Values and variables

In algebra, score = score + 5 has no solution. In Python it is an everyday line: if score was 10, it becomes 15.

The = sign does not mean "is equal to"

Show a maths student the line score = score + 5 and they will tell you it is false: no number equals itself plus five. They are right about algebra, and that is exactly why the line confuses people the first time they see it in a program. In Python, = is not a statement that two things are equal. It is an instruction with a direction. Work out the right-hand side first, then attach the name on the left to the result.

score = 10
score = score + 5
print(score)

15

Read it the way Python does. Line 1: the right side is 10, so the name score now refers to 10. Line 2: the right side is score + 5, which is 10 + 5, which is 15, so score now refers to 15. The old 10 is gone. Line 3 prints what score refers to at that moment. A name that refers to a value like this is called a variable, and the line that gives it a value is an assignment.

A trace table follows each variable line by line

When several variables change, keep a trace table: one row per line of code, one column per variable, and write the value each variable has after that line runs. It is slow, and it works. Most "why does it print that?" questions are settled by a trace table in under a minute.

score = 10
bonus = score * 2
score = 0
print(score, bonus)

0 20

Look at the last two rows. Changing score to 0 did not change bonus. When line 2 ran, Python worked out 10 * 2 = 20 and gave bonus that number. bonus holds the result, not the recipe, so it has no memory of where the 20 came from.

Every value has a type

Values come in kinds, called types. Whole numbers such as 42 are int. Numbers with a decimal point, such as 3.5, are float. Text in quotes is str. True and False are bool. The built-in function type tells you which one you have. The type matters because it decides what an operation does: 2 + 3 adds, while "2" + "3" joins two strings into "23".

print(type(42))
print(type(3.5))
print(type("42"))
print(type(True))
print(2 + 3, "2" + "3")

<class 'int'>
<class 'float'>
<class 'str'>
<class 'bool'>
5 23

Rules for names

A variable name can use letters, digits and the underscore _, but it cannot start with a digit and cannot contain spaces. Capital letters count as different letters, so Score and score are two separate variables. Some words, such as if, for and True, belong to the language and cannot be used as names. Python programmers usually write names in lower case with underscores between words, like lunch_price, and pick names that say what the value means. x is fine for a quick test, and seats_left is better in a program someone else will read.

a = "left"
b = "right"
a, b = b, a
print(a, b)

right left

The last sample swaps two values in one line. Python works out the whole right side, b, a, first, and only then gives the two results to a and b. Many other languages need a third variable to do the same job.