Variables and types

Variables

A variable is a name that refers to a value. You create one with =:

name = "Ada"
age = 19
print(name)
print(age)

= means "make this name refer to this value", not "is equal to". You can point a name at a new value at any time:

score = 10
score = score + 5
print(score)  # 15

Variable names can contain letters, digits and underscores, but cannot start with a digit. Python style is lowercase words joined by underscores, like total_score.

The basic types

• int: whole numbers like 42 or -7.
• float: numbers with a decimal point like 3.14.
• str: text in quotes, like "hello" or 'hello'.
• bool: either True or False.

Use type() to ask Python what type a value has:

>>> type(42)
<class 'int'>
>>> type(3.14)
<class 'float'>
>>> type("hi")
<class 'str'>

Working with numbers

print(10 + 3)   # 13
print(10 - 3)   # 7
print(10 * 3)   # 30
print(10 / 3)   # 3.3333333333333335
print(10 // 3)  # 3
print(10 % 3)   # 1
print(2 ** 10)  # 1024

/ always gives a float. Floats are stored in binary, so some decimals cannot be exact: 0.1 + 0.2 gives 0.30000000000000004. That is normal in every language, not a Python bug.

Working with strings

first = "Grace"
last = "Hopper"
full = first + " " + last
print(full)          # Grace Hopper
print(len(full))     # 12
print(full.upper())  # GRACE HOPPER
print("ha" * 3)      # hahaha

Converting between types

You cannot add a number to a string directly. "Age: " + 19 raises a TypeError. Convert first:

int("42")      # 42
float("3.5")   # 3.5
str(19)        # "19"
int(3.9)       # 3 (cuts off the decimal part)

Reading input

input() shows a prompt and waits for the user to type something. It always returns a string, so convert it if you need a number:

name = input("What is your name? ")
year = int(input("What year were you born? "))
print("Hello", name)

f-strings

The cleanest way to mix values into text is an f-string. Put f before the quote and wrap expressions in braces:

name = "Ada"
age = 19
print(f"{name} is {age} years old.")
print(f"Next year: {age + 1}")
print(f"Pi is about {3.14159:.2f}")  # 3.14

Exercises

a, b = b, a

c = float(input("Celsius: "))
f = c * 9 / 5 + 32
print(f"{c} C is {f:.1f} F")