earlier and examine a use case for the finally clause:

try:
  check_password()
except ValueError:
  print('Wrong Password! Try again!')
else:
  login_user()
  # 20 other lines of imaginary code
finally:
  load_footer()

In the above program, most of our code stayed the same. The one change we made was we added the finally clause to execute no matter if the user fails to login or not. In either case, we use an imaginary function called load_footer() to load the page’s footer. Since the footer area of our imaginary application stays the same for both states, we always want to load it, and thus call it inside of the finally clause.

Note that the finally clause can be used independently (without an except or else clause). This is a convenient way to guarantee that a behavior will occur, regardless of whether an exception occurs:

try:
    check_password()
finally:
    load_footer()
    # Other code we always want to run

Let’s put the finally clause into practice for our Instrument World application!

Instructions

Instrument World maintains a database (in this case a large dictionary) with instrument information that any store can access.

The current program displays information from the database for a particular instrument. Take some time to look over database.py and instrument.py to get better acquainted with the program.

Run the code to examine the output!

Since the database server Instrument World uses can only have a limited number of users connected to it, we want to make sure that we disconnect from it after attempting to retrieve information, even if an exception occurs.

Add a finally clause that calls database.disconnect_from_database(). Observe the output of the exception handling when we hit an exception by running the code!

Change instrument to have a value of 'Kora'. Run the code to observe the finally clause executing even when we don’t hit an exception.

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