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- [Instructor] Now let's take a look

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at how to handle actual exceptions
as they occur at runtime,

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so for this purpose,

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we're going to be looking at a script

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called dividebyzero.py,

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let me go ahead and execute that

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so you can see it in action.

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This script is going to prompt the user

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to enter two numbers,

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a numerator and a denominator
specifically for this script,

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they must be integers,

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so let's say I go ahead and enter 100,

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now for the numerator,
now for the denominator,

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in the preceeding video,

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we saw that if we divide by zero,

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a zero division error occurs.

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So, let's see what happens
if we try to divide by zero

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in this script.

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We get an error message,

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attempted to divide by zero

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but the script doesn't terminate,

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it simply prompts the user

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to enter the numerator once again.

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Now in the preceding video,

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we also demonstrated what happened

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if an integer was expected

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and the user typed a non-integer value.

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In that case, we got a valuer

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but in this script, we
simply get a message

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you must enter two integers

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and the prompt to enter the
numerator appears once again.

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So, finally if I do go
and enter two integers,

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then we can see the result

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of dividing those two values

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and the script terminates.

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So, there's actually three
flows of control going on

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in this script.

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There's the flow of control that occurs

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when we attempt to divide by zero

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and we display a message indicating that

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and then simply continue
by prompting the user

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for input once again.

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There is the flow of control

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that occurs if the user
types non-integer value

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like hello, in which case again,

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we get an error message
and then we prompt the user

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for the numerator once again

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and then there's the flow of control

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for a successful division

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which does in fact lead us
to terminating the script

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after the division is performed.

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So, let's take a look at the code for this

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and those three flows of control.

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So, for the code, we have this script

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and we haven't looked
at a script for a while,

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so just as a reminder,

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we always include a
comment at the beginning

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of the script file

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that specifies the name of the file

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so you can find it on disk

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and we have a doc string that is used

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to describe the purpose of the string.

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Now in this script we have
an infinite while loop

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and the while loop is going
to execute the statement

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in its suite which is a try statement

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in this example and as you can see,

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the try statement has code in it,

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in its suite and then it has a couple

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of except clauses and an else clause,

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so we'll talk about all
the different pieces here.

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Now, in Python, typically
you're going to put

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in the try suite any statements

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that might cause an exception.

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The else suite

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is where you put code
that you want to execute

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if no exceptions occur

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in the try suite, so in a lot
of other programming languages

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that have exception handling,

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you put in the block of code

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for the try, not only the statements

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that might cause an exception,

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but also any statements

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that you don't want to execute
if an exception occurs.

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In Python that second category,

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the statements that you
don't want to execute

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if an exception occurs,

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those get placed in the else block

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and they are in the
else suite I should say

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and those only execute
if no exceptions occur

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in the try block,

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so back up here in the try statement,

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we're going to prompt
the user for some input,

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the input function will obtain
the value the user types,

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give it back to our program as a string,

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we will convert that string to an integer

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and assign it to number one,

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then we'll do the same thing again

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for a second value that will
represent the denominator.

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If both of those successfully execute,

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we will attempt the division.

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If that successfully executes,

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we will print the result of the division

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by executing the else clause

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which will also terminate the loop.

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On the other hand, going
back up above here,

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if either of these first two
statements in the try suite

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causes a value error

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because the user did not
enter an integer value,

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in that case the try suite
terminates immediately

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and control jumps down to the first

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of the except handlers.

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Now if I did not enter an integer,

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the type of exception I'll get

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from these first two
statements in the try suite

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is a value error,

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so that would be the
matching exception handler

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for the particular type of
exception that occurred.

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In that case, we will print
You must enter two integers.

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Now once you reach the end
of an except clause's suite,

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that exception is
considered to be handled.

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All additional except clauses are skipped

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as is the else clause, so once we print

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you must enter two integers,

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the rest of the try
statement will be skipped

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and control will continue
with the first statement

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after the try statement

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but in this case, that's
the end of our loop,

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so we'll go back to the top of the loop

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and we will try everything once again.

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If we successfully read
in the first two values,

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the numerator and the denominator,

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then we'll try to perform the division.

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If the numerator is zero,

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in that case, again the try statement

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or the try suite terminates immediately.

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If we had additional statements

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before this first except clause,

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they would not execute.

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Control flow jumps to the except clauses

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and this first except clause

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is not for zero division errors,

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so again we're talking about
the case of dividing by zero,

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in that case, we skip that except clause

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and go to the next one.

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That except clause does in
fact match a ZeroDivisionError,

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so this one will execute,

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we will print Attempted to divide by zero.

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If there were any further except clauses,

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they would be skipped,

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the else clause will be skipped

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because we're in the
exception handling flow

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of control and we will proceed

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with the first statement
after the enter try statement

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which in this case
means go back to the top

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of the loop and try it all again.

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Now if we successfully
read the two integers

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and successfully perform the division,

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all the except clauses will be skipped

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and the else clause will execute.

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That will print out the
results of the division

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and we will terminate the loop

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at that point and because
we have nothing more

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in our script to execute,

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the script will terminate
in that case as well.