Lesson 2 — Input

By the end of this you can: ask a question, keep the answer in a variable, turn a typed answer into a number you can do sums with, and know why that last step is not optional.

Still plain Python. Everything here runs in the playground: press Run, and when the program asks a question a box appears in the output pane for you to type into. Press Enter to answer.

Asking a question

One function does all of it:

name = input("What is your name? ")
print("Hello, " + name + "!")

What is your name? Ada
Hello, Ada!

Run that and type your own name; the second line changes to match. Three things happen on line 1, in this order:

1. What is your name? is printed — that is the bit in the brackets, and it is called the prompt.
2. The program stops and waits. Nothing below runs yet.
3. What you typed is handed back, and = puts it in name, exactly the way lesson 1 put a 0 in score.

Leave a space at the end of the prompt, inside the quotes: "What is your name? ". Without it you type right up against the question mark, which looks like a mistake because it is one.

The prompt is optional. input() with empty brackets waits for a line without asking anything, which is nearly always worse — a program that sits there silently looks like a program that has crashed.

The answer is always text

This is the one thing in the lesson that catches everybody, so here it is going wrong:

age = input("How old are you? ")
print(age + 1)

How old are you? 12
TypeError: can only concatenate str (not "int") to str

You typed a number. You can see it on the screen. Python still says you tried to join text — because input() always hands back a string, every time, whatever you typed into it. 12 arrived as "12", and lesson 1 already showed what + does to a string.

It is worth saying the rule to yourself as a sentence: input gives me text, even when it looks like a number. The error message will not tell you that — it talks about str and int and never mentions input at all.

Turning text into a number

int(), from the end of lesson 1:

age = input("How old are you? ")
age = int(age)

print("Next year you will be", age + 1)

How old are you? 12
Next year you will be 13

Line 2 takes the text out of age, makes a number of it, and puts the number back under the same name. Once that line has run, age is a number and behaves like one.

You will usually see the two lines written as one, with input inside int:

age = int(input("How old are you? "))

print("Next year you will be", age + 1)

How old are you? 12
Next year you will be 13

Read it from the inside out: input asks and gives back text, int turns that text into a number, = stores it. Both versions are fine. Write the two-line one until the one-liner reads easily.

For a number with a decimal point, int is the wrong tool — it wants a whole number and nothing else. float() is the one that takes both:

temp = float(input("Temperature in Celsius? "))

print("In Fahrenheit:", temp * 9 / 5 + 32)

Temperature in Celsius? 21.5
In Fahrenheit: 70.7

When the answer is not a number

int() is strict, and a person typing at a keyboard is not:

age = int(input("How old are you? "))
print(age)

How old are you? twelve
ValueError: invalid literal for int() with base 10: 'twelve'

ValueError means the value was the wrong shape: int() was given a string, which is what it wants, but the string was not a number. It happens with twelve, with 12 years, with an empty answer, and — annoyingly — with 12.0, because that is not a whole number. float() would take it.

Handling that properly takes an if, which is the next lesson. For now, knowing what the message means is enough: the program did not break, the answer did not fit.

A program that asks more than once

Each input() is its own question, and they happen in the order they are written:

name = input("Name? ")
coins = int(input("How many coins? "))
value = int(input("Worth each? "))

total = coins * value

print(name + " has " + str(total) + " gold")

Name? Ada
How many coins? 3
Worth each? 25
Ada has 75 gold

Both of the last lesson's habits are in there: int() on the way in, because coins * value needs numbers, and str() on the way out, because " has " + total needs text. Getting those two the right way round is most of what input work is.

Now put it on the screen

An answer can decide what a game window says. Ask before the window opens:

from kaypy import *

name = input("Who is playing? ")

kaypy(width=800, height=600, background=[20, 24, 36])

add([text("Good luck, " + name, size=36), pos(40, 40)])
add([text("Press Stop when you are done.", size=20), pos(40, 110)])

Press Run, answer the question, and the window opens with your name in it.

The question arrives in a pop-up box, not in the output pane. That is not a bug and it is worth knowing why: the moment kaypy() opens a window, the game takes the keyboard so that the arrow keys reach the game and not the page. A typing box in the output pane would collect nothing, so the browser asks in a dialog instead. In a plain program, with no window, you get the output pane.

So input() is a way to set a game up — a name, a difficulty, how many rounds — rather than a way to play one. Playing means reacting to keys while the game is running, and that has its own tools later on.

Watch for

input() always gives back a string. Always. Even 7.

Wrap it in int() before doing arithmetic, or float() if the answer can have a decimal point.

TypeError on a line with + nearly always means a forgotten int() above it, or a forgotten str() on the line itself.

ValueError means the typing did not fit — the answer was not a number at all.

Put a space at the end of the prompt, inside the quotes.

The program stops at every input(). If a program of yours seems to have frozen, look for a question it is waiting for an answer to.

Classwork 1 — The Greeter

Start here and press Run:

name = input("What is your name? ")

print("Hello, " + name)

What is your name? Ada
Hello, Ada

Then, running after each step:

1. Ask a second question — a favourite colour — and print both answers in one sentence.
2. Ask for a number of years and store it with int() around the input. Print how old they will be in ten years.
3. Print the name, the colour and the age on three separate lines, with a label on each: Name: Ada, and so on.
4. Run it again and answer ten to the number question. Read the error, and find the word in it that says what kind of thing went wrong.

Stretch: ask for a first name and a last name separately, then print them joined with a space between. Do it twice — once with + and once with commas in print — and see which gives you the spacing you wanted without thinking about it.

Classwork 2 — The Shop

A till that works out a bill.

item = input("What are you buying? ")
how_many = int(input("How many? "))

print("You bought", how_many, item)

What are you buying? sword
How many? 2
You bought 2 sword

1. Ask for the price of one, and store it with float() so that 4.5 works.
2. Work out the total in a variable of its own and print it with a label.
3. Add a delivery charge of 2.50 to the total, and print the two numbers on separate lines — the goods, then the bill.
4. Try answering two to the "how many" question, then 2. Both errors are in this lesson.

Stretch: ask whether the order is urgent and print the answer back. You cannot make the program act on it yet — that needs if, which is lesson 3 — but you can store it and show it.

Check yourself

You run this and type 5, then 3. What does it print?

first = input("First number? ")
second = input("Second number? ")

print(first + second)

First number? 5
Second number? 3
53

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