First Python sessions¶

This section is for people who have never talked with a computer. Welcome on board!

Your first date with Python ¶

Open a terminal on your computer (on Linux hit Ctrl+Alt+T, on Windows hit the 🪟 Win key, type “cmd” and hit ENTER).

In your terminal, type python followed by ENTER. It should say something like:

$ python
Python 3.12.3 (main, Jan 17 2025, 18:03:48) [GCC 13.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

These three “>>>” are called the prompt. The prompt indicates that Python waits for you to say something. Try to say:

123+456

and then hit ENTER. She will answer instantly:

And again she asks you to say something. Talking with a computer is always like this: you say something, she does what you want her to do, and then asks for your next command. This way of talking with a computer is also called the command line.

Next thing. Try the following:

x=123

Python doesn’t seem to answer anything, but actually has stored a value (123) in a variable (named “x”). You have now defined a variable.

Computers are very good in remembering things. From now on you can say:

x+456

And she will tell you again 579. That’s because she now knows that when you say “x”, you mean 123.

In case you aren’t sure what “x” means, she can now remind you. Just type the name of the variable:

Now try to say y instead of x

y+456

You’ll get a longer answer:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'y' is not defined

The first lines are a bit strange (imagine she got angry and therefore talked rubbish), but the last line makes sense, does it? Yes, indeed you never told her what “y” means, so of course she doesn’t understand you. Python is nit-picky regarding how to call things.

Okay, let’s tell her what “y” means to you:

y = 456

And then try again:

x+y

Now she answers 567 because yes, when x is 123 and y is 456, then their sum is 579.

Python is very good in computing. Try the following ones as well:

x*y
x-y
x/y

And a few more:

x*x+5
x*(x+5)
x*x*x

You now know the four basic operators of the Python language: +, -, * and /.

A last thing before we stop for this session:

x=2
y=3
x*y

Yes, Python is not like a fundamental traditionalist, she is perfectly okay with changing your mind about what the names x or y mean. You just redefine them. That’s by the way why they are called variables.

How to end a session with Python? Try to say “exit” and she will tell you. On my Linux computer she says:

>>> exit
Use exit() or Ctrl-D (i.e. EOF) to exit

On Windows computers it’s different. Don’t ask me why…

Keep in mind:

variable¶: A name that you define so that it gets a “meaning” for Python. This “meaning” is usually called the value of that variable.
value¶: The “meaning” or the “content” of a variable. For example 123 or 456 are values.
operator¶: One of the symbols +, -, * and /, which represent a mathematical operation on two values.

Working with text ¶

A variable can hold other values than numbers. For example chunks of text. Today we’ll play with these.

First Python sessions¶

Your first date with Python¶

Working with text¶

Your first date with Python ¶

Working with text ¶