Lesson 04 · Basics

Input and output

A program talks to you through the terminal: print() sends output out, input() brings what you type back in. Step through a five-line program, age.py, that asks your name and age, turns the age into a number with int(), and prints a greeting and an f-string. Then see the classic mistake: forgetting that input() always gives back a string.

12 min9 steps5 questionsBeginner

Fig 4 · Interactive

Loading figure…

or ← →

Check yourself

5 quick questions

Pick an answer to see why. Your score saves with your progress.

01A program runs n = input("How many? ") and you type 3. What is in n?

input() always returns what you typed as a string, even digits. int(n) would give the number 3.

02text holds '15'. What happens when Python runs text + 1 ?

+ cannot join a string and a number, so Python raises TypeError: can only concatenate str (not "int") to str. Write int(text) + 1 instead.

03What does print("a", "b", sep="-") show?

sep is what print() puts between items. It is one space by default; here it is a dash, so the output is a-b.

04age is 15. What does print(f"In a year: {age + 1}") show?

In an f-string Python works out the code inside the braces, age + 1 = 16, and puts the result into the text.

05Which line turns the answer '1.5' into a number you can do sums with?

float() reads decimals. int('1.5') raises ValueError, because '1.5' is not a whole number.

Key terms

Glossary

Output: What a program shows you. print() writes output to the terminal (the standard output stream, sys.stdout).
Input: What you give a program while it runs. input() reads one line you type (from the standard input stream, sys.stdin).
input(): A built-in function that writes an optional prompt, waits for a line and returns it as a string, without the newline.
Prompt: The question input() writes before it waits, such as "Name? ". It is written exactly as given, with no newline.
int() and float(): Built-in functions that turn text such as '15' or '1.5' into a whole number or a decimal number.
print(): A built-in function that writes its items separated by sep (a space) and followed by end (a newline).
f-string: A string with an f before the quotes; code inside {braces} is worked out and its value put into the text.
TypeError: The error Python raises when an operation gets a value of the wrong type, such as adding a number to a string.

Fine print

Notes and sources

On macOS and Linux the command is often python3 rather than python; on Windows the py launcher also works. The $ is the terminal's own prompt: you do not type it.
input() reads one line. The Enter you press ends that line; input() removes the newline it adds and returns the rest, so the answer never contains it.
int() ignores spaces around the digits, so int(" 15 ") is 15, but text that is not a whole number, such as "1.5" or "fifteen", raises ValueError. float() reads decimals.
The figure shortens the error: the real traceback also names the file, the line and the code, and the terminal wraps the long last line.
Behind the scenes print() writes to sys.stdout and input() reads from sys.stdin: the standard output and standard input streams of the program.

Quiz Unlocks after the last step.