1.3 Input and Output

Programs need to communicate with users. The most basic interactions are printing output and reading keyboard input.

print()

print() displays text or variable values on the screen.

Some characters have special meaning in code, so they need escape sequences.

• \\: backslash
• \': single quote
• \": double quote
• \n: newline
• \t: tab

print("\"Hello\nWorld\"")

Output:

"Hello
World"

Placeholder formatting

You can also insert values into a string with placeholders.

• %d: integer
• %f: floating-point number
• %s: string

length = 10
width = 5
area = length * width
print("Area = %d * %d = %.2f" % (length, width, area))

Output:

Area = 10 * 5 = 50.00

A more modern style is the f-string, which can directly reference variables inside a string.

print(f"Area = {length} * {width} = {area:.2f}")

Controlling the line ending

By default, print() ends with a newline. Use the end parameter when you do not want that newline.

num1 = 1
num2 = 2
num3 = 4
num4 = 8

print(num1, end=', ')
print(num2, end=', ')
print(num3, end=', ')
print(num4, end='...')

Output:

1, 2, 4, 8...

input()

input() reads text from the keyboard and assigns it to a variable.

Important: input() returns a str by default. Convert it when you need a number.

import math

r = float(input("Radius: "))
area = math.pi * r ** 2
print("Area = %.2f" % area)

One possible run:

Radius: 5
Area = 78.54

Here, float() converts the input string into a decimal number. math.pi comes from Python’s math module.