Variables & Types

A variable is a name for a value you want to use later. You create one with =:

apples = 5

Now apples means 5 anywhere in your program. You can change it whenever you like:

apples = apples + 1   # apples is now 6
apples += 1           # a shortcut for the line above; apples is now 7

= versus ==

A single = stores a value in a variable. A double == compares two values and gives back True or False. Mixing them up is one of the most common beginner mistakes.

Types of values

Every value has a type. These are the ones you'll use most:

Type	Example	What it's for
int (integer)	5, -12, 4095	Whole numbers: counts, pin numbers, sensor readings
float (floating point)	5.12, 0.5, -3.3	Numbers with a decimal point: voltages, seconds
str (string)	"Hello", 'Fuji'	Text. Wrap it in single or double quotes
bool (boolean)	True, False	Yes/no values: is the button pressed?

number_of_apples = 5        # int
half_an_apple = 0.5         # float
apple_type = "Fuji"         # str
has_apples = True           # bool

You can ask Python for a value's type with type():

>>> type(0.5)
<class 'float'>

Naming variables

• Use lowercase words joined with underscores: button_pin, servo_angle.
• Names can't start with a number or contain spaces.
• Names are case-sensitive: score and Score are different variables.
• Use CAPITALS for values that never change, such as pin numbers: LED_PIN = 27.

Maths

print(5 + 5)     # 10   add
print(10 - 5)    # 5    subtract
print(5 * 5)     # 25   multiply
print(4 ** 2)    # 16   power (4 squared)
print(100 / 20)  # 5.0  divide (always gives a float)
print(17 // 5)   # 3    divide and round down to a whole number
print(17 % 5)    # 2    remainder after dividing

The % (remainder) operator is handy for things that wrap around. For example, hour % 12 turns 24-hour time into 12-hour time.

Converting between types

Sometimes you have a value of one type and need another. For example, a number that arrives from the internet as text has to become a number before you can do maths with it.

x = "5.432"     # a string: it's in quotes
x = float(x)    # now the float 5.432
x = int(x)      # now the int 5 (int() chops off the decimals)
x = str(x)      # back to the string "5"

int() on a string with a decimal point

int("5.432") gives an error, because the text isn't a whole number. Convert to a float first: int(float("5.432")) gives 5.

Joining text

Use + to join strings, and str() to turn numbers into text first. An f-string (a string with an f in front) is often easier: put any variable inside { }.

score = 12
print("Score: " + str(score))    # Score: 12
print(f"Score: {score}")         # Score: 12
print(f"{score} points in {half_an_apple} seconds")

Next: Lists & Dictionaries