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as I mentioned in the end of the
previous video you can't really get that

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calculator to do anything until you
understand how to write our own functions

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in Python now we've been using
functions throughout the course so

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far but now it's time to actually
look at how they work and importantly

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how we can create this for ourselves now in
Python functions are created with the

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word dev followed by the function name
and a pair of parentheses

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because the function introduces block of
code that dev statement must be followed

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by a colon and I'm gonna show you just
how this works with an example so lets define a

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function called python_food so....

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and then we type the code we want in the function so....

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and we can call the function here....notice how intelliJ has found it for us and knows

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that's a function so we put that code in their and we can
run that now obviously it doesn't do a lot it

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just prints spam and eggs as you can see on the screen their but you can see that the keyboard

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dev is followed by the function as I mentioned and parenthesis and then also

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colon as well and then the actual
method code you want executes is indented so the

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statements that make up the body of the
function are indented in the usual way so

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as soon as the indent is back to the left
margin then that new code or any new code at

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that point is no longer
part of the function definition now the

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python_food function as can
see on line 5 is called just like any

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other functions that we have used previously in the course and running the

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program results in a
Pythonic meal appearing on the screen

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as you can see now although it is not obvious here all
Python functions return a result

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that's important so if you don't tell the
function what result to return then its gonna

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return none and we can see this with a
simple change of the program what we're

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going to do is just add a print
surround our function in a print we can

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actually see what is returned from that
functions so run that you could see it still

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done the output on line 2 its is printed
that out but also its return none and

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that's because we haven't defined
the parameter now you wouldn't tell

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Python to print the result of the function that
isn't intended to return anything

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but that example really demonstrates what happens
when you call when you use a function

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call as an argument to another function
so we've use print a lot now in the course

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and you probably very used to it so I'm certain you should be using that and

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taking on one or more arguments using
parenthesis and then of course that

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prints them out but if one of the argument is
a function call then the function is

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executed and its result is used as the
argument to print which is what is

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happening here in this case you can see
the print is print has outputted the word

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none so we've actually called the
python_food function and

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this return none and the print after
executing that function has returned the

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output that came back to it on the screen
to none now obviously that's a pretty boring

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function so let's try make it a little bit more
interesting so if we go back and make some

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changes to so I'm just gonna closed the run window and
see if we can actually center the text

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.....

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......its a

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calculation to get an entry that is going
to print in the middle of the screen so

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what we gonna do their now is instead of
printing as we did there we are going to delete

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that we're going to print a space and
multiply that by the number of spaces

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which we have calculated on line 4 so if
margin and we are also going to print text

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like and we will just get rid of the print on line 8 and make it so its just calling the

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function now that is a bit more useful but now it can only print the same thing o its called but lets just run it first and

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see and you can see there now we actually got
it centering and obviously you need to be in a

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window with 80 characters across to see it centered correctly but you can see

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how it is their is some calculation and is
trying to figure out the center if I move this window a bit

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to roughly simulate 80 characters that will be roughly the middle as you can see there

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we can actually test that by changing
this and by adding a parameter and instead of

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printing the hard coded it on line 3 we
can print the parameter that was passed

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to this methods so what we'll do is we'll
create a new method so we are going to leave

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the python food method their and create another one I'm going to type....

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.....

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......

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.....

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......

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.....

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we are now going to call the function that we've
created three times with a different

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parameter which is going to be
automatically place in the text variable

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and we're using that to figure out how
why to format it all and we should see if we

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run this in that window that I saw that I previously that it will be

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re-centered on that screen incidentally
just while I remember the Python style is

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to have to blank lines before a function
definition and IntelliJ will actually give you

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warning if you only leave one so if you
take this back you can see how that's

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actually complaining and go back and have a look at the code expected 2 bank lines found one so it

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know their should be 2 so if we press enter their we should get rid of that warning and then

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we'll be good to go

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ok so let's try running this and you
can see their the text is now correctly

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centered now in terms of defining the
argument or parameter whichever way you want

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to call it could be called anything we
happen to call  text which happened to

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coincide with the variable name text on line 3 so we
used the same name but that can

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literally be anything you wanted it to be as
long as it's a valid variable name

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obviously the text makes sense here
because that's what we're doing is we

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are formatting and printing it out some texts so I decided to stick with that now when centered

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text is called it needs a single argument
between the parenthesis which you can see

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down here on lines 14 through 16 this argument
used to provide the actual value that

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the parameter text is set to when the
function runs so the first time

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we call it as you saw if we just run again you can see
the first time was spam and eggs and I've centered

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that then next we went through did another
call to its spam spam and eggs and then spam

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spam spam and spam and it correctly
figured out the width for each one how

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wide it needed to be to center it on
the screen and worked each time for us so each

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time we are calling it the argument that is being
passed to the method to the function is

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changing the different value and its
calculating that and printing it on the screen

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and I've use the term parameter an argument
you've heard me both say those in this video

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just to be clear parameters refers to the
variables defined in the function

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definition that's that is here the definition on line 8 and also the one on line 1

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although that hasn't got a parameter their and argument refers
to the actual values used when the

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functions is called which is these down here
line 14 through 16 and may have found the two

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teams used interchangeably with
parameters now often uses instead of argument technically

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speaking it's incorrect but spoken and
for the matter written language is often

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littered with grammatical errors and
this is just one more example so long as you are

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aware of the correct usage if your asked in a job
interview for example then don't worry

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too much about it and know that a lot of
code and a lot of videos that their will

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use the two terms interchangeably but
again technically to be correct

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parameter is the definition it's the
variables defined in the function

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definition itself as you can see on line 8
so we refer to text their

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as a parameter then the arguments are the
actual values used when the function is

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called on line 14 through 16 so its good to know both of them
but you can certainly use both terms

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interchangeably

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now the functions that we've created so far are really
quite simple but they do demonstrate the

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basics of defining a function in Python so center_text also introduced

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the idea of function parameters and how
they used but there's a lot more to

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functions the basic concept is that you
write a block of code once you call it

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whenever you need to perform these
actions in your program that's the

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reason that you're creating functions and
this obviously saves a lot of typing but

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more importantly it makes the actual
code you create much easy to maintain so if

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you find yourself writing the same lines of
code repeatedly in a program that's a

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good indication that could you should
really consider putting that code in

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a function now functions can often
return results we mentioned that

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earlier in this video the face that all Python functions do

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return results and Python will ensure they
return none if you don't specify another value

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before we look at how to do that we
should really fix an error in center text

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so anyone used to printing text in Python would
probably expect center_text to

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behave

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similar to print but if we try and use
it to center a number we actually get an error

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so in other words if we come back here type....and we run it

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we actually get an error so the first two
lines print fine as you can see their spam

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and eggs and spam spam and eggs but when
the code processing got to line 16

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where we used a number as a argument we get an error
on the screen as you can see object

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of type int has no len now the good
thing here is that Python makes life easy

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but not forcing us to worry about
variable types most of the time but there

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are times when we need to be aware of
the type of the variable we are using now here

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when we get the length in characters and
you can see that's the code on line 9

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using len and texts which is the
variable in the parameter the argument

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we're assuming that its character length is actually
meaningful but the thing to remember

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here is the numbers don't have a character length though so  if that sounds odd consider

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that the string spam consist of 4 characters but we cannot really say that the

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number 12 consists of two characters
remember that in a computer 12 is stored

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....so basically in computer
terms 12 would be stored as the

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binary number 1100 this is what would
happen internally or in 16 bit notiation

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it would be 0000 1100 you didn't really
need to know this I'm just trying to point out how they are stored

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differently so that's how a number would
be stored but in hexadecimal it will be

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stored as a C so looking at it in those
terms when we are not storing and using texts

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such what would really be meant by the
length if we're trying to get the length

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of the number doesn't really make sense as can see so we've used the print

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function to display numeric variables
and what happens when we do that is that

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Python automatically calls one of two
functions to produces a string

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representation of that variable

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the two functions are STR and REPR and
if string is defined for an

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object then the print function will use
that otherwise it is going to use this sa REPR

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so what we're going to do is we are going to look at these two functions in more detail when we create

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our own objects later in the course but for
now we can actually make a little bit of

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a change to make sure that this works
so lets just delete what I added their about binary numbers and

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go back to our method now what I want to do is
make a change

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here on line 9 so I want to type in.......and by doing that we should not need to

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change anything else and we should find the
code now works you can see how its

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now work and what's basically happened is
that by adding that code to line 9

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we are explicitly calling STR the STR function to
convert the argument that was passed to

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a string and we're doing that before we
attempt to get its length so consequently

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in the first two cases its already strings
so nothing else has done it still a string

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but in the third case where we passed the
parameters as an integer of the value 12 that

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gets converted to a string first time on line 9 so consequently on line 10 the len

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function is called is now a string that is
the argument that sent to the len function

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and that's why it works so we got now a
really useful function that can be used

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whenever we wanted to print text centered
on the screen

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this is a assuming and 80 character screen of course but it doesn't really act

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as a for replacement for the built in
print function if you remember from

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section 10 print can be used to write data to a file which is something center_text

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function clearly can't do
it at the moment because it's a useful way to

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introduce optional and default parameters what we're
going to do now is modify the function

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so that it could be used

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really pretty much as a direct
replacement for the print function and

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we also have a look at another approach
to the same problem now you already seen how

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can look at the Python source from IntelliJ by using to control click if your in a

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Linux machine or a Windows machine and on a
Mac you can do the same by holding down command

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and clicking it

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so what we are gonna do is we're gonna have a look at
the definition of print so I'm holding down

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command and click print so as you can see that's the

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definition for print so the first thing
to note is that you can actually view the

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real source of the built-in functions
the modules builtins.py

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stubs file and that what that means is that it
only contains descriptions of the

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functions without any actual
implementation so it's used by

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IntelliJ to provide the tool tips when you
were typing print for example showing

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you what arguments but the actual
implementation of the

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functions is actually written in C

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although we can't see all the source code
for the print function as such we can

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see its definition

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in a doc string describing the parameters
so you know its really still useful

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as you can see on the screen there now a .doc string just to be clear is a way of documenting your functions

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00:14:07,450 --> 00:14:11,170
and we are going to be looking at them a little
bit later but here everything between the

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two sets of triple quotes as you can see
from the line 477 through 486 is the doc

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00:14:17,820 --> 00:14:23,490
string print so you can see the first
parameter looking at line 476 first

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parameters is called args and the fact that it's a plural rather
than arg gives an indication of what the

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asterix means so if our parameter is prefixed with a stab then

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Asterix and it represents a variable
number of arguments and what that really means is

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00:14:36,880 --> 00:14:38,510
you can pass as many as you want

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separated by commas now seeing tha down a couple times in the course

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but we haven't really explained in detail so
here is an example what we mean so I'm going to

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00:14:46,430 --> 00:14:57,650
close it down for now so type in.....then this prints the

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5 arguments so if we run that first second 3 4 spam so it prints the five

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arguments on the same line automatically
separating them with a space so what

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we could do is provide the
same functionality to our center

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text function quite easily and will start
doing that in the next video