07. Hello, World

The traditional first program in any language prints Hello, world! and exits. It is tiny, but writing it proves the language is installed, the editor is set up, and the terminal can run files. This chapter writes it, breaks it to see an error, then fixes it.

The whole program

Open exercises/07/01-hello.py in VS Code. The file contains one line:

print("Hello, world!")

In the VS Code terminal, run it:

python exercises/07/01-hello.py

The terminal prints:

Hello, world!

That is the whole program. The rest of this chapter explains each part of that line.

What print does

print is a function — a named piece of code that does a job when you ask for it. The job of print is to write whatever you give it to the terminal, followed by a new line.

The parentheses ( and ) after the name are how you ask the function to run. Anything between them is what you give it.

print("Hello, world!")
#     ^^^^^^^^^^^^^^^
#     this is what we are giving to print

In Python, the things you give a function are called arguments. The program above gives print one: the string "Hello, world!".

Strings

The text "Hello, world!" is a string — a sequence of characters. In Python, strings are written between double quotes " or single quotes '. Both work:

print("Hello, world!")
print('Hello, world!')

The quotes are not part of the string. They mark where the string starts and where it ends. When the string prints, only what is between the quotes appears.

Printing more than one thing

print can take more than one argument. Separate them with commas:

print("Hello,", "world!")

The terminal prints Hello,, a space, then world!, all on one line. print puts a space between each argument and a new line at the end.

For a different separator, build one string yourself with +, which sticks strings together:

print("Hello, " + "world!")

Now the output is Hello, world! with one space between the words, because the space is inside the first string.

A cleaner way to combine text and values is an f-string. Put f before the opening quote and use {} curly braces to drop a value in:

name = "world"
print(f"Hello, {name}!")

Output:

Hello, world!

Comments

A comment is text Python ignores when it runs the program. Comments are for the humans reading the code. A single-line comment starts with #:

# This line is a comment. Python does not run it.
print("Hello, world!")  # A comment can also sit at the end of a line.

Python has no special multi-line comment syntax. To write a longer block of notes, just use several # lines in a row:

# Anything in here is ignored.
# Useful for long explanations
# or for temporarily turning off
# a block of code.
print("Still runs.")

Use comments to explain why the code does something, not what each line does. Good code already says what.

Making and reading errors

Now break the program on purpose. In 01-hello.py, remove the last quote so the line reads:

print("Hello, world!)

Save and run it. Python refuses to run the file and prints an error like this:

  File "exercises/07/01-hello.py", line 1
    print("Hello, world!)
          ^
SyntaxError: unterminated string literal (detected at line 1)

Read errors in this order:

1. The file name and line number (line 1). This is where the problem is. Look here first.
2. The error type (SyntaxError). This is the category of problem.
3. The short message (unterminated string literal). This is what went wrong. "Unterminated string literal" means Python saw the opening quote and started a string but never found the closing one.

Errors are not punishments. They tell you exactly which line to look at and hint at what is wrong.

A note on whitespace and case

Python does care about indentation (the spaces at the start of a line). You will learn why in a later chapter. For now, do not add spaces at the start of any line unless the book tells you to.

Python does care about capital letters. print is a different word from Print and PRINT. The built-in name is lower case.

Homework

The homework files are in exercises/07/homework/. Open each starter in VS Code, read the comment at the top, and finish the file so it runs and produces the expected output.

"The best way to predict the future is to invent it."
                                            — Alan Kay

All good now.

Stuck or finished? Open the homework solutions page for a walkthrough.