Exceptions - Revisions

Added material to give further explanation for beginning programmers.

exceptions.md

@@ -21,7 +21,7 @@ Observe that a `NameError` is raised and also the location where the error was d
 
 ## Exceptions
 
-We will **try** to read input from the user. Press `[ctrl-d]`, (or `[ctrl-z]` if you are using Windows), and see what happens.
+We will **try** to read input from the user.  Enter the first line below and hit the `Enter` key.  When your computer prompts you for input, instead press `[ctrl-d]` on a Mac or `[ctrl-z]` with Windows and see what happens.  (If you're using Windows and neither option works, you can try `[ctrl-c]` in the Command Prompt to generate a KeyboardInterrupt error instead).
 
 ```python
 >>> s = input('Enter something --> ')
@@ -94,7 +94,9 @@ We do the usual file-reading stuff, but we have arbitrarily introduced sleeping
 
 Observe that the `KeyboardInterrupt` exception is thrown and the program quits. However, before the program exits, the finally clause is executed and the file object is always closed.
 
-Note that we use `sys.stdout.flush()` after `print` so that it prints to the screen immediately.
+Notice that a variable assigned a value of 0 or `None` or a variable which is an empty sequence or collection is considered `False` by Python.  This is why we can use `if: f` in the code above.
+
+Also note that we use `sys.stdout.flush()` after `print` so that it prints to the screen immediately.
 
 ## The with statement {#with}