1.1 Data types and expressions

Big ideas

• Python has four everyday types: int (whole numbers), float (decimals), str (text in quotes) and bool (True/False). type(x) tells you which one you have.
• Arithmetic: + - * as usual, / always gives a float, // is floor division, % is the remainder and ** is a power. Normal precedence rules apply; parentheses win.
• A variable is a name bound to a value with =. Assignment is not an equation: x = x + 1 means “make x one bigger”. Comparisons use ==.
• input() always returns a string, so convert with int() or float() before doing math. str() goes the other way.
• f-strings put values inside text: f"{name} is {age}", with formatting such as {price:.2f}. print() takes many arguments and puts spaces between them.

Code you should know

print(7 // 2, 7 % 2, 7 / 2, 2 ** 10)
print(type(7), type(7 / 2), type("7"), type(True))
print(3 + 4 * 2, (3 + 4) * 2)

3 1 3.5 1024
<class 'int'> <class 'float'> <class 'str'> <class 'bool'>
11 14

name = input("Name? ")
age = int(input("Age? "))
print(f"Next year {name} will be {age + 1}.")
print("Age doubled as text:", str(age) * 2)

Next year Nik will be 20.
Age doubled as text: 1919

price = 4.5
qty = 3
print(f"Total: ${price * qty:.2f}")
print(f"{10 / 3:.3f}", round(10 / 3, 2))

Total: $13.50
3.333 3.33

Worked example

x = 17
y = 5
print(x // y, x % y, x / y)
print(str(x) + str(y), x + y)

Common mistakes

• Forgetting that input() gives a string, so "3" + "4" is "34", not 7.
• Using = when you mean == inside a comparison.
• Expecting 10 / 2 to print 5; it prints 5.0. Use // when you want an int.
• Mixing tabs and spaces, or forgetting the colon and indentation later in the course. Set your editor to 4 spaces now.