Why is a string printed although print isn't used?

Question

This is an example program from the Python book that I am using to learn. What does the message += line signify? And how does it print that statement even though we've never used a print there?

This works as expected and returns $number is too high or $number is too low . But how does it do so even without print statement?

And what does message += str(guess) +.... do here, given that we've declared message as an empty string in the beginning ( message = "" )?

import random # We cover random numbers in the
rng = random.Random() 

number = rng.randrange(1, 1000) 

guesses = 0
message = ""

while True:
    guess = int(input(message + "\nGuess my number between 1 and 1000:")) 
    guesses += 1
    if guess > number:
        message += str(guess) + " is too high.\n" 
    elif guess < number:
        message += str(guess) + " is too low.\n" 
    else:
        break
input("\n\nGreat, you got it in "+str(guesses)+" guesses!\n\n")

I tried using message like in the above program in one of my other scripts and it doesn't print statement like it does in the above program.

Answer 1

The call to input() includes message in its prompt.

Addition of strings concatenates them in Python. "ab" + "cd" produces "abcd" . Similarly, message += "stuff" adds "stuff" to the end of the variable's earlier value. If it was the empty string before, "" + "stuff" produces simply "stuff" .

The operator += is an increment assignment; a += b is short for a = a + b . (Some people also dislike the longer expression because it is mathematically implausible unless at least one of the values is zero; but = does not have its mathematical semantics anywhere else in Python, either.)