Asking the User

A program that always does the same thing is a poster. The moment it can ask a question and act on the answer, it becomes a tool. Python's input function does the asking: it shows a message, waits for the person at the keyboard to type something and press Enter, and hands back what they typed. There is one catch, and it produces the first real crash almost every beginner meets: the answer always arrives as text, even when the person typed a number.

Think of a paper form with a box labelled age. People can write anything in that box: 34, thirty-four, or a smiley face. Before anyone does arithmetic with it, a clerk has to check that it is a number and read it as one. In a program, you are the clerk.

input Asks and Waits

Inside the brackets of input goes the question to show. Python prints it, waits, and gives back the typed answer as a string. In the session below, the person types 34. The answer is the text 34 in quotes, and type confirms it is a str.

>>> answer = input("Your age: ")
Your age: 34
>>> answer
'34'
>>> type(answer)
<class 'str'>

Text In, Number Needed

To do arithmetic with an answer, convert it. The int function turns text into a whole number, and float turns it into a decimal: int("34") is the number 34, and adding one gives 35. There is a limit. int accepts only text that looks like a whole number, so int("4.5") is a ValueError. For a value with a decimal point, use float, or Decimal when it is money.

>>> int(answer) + 1
35
>>> int("4.5")
Traceback (most recent call last):
  File "<python-input-4>", line 1, in <module>
    int("4.5")
    ~~~^^^^^^^
ValueError: invalid literal for int() with base 10: '4.5'
>>> float("4.5")
4.5

A Tip Calculator

Here is the first program in this book that responds to a person. It asks for the bill and for the tip percentage, works out the tip, and prints the tip and the total. The bill is money, so it becomes a Decimal made straight from the typed text, exactly as the arithmetic page recommended; Decimal accepts a string. The percentage is a whole number, so it goes through int. The tip is the bill times the percentage divided by a hundred, and the f-strings show both results with two decimal places.

from decimal import Decimal

bill = Decimal(input("Bill amount: "))
percent = int(input("Tip percent: "))

tip = bill * percent / 100
print(f"Tip: {tip:.2f}")
print(f"Total: {bill + tip:.2f}")

Run it with python tip.py on Windows or python3 tip.py on a Mac. With a bill of 48.50 and a tip of 15 percent, the tip is 7.28 and the total 55.78.

Bill amount: 48.50
Tip percent: 15
Tip: 7.28
Total: 55.78

The First Real Crash

Now run it again and type the word forty as the percentage. The program stops with a traceback. Read the last line first: ValueError, invalid literal for int with base 10, and then the word forty. In plain words, int was given text it could not read as a whole number. The line above the message shows exactly which instruction failed: line 4, the one that converts the percentage.

Bill amount: 48.50
Tip percent: forty
Traceback (most recent call last):
  File "/Users/you/python-course/tip.py", line 4, in <module>
    percent = int(input("Tip percent: "))
ValueError: invalid literal for int() with base 10: 'forty'

The program is not wrong for sensible input. The input was not what it expected. Asking again until the answer is usable needs a loop, which arrives in Chapter 4, and handling the error gracefully is the subject of Chapter 8. For now, the skill is reading the message and knowing what it means.