WEBVTT

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So in this video we're going to
do some writing to text files.

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So obviously in the previous video
we covered reading from them.

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So I'm still in the same project
that I used for the last video and

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we are going to create a new path file.

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I'm just going to call this "writing." So
the process of

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writing data to a text file is
very similar to reading from it.

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The files opened but this time the mode
is w from write instead of r for

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read which we saw previously Now
the actual data is written using the print

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function exactly as you want to
display it on the screen but

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only this time we specify a file object
to send it to and it goes to the file.

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And finally it's very important to
close the file when you're finished and

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once again we can use which we
looked at in the previous video so

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we don't have to remember
to do that ourselves.

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So, let's start talking some code.

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We're going to start with cities.

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So, cities equals LA.

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Alice Springs.

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Darwin.

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Melbourne.

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And Sydney.

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And we're going to type with open

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cities.txt for write,
so using a W this time.

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S City and
then we can type four city in cities,

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print city, and
here's a different bit now.

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File equals City underscore file so

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instead of just having it print by itself
we're adding the file part of the line

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which indicates that we want
to write this to a text file.

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And again because we're using
with here on line three

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we don't have to close the file ourself.

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So if we go to our folder and
we just open that up.

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So we're going to go to our
folder that contains the code.

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It's it's this one here you can see the
sample dot txt from the previous video.

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So if I did run this and
whoops, I run this by default,

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which choose the file I I need to actually
right-click it and run it the first time.

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So nothing appeared in the console output
but if we go back to our folder We can see

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this now, cities.text has been created and
we'll have a look at that, double click.

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You see you've got the five
cities that I typed originally in

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the source code on line one.

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So you now know what this parameter was.

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I know we talked about that very
briefly once in the course.

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You can now see how that's used.

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Now another thing you
might have noticed here,

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because we're using the default folder,
cities.txt appeared here as well, so

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intellijack picked up the fact that
there was a file in its folder, and

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it enables you to actually view in
that as well, which is pretty handy.

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And when you specify right mode the file
will be, will be created as be solvent

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doesn't everything exist, but important
if it does exist it will be overwritten.

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Now incidentally there's a slight auditiy
about the path and convention for

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the spaces around operators here.

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Now normally you'd put a space
either side of the equals symbol and

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in a silent operation And it tells you
correctly warns you if you forget, but

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when using named parameters and function
calls, the spaces are frowned upon,

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and you get a warning if you put
them around the equals here.

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In fact, equals here is not
actually the assignment operation.

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But it's used to provide a named
argument in place of the file parameter.

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So in other words if I do something here.

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We got a warning here.

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Now in line.

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That's fine.

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Put that there to get rid of that.

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But if we put a space here and
here we'll actually get

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a realwaring unexpected spaces around
key word slash parameters values.

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So On the we need to not
have spaces to avoid that

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error which I know is a little bit weird.

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So just to confirm the equals here in
this particular case is normally used for

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the and we put eye equals five but there.

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But here its not actually
in the operation but

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it's used to provide a name document
In place of the file parameter.

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So in other words, city_file is
the parameter, it's a named argument.

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Now we saw another named argument earlier
in the print statement when we passed and

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equals in two single quotes, and
we'll be looking at functions and

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parameters in more detail
later in the course.

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So just for now, humor IntelliJ,
remove the spaces around the equal

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When specifying named arguments
like we've done in line five.

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Now before we leave named arguments print
argument that can be set to true or false.

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And the false is false so
data isn't flushed.

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Now to understand what this is for

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we have to look a little bit in more
detail about what is happening.

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When Python writes data to a file.

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Now, external devices on your
computer including the screen

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are much slower than computer memory.

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So, running with things like disc
drives in the screen is buffered.

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So what that means is that the data
is actually written to a buffer, and

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the contents of that buffer is transferred
to the external device in the background.

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So it's something that happens,
While you're sort of doing something else.

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This allows the program to continue
processing without waiting for

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the write to complete.

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And so, everything seems much faster.

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So in other words, as soon as we run
this program, Once the process finished,

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at that point it may still be writing
something in the background to that file.

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Now this, obviously an example,

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you have five words that
would have been pretty quick.

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But you can imagine like a huge
file writing that was perhaps many

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megabytes long.

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It does take the computer
some time to process.

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And that's because it is putting
in that buffer It all then

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comes down to the speed of the device
that you're writing the information to.So

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essentially it means that the data is
actually written to the buffer and then

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the contents of the buffer is transferred
to the external device in the background.

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And again, we're doing that or

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it happens automatically to
allow the program to continue.

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Without waiting for
that write process to complete.

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Sometimes though with that said, you want
the data to be written out immediately.

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So especially if the external
device is a screen, and

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you want the user to be able to
see the output straight away.

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So with buffering, data could be sent
to the screen from the buffer for

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example, and then immediately overwritten
by subsequent data from the buffer.

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So this wouldn't be apparent
in our example so far.

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Because it already using relatively
small amounts of data but

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displaying the last on the screen
could suffer from that effect.

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As the scrolls up the screen
lines of text appear to flicker.

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Now, closing a file causes the buffer
to be flushed automatically.

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But if you wanna ensure
your data's written sooner,

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you can pass flush equals true to cause
the data to be written immediately.

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Generally though with that said, it's not
something you'd, you really should worry

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about, but the ability to control
it through a window is important

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enough that the benefits Python introduced
the flash parameter in Python three.

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Remembering that with that said that
Python was but Python three was released

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in 2008, and although they may not seem
very a longer time ago A modern computer

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is three times as fast as they were
then and interestingly enough a modern

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mobile phone more than likely has a faster
processor than a 2008 desktop computer.

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So things have come a long way and
the point we're making here is

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you probably generally speaking don't
need to use the first parameter.

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But it's there if you decide to use it.

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So you go about doing something
like flash equals true.

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That's actually how you do it if you
wanted to actually use that flush command.

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And generally we don't recommend
that you need to do that for

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most of the situations.

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So okay data from the last video.

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And we've now seen how to write our
little file worked earlier in this video.

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But before we move on it's probably
useful to see if we can read back

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the data we've just written.

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So let's that bit out, because
we've already now written the data.

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We don't need to call them again
because the file already exists so

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we're going to start with an empty list.

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And with open, with open cities.txt for

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read as city_file.

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This is our first study,
for city in city_file.

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You can try something
like cities.append(city).

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Then we can do a print(cities).

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Then maybe, for
city in cities and print(city).

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So now we're going through
opening the file using with.

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We're going through each city,
we're appending it to our list.

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Then we're printing up the list and you're
just going through each entry in the list

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and printing it again just to confirm
that we can save the individual cities.

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So if we run this

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you can see we've got notice that slash
n exists on all of those as well.

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And then we can see
that the five examples.

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So in other words this
works after a fashion.

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But we have those,

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these additional new line characters up
here in the list but also after each line.

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And one good way that we can remove
that is we can use the strip function,

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from a string to remove those
characters from a string.

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Instead of waiting until you print it out,
I'm doing something in the print here,

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like putting end equals single quote
single quote there to remove it.

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It can be a good idea to remove it
at the time we're importing it.

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So we can do something like,
append, and we can put .strip

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.strip/n.

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So if you then run that We see
we get a much better result.

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You notice the /n doesn't
appear in our list anymore.

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And obviously,

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each entry is outputted without the any
line character which is pretty cool.

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So, to see how strip's used let's
temporarily create a new file.

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We're just going to call it scratch pad.

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So, if you happen to be using
idol you can just type these

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commands directly into idol.

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Assuming you.

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You need to do just what I have done and
create a new file.

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We do something like
print Adelaide.strip A.

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Then we run that and

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it will say problem with running it again
so I'm going to just right click that.

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And run scratchpad.

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And notice how it's taking the a off,
so it's stripped off the a.

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So what strip does, it removes characters
from the beginning or end of a string and

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only from the beginning or end.

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That's an important thing.

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So for example, if I went back now and
changed this To del,

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which you can see.

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I ran that.

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So you can see what's happened there.

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The intention here when we ran it was to
remove the del, the second, third, and

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fourth characters from.

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But what actually happened
was it ignored those.

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But, it did remove the DE on the end,
and it ignored the L.

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So, very important that's a partial
match and its only returning, stripping

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anything that's in there even its only
a partial match as you can see there.

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So, back to our program again.

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Back to writing, and I'll just
select that before I forget again.

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So on line 11,
our program now strips slash n,

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which is the new line character,
before they're appended to the list.

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So one thing to note is that Python
is aware of which operating system

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it's running on, and it handles what
is referred to as universal newlines.

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Now on UNIX systems, such as Linux and a
Mac, New lines are written using a single

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character, where as Windows uses two
characters which is a /r or a /m.

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Sometimes now it's written as CR-LF for
carriage return and line feed, back

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from the the days when printers needed
codes to correctly position the paper.

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For the next line of text.

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But the good thing is the point we're
making here is that Python's aware of this

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and automatically takes care of
the different conventions for

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handling line breaks for you for
different operating systems.

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But, this is the important thing,
as long as the files open in text mode.

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Does need to be opened in text mode for
that to happen.

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So we're going to finish the discussion
on text files by observing that

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anything that you can print out on
screen can be written to a text file.

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However, it's not always possible to
read it back in the original form.

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And to show you that,
you're going to run the following program.

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And we'll see exactly what we mean.

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So down here,
I'm gonna type Imelda equals Mayhem.

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[NOISE]
My 2011.

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And it's one, you saw this
obviously in the earlier lectures.

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[NOISE]

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Do a couple more.

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Okay so
there's the data we want to [INAUDIBLE] so

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if we do something like with open.

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Three, double and the order as before.

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And we do something like print,

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file equals file.

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So that's obviously gonna run and
print out our top.

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So if we run that, just for simplicity
I'll change that to a text file, .txt.

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And if we run that, and there's our .txt.

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We'll have a look at that.

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And you can see that,
just make a bit space here.

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So the contents of the file probably look
like a good representation of our tuple,

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but there's no easy way to read
it back into a tuple variable,

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because it's now stored as a string.

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So Python does have an eval function
that will evaluate any expression

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to a string passed to it, so

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we could get the contents back from the
file into a tuple variable by using eval.

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So we'll just close it down.

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And we'll just comment this
out now because we know what

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file we're going to read the file again.

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So I'm going to put with open, and in
order three dot TXT, we'll read this time.

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As imelda_file contents

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equals imelda_file.readline.

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And we can do something like
imelda equals eval contents.

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We can do print imelda Title,

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artist, year, tracks equals imelda.

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print title, print artist, print year.

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Okay so if you run

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that So it does allow us,

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in this case, to retrieve
the contents of the type of the file.

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But there are much better
ways to do it as we'll see.

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So the use of evals is not a good idea
when dealing with data external to your

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program, and that's because the contents
of the file could be changed, and frankly

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could contain damaging instructions and
your program would happily execute them.

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It's really important to design
programs with security in mind and

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not to leave vulnerabilities in your code.

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So I'm gonna finish the video here, now.

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In the next video we're gonna talk
about appending to a text file.

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And then we're going to come
up with a challenge, for

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what we've learned to do with reading,
writing, and appending text files.

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So we'll do that in the next video