Build Small Interactive Python Programs

Input, Process, Output

Almost every program follows the same three-step shape, called input, process, output (IPO for short):

1. Input: get the data, here with input().
2. Process: work with it: convert, calculate, combine.
3. Output: show the result with print().

You know these shapes from chapter 3: input and output are parallelograms, and the processing step is a rectangle. Now each shape becomes one line of code:

123number = int(input())
double = number * 2
print("Double:", double)

Think of a juice machine: fruit goes in (input), the machine squeezes it (process), and juice comes out (output).

Plan before you code

For every program in this topic, start by answering three questions:

• What goes in? Which values does the user type, and are they text, whole numbers or decimals?
• What happens? Which calculation or combination?
• What comes out? What exactly should the screen show?

Once you can answer them, the code is just three short groups of lines. This is the 4-step method from chapter 3, made concrete.

Input, process, output: plan all three, then write the code.

A Personal Greeting Program

Let's build a complete program that greets the user and tells them their age next year.

Plan:

• In: a name (text) and an age (a whole number).
• Process: add 1 to the age.
• Out: a friendly message.

12345name = input()
age = int(input())
next_age = age + 1
print(f"Hello, {name}!")
print(f"Next year you will be {next_age}.")

Try it with your own name and age.

What each line does

Make it your own

Change the program so it also says how many years until they turn 100:

123name = input()
age = int(input())
print(f"{name}, you will be 100 in {100 - age} years.")

The calculation sits right inside the f-string, as you learned in chapter 5.

Think of this program as a birthday card machine: give it a name and an age, and it writes the card for you.

A complete program: read the inputs, work with them, then show a clear message.

A Simple Calculator

Now a calculator: the user types two numbers, and the program shows what you get with each operator.

Plan:

• In: two numbers, possibly with decimals, so float().
• Process: add, subtract, multiply and divide.
• Out: one line per operation.

123456a = float(input())
b = float(input())
print(f"{a} + {b} = {a + b}")
print(f"{a} - {b} = {a - b}")
print(f"{a} * {b} = {a * b}")
print(f"{a} / {b} = {a / b}")

Each line shows the full sum, so the user can check it. Because we used float(), every number shows a .0.

Whole numbers only?

If the calculator is only for whole numbers, use int() and the extra operators from chapter 5:

1234a = int(input())
b = int(input())
print(f"{a} // {b} = {a // b}")
print(f"{a} % {b} = {a % b}")

What could go wrong?

If the user types 0 as the second number, the division stops with a ZeroDivisionError. In chapter 8, you'll check for that with if before dividing.

Common beginner doubt: "Why does 8 show as 8.0?" Because float() turned it into a decimal number, and Python always shows floats with a point. Use int() if you only want whole numbers.

Think of this program as a pocket calculator with only one button: "show me everything".

A calculator is input (two numbers), process (the operators) and output (one line each).

A Unit Converter

Converters are perfect input programs: a value goes in, a formula changes it, and the new value comes out.

Kilometres to miles

One kilometre is about 0.621371 miles.

123km = float(input())
miles = km * 0.621371
print(f"{km} km is {miles:.2f} miles")

Notice the named value: miles = km * 0.621371. You could write 0.621371 straight into the print(), but a clear name makes the code easier to read and to check, as you learned in the Comments chapter.

Minutes to hours and minutes

With // and % from chapter 5, you can split a number into parts:

1234total = int(input())
hours = total // 60
minutes = total % 60
print(f"{total} minutes is {hours} h {minutes} min")

Think of changing money: 135 rupees in 60-rupee notes is 2 notes, with 15 rupees left over. That's // and %.

Check your converter

Test with a value whose answer you know: 60 minutes should give 1 h 0 min, and 0 km should give 0 miles. If those are right, try the tricky ones.

A converter reads a value, applies a formula, and shows the result neatly.

Practice: Interactive

Solve these to lock in what you learned in this topic. Each one runs in your browser.

1. 56.Python Program to Calculate Age in MonthsEasySolve
2. 57.Python Program to Split a Bill Between FriendsMediumSolve
3. 58.Python Program to Make a Shopping Receipt from InputHardSolve