Part 01 - Getting Outputs from your Program

This is the second part of my Introductory Python Programming Tutorial. The first was Part 00 which can be found here and explains how to install Python.

All computer programs and systems boil down to a very simple model, basically there has to be some sort of input, whether that's from a user controlled device such as a keyboard or mouse, an automated device such as a sensor, or even an external device of some kind that sends a signal to our system.

This then is either processed itself or causes some sort of processing to take place. Finally, once a result is arrived at this then needs to be communicated back to the user or other computer systems by outputting some sort of value.

Basically, we have Input --> Process --> Output there may be other things which occur too, in terms of storing things, but every computer system and every computer program essentially boils down to a version of this model. So the first thing we are going to do is make Python tell us a story, in other words, we're going to get it to output some text and other things.

So we are going to use this to make the program output some text in the Python shell, initially, we are going to start with the simplest possible program which just outputs some text like we did in the previous section. Start up the Python shell like we did in Part 00 (select Idle, Python(Idle), Python(GUI) or something similar from your equivalent to the Start Menu). This will bring up the interactive Python Shell:

The Python Shell

Click on the File Menu in the Shell window and select the entry that reads either New File or New Window (it varies depending on your version of Python). This will open a new empty editor window, which is where we are going to write our code.

Blank Editor Window

Type the following Python code on the first line of the file:

print("Hello world!")

Editor window with line of code

If we run this it will just output the text which is between the two quote marks ". In technical terms, this is a string, which is a piece of text that we can output, manipulate or pass to some other part of the program so that it can do stuff to it. We can run the code either by clicking on the Run Menu and then choosing the bottom entry Run Module or by hitting the F5 key. Either way, you will be asked if you want to save your source code, you do so choose yes, give it a name such as hello (if you are on an older version of Python you may need to include the .py at the end as this didn't get added automatically in earlier versions). When you finish saving it, you should see the output appear in the Shell window, something like this:

Shell window with output

We can change the code so that it will output other text, by altering what lies between the quote marks. Do that now and rerun the code to make sure it works successfully (you will be asked if you want to save as you go, choose yes each time). Whenever you change your code, you must rerun it to see the changes take effect.

We can make the code output over multiple lines in a number of ways. Firstly we can add a print function for each line of text we want to output. The last verse of the poem "The Masque of Anarchy" by Shelley reads:

Rise, like lions after slumber
In unvanquishable number!
Shake your chains to earth like dew
Which in sleep had fallen on you:
Ye are many-they are few!

We can turn this into a program in a number of ways. Firstly as stated we can use multiple print functions:

print("Rise, like lions after slumber")
print("In unvanquishable number!")
print("Shake your chains to earth like dew")
print("Which in sleep had fallen on you:")
print("Ye are many-they are few!")

Each print automatically outputs a newline character after the text has been displayed, making each string display on a new line. This is behaviour we can modify if we so choose (more on this in a later section).

The second way in which we can display the poem on multiple lines is to put triple quotes around the whole verse, and use a single print function. This will output whatever is inside the string, using any newline characters that make up the string.

print("""Rise, like lions after slumber
In unvanquishable number!
Shake your chains to earth like dew
Which in sleep had fallen on you:
Ye are many-they are few!""")

Adding extra line breaks inside the string, is then reflected in the output from the print function. Try putting the cursor at the end of one of the lines of poetry and hitting enter, then rerun the code.

We can also get the same effect using a single print function, by including all of the lines of text in a single string and adding in the newline character ourselves (newline is a backslash followed by an n - \n).

print("Rise, like lions after slumber\nIn unvanquishable number!\nShake your chains to earth like dew\nWhich in sleep had fallen on you:\nYe are many-they are few!")

In the next part we'll look at making more complex use of the Python print function. Meanwhile have a play with print and see what you can do.

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My name is Dave Ames. I've been a teacher for 25 years and for the last few of those I've been teaching both children and other adults, especially teachers to program in a variety of environments, but mostly Python.

I'm intending these to be an ongoing series of Introductory Python Programming Tutorials, that are small enough to be easily digestible.