13.8. The finally clause of the try statement

A common programming pattern is to grab a resource of some kind — e.g. we create a window for turtles to draw on, or we dial up a connection to our internet service provider, or we may open a file for writing. Then we perform some computation which may raise an exception, or may work without any problems.

Whatever happens, we want to “clean up” the resources we grabbed — e.g. close the window, disconnect our dial-up connection, or close the file. The finally clause of the try statement is the way to do just this. Consider this (somewhat contrived) example:

import turtle import time def show_poly(): try: win = turtle.Screen() # Grab/create a resource, e.g. a window tess = turtle.Turtle() # This dialog could be cancelled, # or the conversion to int might fail, or n might be zero. n = int(input("How many sides do you want in your polygon?")) angle = 360 / n for i in range(n): # Draw the polygon tess.forward(35) tess.left(angle) time.sleep(3) # Make program wait a few seconds except Exception as e: print("insufficient number of sides") print(e) finally: win.bye() # Close the turtle's window show_poly()

In lines 20–22, show_poly is called three times. Each one creates a new window for its turtle, and draws a polygon with the number of sides input by the user. But what if the user enters a string that cannot be converted to an int? What if they close the dialog? We’ll get an exception, but even though we’ve had an exception, we still want to close the turtle’s window. Lines 17–18 does this for us. Whether we complete the statements in the try clause successfully or not, the finally block will always be executed.

Notice that the exception is still unhandled — only an except clause can handle an exception, so our program will still crash. But at least its turtle window will be closed before it crashes!

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