Chapter 3

First Programs

3.1 Output

In our first Python program, we saw the print function which prints whatever message is put between the parenthesis. Here it is again:

print("Hello World!")

print is the main way for doing output in Python. Of course, we can have multiple print statements in a program. The following program has three prints:

print("Welcome to this program!")
print("Hello World!")
print("Bye bye!")

When a program has more than one line like this, Python will do them one by one. This is an important point of programming. Unless we tell Python otherwise, it starts with the first instruction, then goes through them in order until it gets to the end.

So this program will print the first line, then the second, and then the third. The output of this program looks like this:

Welcome to this program!
Hello World!
Bye bye!

By the way, in this book, we will display code in the blocks with a light grey background, and what the programs output with a darker background like this.

As you can see, this program prints three lines of output, one for each of our three print statements. We will look at more things we can do with print statements in a bit, but first let's look at getting user input.

3.2 Input

We can also do input in Python, when we want to ask the user for information. Most programs take some sort of input, which allows us to control what the program is doing, or what values it is calculating with.

This can be done with the input function. Like print, input can take a message inside of parenthesis. In the case of input, this message is a question to give the user, called a prompt.

Here is an example of how input works ¹:

input("How are you feeling today? ")

When we run this program, it will print the prompt to the screen for us, and then wait for us to type something in. To give the program the input it's waiting for, we have to type into the shell window at the bottom of the screen. When you type something in and hit enter, it will take the input:

As you can see, Thonny colors what we are typing in blue, and what the program prints as black. Here the input we gave the program was the words "Pretty Good".

We can only type one line of text. As soon as we hit enter, Python moves on from the input instruction. In this case, there is no next instruction so the program finishes.

This program does not actually do anything with the input we give it. In the program above, whatever the user types in can't really affect the program at all. In order to do something with input, we must put it into a variable.

3.2 Variables

We talked about variables briefly when we were talking about algorithms back in Chapter 1. Here we will talk about how to use them in Python.

Variables in programming are names that we associate with some piece of information. Variables let us refer back to something that was created earlier on in a program. They also let us save whatever the user inputs, so we can keep track of it.

The way that a variable is created in Python is by putting the name on the left hand side, then an equals sign, and finally the thing that you want to store in the variable. For instance, if we want to save our user's input in a variable, we could do it like this:

answer = input("How are you feeling today? ")

Now when we run this program, it will ask us the question, and wait for us to enter a response. It will then save whatever we give it into the variable called answer. We can now change the program so that it prints it back to us:

answer = input("How are you feeling today? ")
print("You said")
print(answer)

Here is an example of the output of this program:

How are you feeling today? pretty good
You said
pretty good

The text that we typed is in a different color so that you can see what the user types in this example. The white text is what the program itself is printing out.

There are a couple of things to note about this program. First, we have saved the input we typed into the variable called answer. We can then print this variable out on the third line of the program. This line of code is worth talking about:

print(answer)

Notice how this did not actually print the word "answer". When we print a variable, it doesn't print the variable's name, it prints the variable's value. Whatever got stored in the variable (which is whatever we typed in), gets printed here.

Also, notice how there are no quotation marks around "answer" in the print command. If we put quotation marks in, it would actually have printed out the word "answer". We have to use quotation marks to print some message out exactly, and no quotation marks when we want to get the thing stored in a variable.

There are some rules for naming our variables. The name of a variable has to be made of letters, numbers and underscore characters. They cannot begin with a number and cannot have spaces in them.

These are examples of legal variable names:

• price
• price_in_dollars
• priceFor2

And these are not legal:

• full-price (the - symbol is not allowed)
• 2_times_price (can't start with a number)
• price in dollars (no spaces are allowed)

Variables also should not be named something that already means something in Python. That means that you should not name a variable print or input. There are lots of other names in Python that mean things and we will see them as we go.

Notice that Thonny colors print and input differently than other things. If the new variable you just made also shows up colored like this, then it means something special and you should pick another name!

3.3 More on Printing

In the program above, we printed our message on two different lines, which looks kind of weird. Instead, we can print it on one line, using just one print instruction. To do that, we can pass the message and the variable to print on one line, separated by a comma. That would look like this:

answer = input("How are you feeling today? ")
print("You said", answer)

When we run this program, it gives us this:

How are you feeling today? pretty good
You said pretty good

There is no limit to how many things we can print like this — we can just keep adding things and putting commas between them. Like if we want to also print "Bye!" so the user knows the program is done, we could add that in:

answer = input("How are you feeling today? ")
print("You said", answer, ". Bye!")

Now the program prints this:

How are you feeling today? pretty good
You said pretty good . Bye!

Notice that Python automatically puts a space between the things that we are printing. This is often helpful, but in this case makes the output look kind of weird since there is a space before our period. When we want to avoid this, we can also give the text sep="" to print. This tells Python to separate the things it's printing with nothing at all. Now the program looks like this:

Program 3.1

answer = input("How are you feeling today? ")
print("You said ", answer, ". Bye!", sep="")

And it will output the following:

How are you feeling today? pretty good
You said pretty good. Bye!

Note that we had to now put a space between "You said" and the variable because now there isn't one put in automatically. Some people aren't too bothered about details like this, but I like to get the spacing to look exactly right for the program's output.

3.4 Example: Greeting Program

Now let's create a slightly longer program which will need two variables. We'll talk about how the program will behave first, and then talk about how to write it.

We want the program to ask the user for two things:

1. Their name
2. What day of the week it is

It will then give them a personalized greeting wishing them to have a good day. For example, if we put in "Nicole" and "Thursday", then it would print this:

Hello Nicole!
Have a great Thursday!

However, if we put in "Tim" and "Monday" when the program asks our name and what day it is, then it will print this:

Hello Tim!
Have a great Monday!

This is an important point in programming — what the program does will depend on the input given to it. It means that we can't just write the program like this:

print("Hello Nicole!")
print("Have a great Thursday!")

If we did, then it works if your name is Nicole, and it happens to be Thursday, but it won't work in any other case. You also can't just replace "Nicole" and "Thursday" with your own name and day. If you do, it will work for you that day, but not in any other situation.

What we want is to have the program do the right thing in every situation. For that, we need to put the name and the day into variables. We will need one for each thing. One variable generally keeps track of just one piece of information.

We will start by asking the user their name and storing the result into a variable:

name = input("What is your name? ")

Next, we need to ask them for the other piece of information we need, the day of the week:

    day = input("What day is it? ")

Now we have these two variables, which we have called name and day. The next step is to do the printing. Now we will use our variables so that whatever answers they gave to those questions will be repeated:

print("Hello ", name, "!", sep="")

This will print "Hello ", followed by the user's name, and then an exclamation point, with no spaces in between (so the exclamation shows up right after their name).

Now it should greet them by name no matter what they put in. We can do the same thing to wish them a good day:

print("Have a great ", day, "!", sep="")

Below is the whole program, with a comment at the top. It's usually a good idea to put a comment at the top of your code explaining what the point of the program is.

Program 3.2

# this program gives the user a custom greeting
name = input("What is your name? ")
day = input("What day is it? ")

print("Hello ", name, "!", sep="")
print("Have a great ", day, "!", sep="")

Notice the program also has a blank line in it. Blank lines are ignored just like comments are. It's common in programs to put a blank line between different sections of code — kind of like paragraphs in a paper.

Below is an example run — though of course what it prints exactly depends on what you tell it!

What is your name? Mary
What day is it? Friday
Hello Mary!
Have a great Friday!

Footnotes

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Licensed under a Creative Commons Attribution 4.0 International License.