Taking Input from the User

1A program that can only add 2 and 3

Here is a program that adds two numbers. It works, and the answer is right:

a = 2
b = 3
c = a + b
print('The sum is', c)

The sum is 5

Now ask a harder question: who is this program for? It adds 2 and 3. It will add 2 and 3 today, tomorrow, and every time anyone ever runs it. If your friend wants the sum of 47 and 68, this program is of no use to them at all.

They could open the file and edit the two lines. But then they are not using your program, they are rewriting it — and they would need to know Python to do it. A program that has to be edited before each use is barely a program.

2input() lets the user choose

Python gives us a function for exactly this: input(). It allows the person running the program to enter a value of their own choice, while the program is running. Not while it is being written — while it is running.

When Python reaches an input(), three things happen:

Watch that trip below. Use Next step to take it one stage at a time, or Play all to see it run straight through. You can change what gets typed before you start:

3Whatever you type arrives as text

This is the part that catches everybody. input() does not care what the typing looks like. Digits, letters, spaces — it collects the keystrokes and hands them back as one piece of text, which Python calls a string.

So if the user types 25, the variable does not hold the number 25. It holds the text '25'. You can check this yourself with type(). Press Run: the program will stop at input() and a box will appear in the output — type a number there and press Enter, exactly as a real user would:

age = input()

print(age)
print(type(age))

<class 'str'> — str is Python's name for text. Even though you typed digits and nothing else, what came back was text.

4Which breaks our sum

Now put input() into the adding program and watch it go wrong. The user types 10 and 5, and the program prints 105:

a = input()
b = input()
print(a + b)

10
5
105

Nothing is broken and there is no error — which is what makes this so confusing. The reason is that both boxes hold text, and + does a different job depending on what it is given. Given two numbers it adds them. Given two pieces of text it joins them end to end.

Flip the switch and watch the same two keystrokes give two answers:

5The fix: int() and float()

Python can convert text into a number, as long as the text really does look like one. There are two functions for it, and which you use depends on the kind of number you are expecting:

You wrap the input() in whichever one you need. Python works from the inside out: input() runs first and produces text, then int() turns that text into a number, and only then does = store it:

a = int(input())

# Python does it in this order:
#   1. input()      → the user types 10, so this gives '10'  (text)
#   2. int('10')    → gives 10                               (a number)
#   3. a = 10       → the box now holds a number

Here is the adding program, finally working for any two numbers. Run it and it stops twice — type a number each time it asks:

a = int(input())
b = int(input())

c = a + b
print('The sum is', c)

Run it again with two numbers of your own. The program has not changed — and it never needs to again. It now works for any two whole numbers, for anybody who runs it. That is the difference between a program that solves one sum and a program that solves the problem.

marks1 = float(input())
marks2 = float(input())

average = (marks1 + marks2) / 2
print('Average =', average)

6Recap