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- [Instructor] In this video,

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I'd like to talk about the finally clause

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of a try statement.

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Now the finally clause,
is guaranteed to execute

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if the normal flow of
control of your Python code

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enters a try statements suite.

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So if I go into the try
statement and start executing,

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and the try statement has a
finally clause at the end,

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that finally clause is
guaranteed to execute

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with the only exception to that being,

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if you manually terminate the code

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by calling system.exit for example.

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Now, if you're coming into Python

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from a language like C# or Java

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that has a finally clause
on the try statement,

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you're already familiar
with the basic concept.

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If you're not, the idea is

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that in a try statement,
you might acquire a resource

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and then you want to use that resource

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and maybe an exception could occur

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if you're working with that resource

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and sometimes it will
and sometimes it won't,

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but you want to absolutely guarantee

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that you give that
resource back to the system

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before the try system terminates,

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so resource deallocation code

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would typically be placed
in a finally block or suite

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as we call it in Python.

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Now in Python, those types of resources

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are typically acquired in with statements

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like we demonstrated with
files earlier in this lesson,

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and with statements automatically

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give those resources back to the system.

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So there's less need for a finally block,

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unless you're dealing with
some sort of resource,

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that does not have that automated support

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for working in a with statement.

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So it's there in case you need it,

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but in the vast majority of cases

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for those types of resources,

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we'd use with statements in Python.

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But let's go ahead and demonstrate
the basic finally block

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and we'll use a mechanical
example for this purpose.

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Here I've pasted in a try statement.

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In the try suite, we have
just a print statement

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that will print out try suite
with no exceptions raised

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and you can see I am not doing something

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that would cause an exception here.

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I'm not, for example,
converting the string hello

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to an integer which cannot happen

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and therefore would cause an exception.

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I'm not using the raise function
that's built into Python

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to manually cause an exception either,

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so the try block will
execute successfully here,

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which means the except
clause will be ignored.

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So this will not execute,
will not get printed.

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The else cause will execute

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because as long as no exceptions
occur in the try block,

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the else clause will execute.

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So else executes because no
exceptions in the try suite

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that will be printed, and because
we executed the try suite,

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the finally block or the
finally suite will also execute

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printing finally always executes.

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So we should see the first
print statements output,

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the third print statements output

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and the finally print
statements output as well

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and if you check these, you'll see that

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that is indeed the case in this example.

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Now let's once again, take
a look at a try statement

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this time for a try suite

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in which an exception actually does occur.

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So here I've pasted in
another try statement.

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In this try statement, we
will print try statement

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that raises an exception,

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and then we will attempt to
call the built in function int

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to convert the string hello to an integer.

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Now of course the string hello

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is not a valid integer
representation as a string

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and therefore this will
cause a value error.

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Now the try suite will
terminate at this point,

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which is known as the raise
point of the exception

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that occurs.

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Any statements that follow
the highlighted statement here

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will not execute when
that exception occurs

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and flow of control will continue

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with the except clause
for the value error.

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So we will print out try suite
that raises an exception,

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then we will jump down here

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and print out a value error occurred

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because an exception occurred,

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the else suite will be ignored,

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but the finally suite always executes.

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So in this case, we expect

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to see the first print statement execute,

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the except clauses
print statement execute,

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and the finally clauses
print statement execute

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and as you can see here,
those are the three prints

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that we did indeed get.

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Now going back to what I said earlier

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about this issue that in Python,

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most resources that need to be deallocated

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when you're done using them
will work with a with statement.

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So it's not uncommon if you're trying

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to deal with exceptions that might occur

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from those resources like
working with a file on disk

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to wrap a with statement
in a try statement.

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So let me first demonstrate
an exception that occurs

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if we try to open a file
that does not exist.

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So first thing I'm gonna do
here is just clear my screen

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so that I can get back to the top here

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and let's say we want to open
a file that does not exist

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in my system.

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I misspelled grades here on purpose.

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In that case, I get an exception.

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A file not found error indicating
that a problem occurred.

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Now normally I would not
execute this function call

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in a snippet by itself,

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instead I would probably
have that in a with statement

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that says go ahead and open that,

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assign the open file to a
particular variable name

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and then process the data
in that file in some way.

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So let's take a look at what you might do

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to combine a with statement
with a try statement.

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So here we have a try statement

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that's going to attempt

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to open the gradez.txt file for reading.

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This file does not exist.

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If it does get opened,
it would be assigned

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to the variable accounts

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and then we would do some processing

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of that file of information.

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Now in this case, the file does not exist,

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so because of the fact that
we've put the with statement

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in the try suite, we can look for

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and deal with that exception.

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So if the exception
occurs, the except clause

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for the file not found error
will match the exception

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that occurred when we tried to call open

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and we will print

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the file name you
specified does not exist.

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So let me go ahead and
execute that snippet,

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and indeed our print statement
in the except clause executes

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rather than us displaying

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this ugly file not found error trace back

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that we saw back in snippet four.

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So of course if we're
creating larger applications

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and scripts that other
people are going to execute,

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we're probably going to want

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to use some more exception handling

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to make it a little bit nicer

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for the user interacting with
that application or script.