Variables and Data Types

The chair program in Your First Python Program had the number 12 in it three times. When Mr. Diaz asked for wider rows, two of the three got changed, and the printed total quietly disagreed with the room. The fix is older than Python: give the value a name, put the value in one place, and let the rest of the program ask for it by name.

chairs_per_row = 14
rows = 8
audience_chairs = rows * chairs_per_row

A variable is a name bound to a value. Change the value at the top and every line that uses the name changes with it — including the lines you have not written yet.

Assignment is not equality

= does not mean “is equal to”. It means put this value in this name, right to left. So this is not nonsense:

total = 0
total = total + 45

Read it as: work out total + 45 using the value total has right now, then store the answer back in total. That pattern — the accumulator — is the engine of half the programs in Repetition. The test for equality is ==, and mixing up the two is a rite of passage; see Boolean Logic.

The four types you need first

Every value in Python has a type, and the type decides what the value can do.

Type	Looks like	Use it for	Watch out
int	78, 0, -4	counting things: marks, days, chairs	int division with / gives a float
float	67.4, 1.0	measuring things: kilograms, averages	0.1 + 0.2 is not exactly 0.3
str	"Priya", "14"	text: names, titles, anything typed	"14" is text, not a number
bool	True, False	answers to yes/no questions	capital letter, no quotes

mark = 78
average = 67.4
name = "Priya"
is_overdue = True

Nothing declares those types. Python works out the type from the value, and type(mark) will tell you <class 'int'> if you ever want to check. That convenience has a cost, which the next section is about.

Everything from input() is text

input() always hands back a str, even when the person typed digits. This is the single most common surprise in Unit 1:

answer = input("Minutes practised: ")
minutes = int(answer)

Without that second line, answer + 10 fails and Python says exactly what is wrong:

TypeError: can only concatenate str (not "int") to str

int() and float() do the conversion. They also refuse loudly when the text is not a number — int("seven") raises ValueError: invalid literal for int() with base 10: 'seven', which is Python being helpful, not hostile. Input and Output shows how to catch that instead of crashing, and Name That Error is five minutes a day spent making messages like that one readable at a glance.

Where the value actually lives

The name is not the value. When Python runs mark = 78, it stores the number in RAM and keeps mark as a label pointing at it; when it later computes mark * 2, that arithmetic happens in the CPU. Nothing about this is metaphorical — it is why a program forgets everything the moment it closes, which is the problem Files and Persistence exists to solve.

Underneath, RAM holds nothing but binary, so every type above is a convention for reading a pattern of bits. 78 is stored as 1001110. Text is stored the same way through a character set: Unicode gives every character a number, and ord("P") will show you that the letter P is number 80. The type is how Python knows which convention to use — which is why "14" and 14 can look identical on screen and behave nothing alike.

Two conventions your future self will thank you for: names that say what they hold (chairs_per_row, not c), and SCREAMING_SNAKE_CASE for values that are never meant to change, like FILE_NAME. Practise all of it in Variables and Types Practice.

Curriculum connection

A1.1

use constants and variables, including integers, floating points, strings, and Boolean values, correctly in computer programs;

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A1.2

demonstrate an understanding of how a computer uses various systems (e.g., binary, hexadecimal, ASCII, Unicode) to internally represent data and store information;

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A1.3

use assignment statements correctly with both arithmetic and string expressions in computer programs;

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C1.4

identify the computer components involved in executing programming operations (e.g., assignment statements store a value in RAM, arithmetic operations are performed in the CPU).

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