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That's enough for the interactive
shell for now. Let's write our first program.
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In IDLE, click on File > New File.
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This makes the file editor window appear.
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Enter the following code: I'll ex[plain what it  does afterwards.
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# This program says hello
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and asks for
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my name. Add a blank line, followed by
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print('Hello world!')
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print('What is your name?')
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# ask for their name
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myName = input(), print(
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'It is good to meet you, ' + myName
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print('The length of your name is:')
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print(
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len(myName))
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print('What is your age?')
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# ask for their age
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myAge = input()
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print('You will be ' +
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str(int(myAge))
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+ 1 +
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' in a year.' Save this program as hello.py.
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Python programs are just text files that have the .py
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file extension. Then click on Run >
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Run Module or just press F5 to run the
program.
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"Hello world! What is your name?" Al.
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"It is good to meet you Al. The length of your name is
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2. What is your age?" Thirt-
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26. Yeeeah.
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I am 26. You will be 27 in a year."
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Ah, I remember when that was true.
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Okay, that's the entire program. One thing
to keep in mind
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is that Python will start at the first
instruction at the top of the program,
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and then move downwards executing each
instruction entered
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Think of it as putting your finger on the top
instruction
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and as each instruction is executed
moving your finger
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down to the next instruction. This is just
like how you might move your finger
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along the lines of a book that you're reading.
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Your finger is called the execution. The
execution
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in Python starts at the top and moves
down.
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So the first line with the
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pound sign is called a "comment". Python ignores comments.
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You can use them to write notes to yourself or reminders
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about what the code is trying to do or
what the entire program does.
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Blank lines are completely ignored by
Python. You can just use these to space
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out your code.
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In fact, let's add a couple more blank
line right here
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just to group together the
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code that's relevant to asking for your
name, and
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the code that ask your age. The
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print() function displays the string value
inside the parentheses
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on the screen. So the line print('Hello
world!')
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means print out the text in the string
'Hello World!'.
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Functions in Python are like mini-programs in your program.
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They contain code that you various things
and for your convenience Python
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comes with a lot of them.
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In code, a function is its name,
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followed by parentheses and
optionally
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sometimes there are values passed
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to the function inside the parentheses.
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In this context these values are called
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arguments. But really, values and arguments are the exact same thing.
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This is called calling the print()
function.
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We have code like this here. So the next
instruction is also a call
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the print() function. This time it's
passing the 'What is your name?'
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string to the print() function. There's
also a comment at the very end of the
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line. You can always add comments
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to the end of an instruction since everything after that
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pound sign will just be ignored by Python.
When the input function
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is called, Python waits for the user
to type some text on a keyboard and
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press Enter.
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Just like a variable will evaluate to
the value it contains,
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calls to the input() function evaluate to
the string value
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of what the user typed. So if I typed in
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"Al" when input() was called, this
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myName variable assignment statement
would just evaluate to 'Al'.
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And then the string will get stored
inside myName.
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Remember, expressions can always evaluate down to a single value.
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So if 'Al' is stored in myName
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and this is what that call to the
print() function looks like
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first my name evaluates to the value
inside it
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and then, this is just a string
concatenation,
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so the next step looks just like this:
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This is the string that gets passed to the
print() function call.
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This next line introduces a new function
called
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len(). What len() does is it takes a string
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argument and then evaluates
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to the integer value of the length of the string,
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which is the number characters in a
string. Let's just experiment with this
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function in the interactive shell for a
little bit.
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so I can call the len() function
and pass it
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'Al' and it returns 2.
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Or 'Albert' to return 6. Or
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length... the blank string and that returns
0.
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and then I can just use these function calls to len() anywhere
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I could using integer. So I could say, you
know,
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type out my name ten times, it's
length would be 20.
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So then we have some more print() and
input() calls.
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This time we're saving whatever the user
types
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into the variable myAge. And then we
have this line,
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which introduces the str()and int()
functions.
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Now this seems really complicated, so we'll just take this one step at a time
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The str() and int() functions return
string and integer
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values of whatever you pass them. This is
really handy if you need to convert
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between data types. So in the interactive shell
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you can have something like str()
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of the value 26, and that returns a string
value
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with 26 in it. And if you have to go the opposite route,
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you can call the int() function and pass it
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a string value like '1234'
and it will return in integer value.
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There's also a float() function that you
can use
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to convert something to a floating-point
value.
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So you can pass a string to it, it'll return a float value.
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Or if you pass an integer value to it, it
will return
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a floating-point version value of that
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integer. Notice it has the decimal
point and .0.
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This may seem pointless but remember
that the input() function always returns a
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string value, even if the string value something like
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'26'. We can't do math on a string.
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That returns an error. First we have
to get in integer value
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of it. So that's why we passed it to the
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int()function. And just like you can't do
addition on strings, you can't do string
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concatenation on integers.
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So we're going to have to call the str() function
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to convert that into a string value. This is
What the
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entire last line
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looks like when it- when Python evaluates it. First
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the myAge variable will evaluate to the
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value that's inside of it. Then that gets
passed to the
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int() function, which returns an integer
form of it.
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We add 1 to that integer. We have to convert that
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back to a string value, because we
want to concatenate
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the other streets next to it. And then
after that, it's just the string
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concatenation.
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And so
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all this just reduces down
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to this string, which we pass to the
print() function call,
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which then gets it displayed on the
screen. That's how the entire program works.
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Just to recap, the file editor is where
you type in code for a program.
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The execution starts at the top and then
proceeds
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down, executing each instruction in turn.
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Comments begin with a pound sign
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and are just notes that are ignored by
Python. They're just notes for the
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programmer.
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Functions are kind of like mini programs
that your program can run
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to execute some code that does some
specialized thing. In this case
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the print() function care has code for
making strings appear on the screen.
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There's a print() function for
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making text appear on screen. There's an input()
function for getting text from the
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keyboard.
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And there's also a str(), int(),
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and a float() function that returns an
integer, string,
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or float version the values that we
passed them.