Input and Types: Text and Numbers Are Different Things

Letting the program listen

So far your programs have known everything in advance. input() is how you let a person hand something to the program while it's running.

name = input("What's your name? ")
print(f"Hi, {name}!")

If you type Maya and press Enter, the output is:

What's your name? Maya
Hi, Maya!

input() does three things, in order:

1. Prints the prompt you gave it (the text inside the parentheses).
2. Stops the program and waits until you type something and press Enter.
3. Hands back whatever you typed, so you can put it in a box.

That middle step surprises people. Your program is genuinely frozen at that line. Nothing after it runs until Enter is pressed.

The trap that catches everybody

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

Type 18 and you get:

TypeError: can only concatenate str (not "int") to str

In plain words: you tried to add a number to a piece of text, and Python refused.

Here's the fact underneath it, and it's worth memorizing:

input() always hands back a string, no matter what the person typed.

You typed 18. Python gave you "18" — the character one followed by the character eight. Not the number 18. It looks identical on screen. It is not the same thing.

Why does that break? Because + means two different things depending on what's on either side:

print(3 + 4)
print("3" + "4")

Output:

7
34

For numbers, + adds. For strings, + glues them end to end. When Python sees text on the left and a number on the right, it has no way to know which one you wanted, so it stops instead of guessing. That's a kindness. A silently wrong answer is far worse than an error message.

Converting between types

int() turns text into a whole number:

age = int(input("How old are you? "))
print(f"Next year you turn {age + 1}")

Typing 18 gives:

How old are you? 18
Next year you turn 19

Read that first line from the inside out: input(...) runs first and produces "18", then int(...) converts it to 18, then that lands in age. Nested calls always work inside first.

The three conversions you'll use constantly:

print(int("18"))
print(float("1.5"))
print(str(18) + " years")

Output:

18
1.5
18 years

Two conversions that fail, so you recognize them later:

• int("abc") raises ValueError. The type is right — you did hand it a string — but there's no number hiding in those letters.
• int("1.5") also raises ValueError. int() won't parse a decimal point out of text. Go through float first: int(float("1.5")) gives 1, and note that it chops the decimal part off rather than rounding.

Two kinds of division

print(7 / 2)
print(7 // 2)
print(7 % 2)

Output:

3.5
3
1

• / is ordinary division, and the result always has a decimal point, even when it divides evenly. 6 / 3 gives 2.0, not 2.
• // divides and throws away the remainder. Useful for "how many whole boxes fit."
• % is the remainder itself, pronounced "modulo." Useful far more often than you'd guess — a number is even when n % 2 == 0, and it's the standard way to ask "does this divide evenly?"

Operator quick reference

print(2 + 3)
print(2 - 3)
print("ab" * 3)
print(2 ** 10)

Output:

5
-1
ababab
1024

* between a string and a number repeats the string, which is occasionally handy for drawing separator lines: print("-" * 40).

** is "to the power of." 2 ** 10 is 2 multiplied by itself 10 times.

A complete small program

weight = float(input("Weight in kilograms: "))
height = float(input("Height in meters: "))
bmi = weight / (height ** 2)
print(f"Your BMI is {bmi:.1f}")

With 70 and 1.75 typed in:

Weight in kilograms: 70
Height in meters: 1.75
Your BMI is 22.9

Two things to notice.

float instead of int, because heights have decimal points and int("1.75") would fail.

{bmi:.1f} inside the f-string is a format specifier: keep one digit after the decimal point. Without it you'd see 22.857142857142858, which is technically correct and useless to a human. :.2f gives two digits, and so on. You'll use this every time you print money.

Ten-second check

What does print("3" + "4") output?

The answer is 34. Both sides are text, so + glues rather than adds. To get 7, convert first: print(int("3") + int("4")).

Every time you read a number from a person, a file, or the internet, it arrives as text. Converting it is your job, not Python's.