Lesson 2 – Strings, functions and programs¶

Working with text ¶

A variable can hold other values than numbers. For example chunks of text. Python calls such chunks of texts strings because they are actually strings of characters.

Every letter (“a”, “b”, “c”, …) is a character, but also every dot and comma or exclamation mark. These are called punctation characters. Even a space (' ') is a character.

You recognize a string because it is surrounded by quotes (').

>>> x = 'Joe'
>>> y = 'Jill'

You can combine two strings with the + operator.

>>> x + y
'JoeJill'

>>> 'Hello, ' + x
'Hello, Joe'

The + operator, when applied to strings, just concatenates the two strings, i.e. “glues them together one after the other”. You can do this with more than two strings in a single operation:

>>> 'Hello, ' + x + ' and ' + y + " !"
'Hello, Joe and Jill !'

The other three operators (- and * and /) are not allowed for strings:

>>> x - y
Traceback (most recent call last):
...
TypeError: unsupported operand type(s) for -: 'str' and 'str'

>>> x * y
Traceback (most recent call last):
...
TypeError: can't multiply sequence by non-int of type 'str'

>>> x / y
Traceback (most recent call last):
...
TypeError: unsupported operand type(s) for /: 'str' and 'str'

Anything surrounded by quotes is a string. When a value is surrounded by quotes, then it’s a string even when it looks like a number:

>>> x = '123'
>>> y = '456'

You might expect that the sum of x and y would be 579, but look:

>>> x + y
'123456'

As we said, the sum of two strings is just their concatenation.

Exercise: What is the output of the following command?

'123 + 456'

Yes, Python just does not try to understand what’s inside the quotes. For Python anything between quotes is just a string of characters. And even operators (like +) or digits (like 1, 2 or 3) are just characters.

Introducing functions ¶

Now it’s time to learn about functions. A function, in Python, is something you can call in order to ask something, and which will return you the answer to your question as a value.

You recognize a function because its name is followed by a pair of parentheses (()). The values between these parentheses are called the arguments. These arguments are a part of your question, of what you ask from the function.

For example there is a function called len(), which returns the lenth of a string. This function expects one argument: the string for which you want it to report the length.

>>> x = 'Joe'
>>> y = 'Jill'
>>> len(x)
3
>>> len(y)
4

Another function is sqrt(), which returns the square root of a number. The sqrt() function is not built-in, so you need to import it from the math module.

>>> from math import sqrt
>>> sqrt(4)
2.0

Two other function are print() and input(), and we will use these in the following lesson.

Your first program ¶

Until now we have been typing every command one by one into the Python interpreter running in our terminal. That’s a nice way to talk with your computer, but there is better.

You can write your commands into a file. A file that contains such commands is called a program file.

When the commands in a program file are written in the Python language, the file is usually called with the ending “.py”.

Let’s create our first program file. Open a terminal and type:

idle hello.py

This will open a window with a text editor. Type the following text into your file:

print("Hello, world!")

Now close the window. It will ask you whether you want to save the file:

And of course you say Yes.

Now you are back in the terminal. From here you can execute your program. Type the following and hit Enter:

python hello.py

The result will be very quick. Your program will print the following text to the terminal:

Hello, world!

And that’s already the end of your first program! The terminal is already waiting for your next instruction.

Let’s make your program a bit longer. Start your editor again on the hello.py file and change the content into the following:

name = input("Please type your name and hit ENTER: ")
print("Hello, " + name)

Once more, close the editor window, say “Yes” to save your file, and execute your program.

Congratulations! You just wrote your first program. You might copy this file to another computer and execute it there. That’s basically what software developers do: they write programs into files and sell these files to their customers.

Keep in mind: When you type simply python in your terminal, it starts the Python interpreter, which prompts you to enter your instructions and exectues them immediately one by one. This is also called “interactive”. But when you add the name of a file after the python in your terminal, Python does not run interactively but reads your instructions from the file.

Input and output ¶

The print() function prints a value to the terminal. This is sometimes called an output.

The input() function does the opposite: it asks a value from the terminal. It waits patiently until the user presses ENTER.

When you give an argument to the input() function, it prints this argument to the terminal before waiting for the user input.

You may call the input() function without an argument. Try it by modifying your program as follows:

name = input()
print("Hello, " + name)

This version of your program is admittedly less user-friendly. Because it doesn’t tell the user what they are expected to do.

More about strings ¶

We said that you cannot multiply two strings, but you can multiply a string by a number:

>>> x = '123'
>>> x * 3
'123123123'

>>> print('=' * 80)
================================================================================

You may use double quotes instead of single quotes when defining a string.

>>> x = "Joe's"
>>> y = "Jill's"

>>> print('This is ' + x + " book.")
This is Joe's book.

Keep in mind ¶

string¶: A chunk of text, a sequence of characters.
character¶: Every piece of a chunk of text.
function¶: Something you can call in order to ask something, and which will return you the answer to your question as a value.
program file¶: A file stored on a computer and containing commands to be executed by the computer.

Lesson 2 – Strings, functions and programs¶

Working with text¶

Introducing functions¶

Your first program¶

Input and output¶

More about strings¶

Keep in mind¶