Input and Output

Reading from the keyboard with input(), writing with print(), and turning text into numbers.

Standard input and standard output

Every program the operating system starts gets three standard channels, called streams, already opened for it. Standard input (stdin) is where the program reads text from — by default the keyboard. Standard output (stdout) is where it writes its normal results — by default the terminal window. A third stream, standard error (stderr), is meant for error messages and warnings, so they stay visible even when the normal output goes elsewhere.

In Python, input() reads from standard input and print() writes to standard output. The streams are also available as objects in the sys module: sys.stdin, sys.stdout and sys.stderr. 

import sys

sys.stdout.write("Written to stdout directly\n")
print("Written by print()")
Written to stdout directly
Written by print()

Redirection

Because the program only sees "a stream", the shell can connect it to something other than the keyboard and screen, without any change to the code. < feeds a file into standard input, > sends standard output to a file, and | connects the output of one program to the input of the next: 

python3 greet.py < names.txt      # read input from a file
python3 report.py > report.txt     # save output in a file
echo "Ada" | python3 greet.py        # pipe another program's output in

Writing output with print()

print() accepts any number of arguments of any type. It converts each one to text (with str()), writes them with a space in between, and ends with a new line. With no arguments it prints just an empty line. 

city = "Nairobi"
print("Temperature in", city, "is", 24.5, "degrees")
print(3, True, None)
print()
print("After an empty line")
Temperature in Nairobi is 24.5 degrees
3 True None

After an empty line

The sep and end parameters

Two keyword arguments change what print() puts between and after the values. sep is the separator (default " "); end is written at the very end (default "\n", a new line). Setting end="" lets the next print() continue on the same line. 

print("2026", "09", "30", sep="-")
print("a", "b", "c", sep="")
print("Loading", end="")
print("...", end="")
print(" done")
print("x", "y", sep=", ", end=".\n")
2026-09-30
abc
Loading... done
x, y.

Printing to standard error

The file parameter chooses the stream. Warnings and error messages belong on standard error, so that they do not end up in a file when the output is redirected: 

import sys

print("Warning: disk almost full", file=sys.stderr)

Reading input with input()

input(prompt) writes the prompt to standard output, waits until the user types a line and presses Enter, and returns that line without the new line at the end. The program pauses while it waits. 

name = input("What is your name? ")
print(f"Hello, {name}!")
What is your name? Ada
Hello, Ada!

In the output above, Ada is what the user typed; a terminal shows it right after the prompt. input() adds nothing to the prompt, so end it with a space, or the answer will be glued to it.

input() always returns a string

Whatever the user types — a name, a number, nothing at all — input() returns a str. An empty line gives the empty string "". So arithmetic on input fails, or worse, silently does something else: 

age = input("Age: ")
print(type(age))
print(age * 2)      # string repetition, not multiplication!
print(age + 1)
Age: 36
<class 'str'>
3636
TypeError: can only concatenate str (not "int") to str

Converting text to numbers

Type conversion (also called casting) creates a value of one type from a value of another. You do it explicitly by calling the type: int(), float(), str(), bool(). The usual pattern is to wrap input() directly: 

age = int(input("Age: "))
height = float(input("Height in meters: "))
print("Next year you will be", age + 1)
print("You are", str(round(height * 100)) + " cm tall")
Age: 36
Height in meters: 1.75
Next year you will be 37
You are 175 cm tall

A few details are worth knowing. int() and float() ignore spaces around the number and accept a sign. int() also takes a base as its second argument. But int() does not accept a decimal point: convert to float first if you need that. 

print(int(" 42 "), int("-7"))
print(float("2.5"), float("1e3"))
print(int("ff", 16), int("101", 2))
print(int(float("3.5")))
42 -7
2.5 1000.0
255 5
3

Python also converts some types implicitly: in 2 + 0.5 the integer is turned into a float automatically. It never does that between numbers and strings, which is why the conversion above is always needed.

When the input is not a number

If the text cannot be converted, int() and float() raise a ValueError, and the program stops: 

quantity = int(input("Quantity: "))
print("Ordered", quantity)
Quantity: twelve
ValueError: invalid literal for int() with base 10: 'twelve'

To keep the program running, catch the error with try and except. The details are on the page about error handling; the basic shape is: 

text = input("Quantity: ")
try:
    quantity = int(text)
    print("Ordered", quantity)
except ValueError:
    print(f"'{text}' is not a whole number.")
Quantity: twelve
'twelve' is not a whole number.

Several values on one line

input() reads one whole line. To let the user type several values on it, split the string, then convert each part. 

line = input("Width and height: ")
width, height = line.split()
area = int(width) * int(height)
print("Area:", area)
Width and height: 3 4
Area: 12

If there is no more input at all — the input file has ended, or the user pressed Ctrl+D — input() raises an EOFError.

Formatting output

Calculations often produce numbers with more digits than you want to show: 2.95 * 3 is 8.850000000000001. Format the number when you print it. An f-string with a format specifier changes only how the value is displayed; round() actually creates a new, rounded number. 

item = input("Item: ")
price = float(input("Price: "))
quantity = int(input("Quantity: "))

total = price * quantity
print(total)
print(f"{quantity} x {item:<10} {total:>8.2f}")
print(f"Total with 21% VAT: {total * 1.21:,.2f}")
Item: Notebook
Price: 2.95
Quantity: 3
8.850000000000001
3 x Notebook       8.85
Total with 21% VAT: 10.71

The format specifiers for width, alignment, decimals and thousands separators are explained under f-strings on the Strings page.