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Chapter 2


Getting Started

Learning Objectives
  • Understand what an interpreter, IDE, and shell do.
  • Learn how to install Python on your computer.
  • Know how to write and run simple Python programs.
 

2.1 Interpreters and IDEs


As we learned in the last chapter, computers only directly understand programs written in machine code. However, nearly all programs are written in a high-level language. For this to work, we need an interpreter program to translate the code into what the computer understands.

So in order to run our programs, we have to install the interpreter for the language that we want to use. Because we will be using Python, we need to install the Python interpreter. If we just give Python programs straight to our computer, it won't know what to do with them. We need to interpreter to run them.

We will also be installing another program along with an interpreter. This program is something called an Integrated Development Environment (IDE). An IDE is a program that lets you type in the program you are writing. You could write your programs in any old program, but it's generally much easier to use a program geared just for that purpose. For example, an IDE will give you a button to pass your program to the interpreter, highlight keywords in the language and make it easier to see when you have errors.

The IDE we will be using here is a simple one called Thonny, which is easy to use and get started with. It also includes Python with it, so you only need to install one thing. The choice of IDE doesn't really matter too much, if you want to use a different one for some reason (for instance if you have one from another class), you can follow the rest of the book using that too.

The rest of this chapter will guide you through setting up Python along with our IDE for whichever type of computer system you have. Then, we will see how to use it to run some Python code!

 

2.2 Installing Python on Windows


To install Python along with Thonny on Windows computers, go to the Thonny website. Then click the Windows link in the download box at the upper right. Choose to save the file. When the download is finished, run the installer program.

Click next, select the agreement, choose where to install it, and wait for the installer to finish. Once it is done, you should have Thonny, along with its Python interpreter installed.

 

2.3 Installing Python on Mac


To install Python along with Thonny on a Mac OSX computer, go to the Thonny website. Then click the Mac link in the download box at the upper right. That should download a .pkg file that includes the Thonny IDE as well as the Python interpreter.

When the download has finished, you should see the Thonny icon in a window. Drag this icon into your applications folder to copy it to your computer. You should then have it installed and be able launch Thonny from your Application menu to start programming.

 

2.4 Installing Python on Linux


While all recent versions of Linux come with Python, they do not come with Thonny. To install it, open up a terminal and run the following command:


bash <(wget -O - https://thonny.org/installer-for-linux)

Then hit Enter at the prompt to continue.

That should download the latest version, and install it on your computer. You should be able to find Thonny amongst your installed applications. You could also launch it by running the command:


~/apps/thonny/bin/thonny
 

2.5 The Shell Window


You should now have Python and Thonny installed. When you run it, you should see a window something like this:

The main Thonny window has two parts, at the top is the area for
writing a program, and at the bottom is the shell window for giving
interactive commands.
The main Thonny window

The main window has two main parts. The top is the file area. This is where you will type in the program that you will create. This is empty right now, and called "<untitled>".

The bottom area is called the Shell. This is a window where you can pass Python code to the Python interpreter. Any code you put in here will be run right away and the results will be given to you. Here is an example:

The shell responds to the line '3 + 4' with an swer of 7.  It
responds to the line of 'print(Hi!)' by printing the text 'Hi!'.
Some examples put into the shell window

As you can see, when we put 3 + 4 into the shell, it gives us the answer, 7. Likewise when we put in the command print("Hi!"), it prints what we told it to. What's happening here is that these are small amounts of Python code. When we put them in, the shell window passes them to the Python interpreter, which runs them. Any results are displayed back in the shell. It is called a "shell" because it sort of surrounds the Python interpreter and acts as our interface to it.

Generally, the top file area is for writing a program that you will run all at once. This window saves what you put there so you can run it over and over again as you work on a program. The bottom shell area is for trying things out and experimenting. Unless you copy and paste it some place else, the things you put into the shell window are not saved.

As you can see from the first example in the screenshot above, the shell can work as a calculator. Try putting a few other simple math expressions in and see how the shell gives you results back.

 

2.6 Our First Program


Now we are ready to write our first program. The goal of the first program is just to print the text "Hello World" to the screen.  . The code for this program is the following:

Program 2.1

# this is our first program
print("Hello World!")

You should type this program into the top window of Thonny. Then we can run the program. This can be done in one of three ways:

Before the program can run, it will ask you to save it. When saving your program files, you should put them some place where you will be able to find them again. You should also always name them something ending with the ".py" extension.

Once the program is saved, it will run. You should then see the results in the shell window:

The program text is in the file area, and the results appear
in the shell area.  The result is that the text 'Hello World!' has
been printed into the shell.
The results of running the program
 

2.7 About the Program


Now that we have seen how to run the program, we will talk about the program itself a little bit. This program consists of two lines. The first line says:


# this is our first program

This line is a comment. Any line that starts with the # symbol is a comment in Python  . When the interpreter gets a comment line, it completely ignores it, and moves on to the next line. The sole purpose of comments is to leave little notes in the code, for any people reading. They are meant to explain things about how the program works. This program is so short and simple that the comment is not really needed, but as we work on more complex programs, they'll become more helpful.

The second line of the program says:


print("Hello World!")

This is the line that actually tells the Python interpreter to do something. Python comes with lots of commands called functions built in that cause it to do different things. One of these is print. The parenthesis mark the things that will be printed. In this case, it's just the message "Hello World!".

Both the parenthesis and the quotation marks are needed for the program to work. You can change the message inside the quotation marks to whatever you want though. Try changing it so that it prints out your name.

 

2.8 When Things Go Wrong


We said that the parenthesis and quotes are needed, but what happens if we get rid of them? In these cases, the Python interpreter will not be able to figure out what to do with the code, and will give us an error message. For instance, if we get rid of the quotation marks, we get this:

The program is missing quotation marks and gives the error
'invalid syntax'
Missing quotation marks

Here the program did not run successfully. Instead, the shell gives us the error SyntaxError: invalid syntax. There is also an "Assistant" window which is a feature of Thonny to help us figure the error out. In this case, it's not terribly helpful. The specific problem here is that Python has no idea what to do with the exclamation mark since that doesn't mean anything in Python code.

If we get rid of the parenthesis instead, we get a different error:

The program is missing parenthesis and gives the error
"Missing parenthesis in call to print.  Did you mean print("Hello World!")?
Missing parenthesis

Here the error is much easier to figure out. It actually tells us what is missing and even suggests a fix for it. Even though the two errors were pretty similar, one would be much easier to figure out.

Errors can be stressful as a beginning programmer. Even if your code is 99% correct, one mistake can prevent the interpreter from being able to figure it out. As you get more experience with programming, they get easier and easier to fix. In the meantime, you can always ask your friends or instructor for help.

Now that we are able to write and run Python programs, we will begin delving into learning the language and starting to solve problems with it!

Chapter Summary
  • An interpreter is a program that translates programs so that they can be executed by the computer. An IDE is a program that lets you write programs and passes them to the interpreter.
  • The Thonny IDE can be installed on Windows, Mac, or Linux. It has a file window for writing programs, and a shell window for running commands interactively.
  • Comments are lines starting with a # and are little notes that are put into programs.