Values and variables

Almost every program works with values: a temperature, a price, a name. Python distinguishes different kinds of values, and the kind decides what you can do with a value. So that a program can use a value again later, it stores it under a name, and such a name is called a variable.

Three kinds of values

For a start, three kinds are enough. Python calls them data types:

With the function type you ask Python which type a value has:

print(type(42))
print(type(3.5))
print(type("42"))

Look at the last example: "42" is in quotation marks and is therefore a piece of text that happens to consist of two digits, not a number.

Variables

You create a variable by assigning a value to a name with =:

temperature = 21.5
city = "Osnabrück"
print("In", city, "it is", temperature, "degrees.")

In Python, = does not mean "is equal to" as in mathematics but "gets the value": Python works out what is on the right and stores the result under the name on the left. From then on, the name stands for this value wherever you use it.

A variable can get a new value later. The old one is then gone, and from this line on the new one applies:

count = 1
print(count)
count = count + 1
print(count)

The line count = count + 1 is nonsense in mathematics but clear to Python: take the current value of count, add one and store the result under count again.

Good names

Names follow a few fixed rules: they consist of letters, digits and the underscore _, do not start with a digit and contain no spaces. Case matters, so price and Price are two different variables. Words like if, for or class are reserved because they have a meaning in Python itself.

2nd_place = "Ada"

This name starts with a digit, and that is why Python stops with a SyntaxError before the program even runs.

Beyond the rules there are habits that almost everyone follows: names are written in lower case, several words are joined with an underscore (total_price), and a name says what the variable holds. x and a are quick to type, but seconds_per_day still makes sense a month later.

The type decides what happens

The plus sign shows well why the type matters. With numbers it adds, with text it joins the pieces together:

print(42 + 1)
print("42" + "1")

And if you mix both, Python does not know what you want and reports a TypeError:

print("42" + 1)

How to turn text into a number and back is part of the next lesson.

Takeaways

• Values have a type: int for whole numbers, float for decimal numbers with a point, str for text in quotation marks.
• name = value stores a value under a name; a new value replaces the old one.
• Names are in lower case, join words with _ and say what the variable holds.
• What an operator like + does depends on the type of the values.