Setup and your first program

A computer does exactly what it is told, one instruction at a time, and nothing more. A program is a list of instructions written for a computer, and a programming language is a language with strict rules for writing those instructions. This course teaches the programming language Python, one of the most popular languages in the world because its programs are short and read almost like English.

Your computer does not understand Python on its own. It needs a program called the Python interpreter, which reads Python instructions and carries them out. “Installing Python” means installing the interpreter. Today you will also install a code editor, an app for writing program files. Any plain text editor works, but a code editor colors the parts of your code and catches some mistakes as you type.

Install the tools

1. Go to python.org, open Downloads, and download the newest Python 3 for your computer (Windows or macOS). Any version 3.12 or newer works for this course. On Linux, Python is usually installed already.
2. Run the installer. On Windows, if the installer shows a checkbox to add Python to PATH, tick it; that lets the terminal find Python.
3. Install VS Code from code.visualstudio.com, open it, and install the Python extension from the Extensions panel.

Check that it worked

To talk to the interpreter you use a terminal: a window where you type a command, press Enter, and read the reply as text. Every computer has one (Terminal on macOS and Linux, PowerShell or Terminal on Windows), and VS Code has one built in: choose Terminal → New Terminal.

Type this command into the terminal and press Enter:

python --version

The command has two parts: python names the program to run, and --version asks it only to report which version it is. If Python is installed, the terminal answers with one line that starts with Python 3. followed by the rest of the version number. Version numbers change often, so yours will not match anyone else’s exactly; what matters is that it starts with Python 3 and is 3.12 or newer.

Now vary one thing: the computer. On macOS and Linux the command is often python3 instead of python, and on Windows py sometimes works when python does not. Each is the same interpreter under a different name:

python3 --version
py --version

Use whichever one answers with Python 3, and use that same word everywhere this course says python.

You have Python installed. What is the fastest way to try one small instruction and see what Python does with it?

Python has an interactive mode for exactly this. Type python (with nothing after it) in the terminal and press Enter. The terminal changes: a few lines about the version appear, then a new prompt, >>>. That prompt means “Python is listening; type one instruction.” This mode is called the REPL, short for Read, Evaluate, Print, Loop: Python reads what you type, evaluates it (works out its value), prints the result, and loops back to >>> for the next one.

The simplest thing to type is an expression: a piece of code that Python can work out to a value, such as 2 + 3. Python uses + to add, - to subtract, * (an asterisk) to multiply, and / to divide.

>>> 2 + 3
5
>>> 10 - 4
6
>>> 6 * 7
42
>>> 7 / 2
3.5
>>> 8 / 2
4.0

Now vary one thing: the order of the operations. Python follows the same order of operations as math class: multiplication and division happen before addition and subtraction, and parentheses go first.

>>> 2 + 3 * 4
14
>>> (2 + 3) * 4
20

In 2 + 3 * 4, Python multiplies 3 * 4 first to get 12, then adds 2 to get 14. In (2 + 3) * 4, the parentheses make Python add first to get 5, then multiply by 4 to get 20. The numbers are the same; only the order changed, and so did the answer.

To leave the REPL, type exit() and press Enter. The >>> prompt disappears and you are back at the ordinary terminal.

>>> python --version
Traceback (most recent call last):
  ...
NameError: name 'python' is not defined

>>> 9 - 2 * 3
>>> (9 - 2) * 3
>>> 9 / 3

>>> 9 - 2 * 3
3
>>> (9 - 2) * 3
21
>>> 9 / 3
3.0

The REPL forgets everything when you close it. Real programs need to be saved, run again tomorrow, and shared with a friend. For that you write the instructions into a file.

A script is a file of Python code that the interpreter runs from the first line to the last. Python script files end in .py. You write the file in your editor, save it, and then ask the interpreter to run it from the terminal.

print("Hello, world!")
print("I am learning Python.")

python hello.py

Hello, world!
I am learning Python.

Scripts and the REPL do not behave the same in one important way. Vary one thing: put a bare expression in a script, as you would in the REPL.

print("Adding 2 and 3:")
2 + 3

Adding 2 and 3:

Line 2 is never shown. The interpreter still works out 2 + 3, but a script only shows what you explicitly tell it to show; the automatic “print the value” step belongs to the REPL. To see the answer in a script, ask for it with print:

print("Adding 2 and 3:")
print(2 + 3)

Adding 2 and 3:
5

print("Start")
4 * 5
print(4 * 5)

Start
20

Scripts only show what you print, so print is the most important tool of the day. It is worth knowing exactly how it treats text and numbers.

print is a function: a named instruction that does one job when you use it. Using a function is called calling it, and you call a function by writing its name followed by parentheses: print(...). The value inside the parentheses is the argument, the thing you hand to the function to work on. print displays its argument in the terminal and then moves to a new line.

Text in Python goes inside double quotes, like "Hello". A piece of text in quotes is called a string. (Single quotes work too; Day 3 covers strings in depth.) Numbers are written without quotes.

The quotes change everything. Python treats whatever is inside quotes as plain text to copy exactly, and it works out anything outside quotes as an expression.

print("Total cost:")
print(4 + 5)
print("4 + 5")

Total cost:
9
4 + 5

Now vary the number of arguments. print can take several, separated by commas; it prints them on one line with one space between each. And print() with nothing inside prints an empty line.

print("Tickets:", 3 * 12)
print()
print("Thanks for visiting!")

Tickets: 36

Thanks for visiting!

Line 1 has two arguments: the string "Tickets:" and the expression 3 * 12. Python works out 36, then prints the text, a space, and the number. Line 2 prints a blank line, and line 3 prints the last string.

print("Price:", 2 * 5, "dollars")

Price: 10 dollars

Your scripts so far have had a few lines each, and their output appeared in the same order as the lines. That is not a coincidence, and it is the rule every Python program follows.

Each line of instructions in a program is a statement: one complete instruction, such as one call to print. When you run a script, the interpreter starts at line 1 and carries out the statements one at a time, from top to bottom. It finishes each statement before starting the next, and it never skips ahead or rearranges them to make more sense.

print("Wake up")
print("Eat breakfast")
print("Walk to school")

Wake up
Eat breakfast
Walk to school

Now change one thing: move the last line to the top. The statements are the same, but the order is not:

print("Walk to school")
print("Wake up")
print("Eat breakfast")

Walk to school
Wake up
Eat breakfast

Python ran the lines in exactly the order you wrote them, even though walking to school before waking up makes no sense. Python cannot know what you meant; it follows the order on the page.

print(3)
print(1)
print(2)
print("Liftoff!")

print(3)
print(2)
print(1)
print("Liftoff!")

3
2
1
Liftoff!

Next week you will open today’s files and wonder what they were for. Programmers solve this by leaving notes in the code itself, written for people, not for Python.

A comment starts with # (the hash sign) and runs to the end of that line. The interpreter ignores everything from the # onward. A comment can fill a whole line, or follow code on the same line, where the code before the # still runs.

# Print a shopping receipt
print("Apples:", 3 * 2)
print("Bread:", 4)  # one loaf
# print("Milk:", 2)
print("Done")

Apples: 6
Bread: 4
Done

Now vary where the # sits: inside quotes, it is just a character in the text, not the start of a comment.

print("Room #12")  # the # before 12 is part of the text

Room #12

The first # is inside the string, so it prints. The second # is outside the quotes, so the rest of the line is a comment.

# Team scores
print("Red team:", 10 + 5)
# print("Blue team:", 8)
print("Green team:", 7)  # print("Gold team:", 9)

Red team: 15
Green team: 7

So far, every line you have seen starts at the very left edge. That is not just tidiness. In most writing, extra spaces at the start of a line are harmless. In Python they carry meaning.

Every language has rules for how sentences must be built; “dog the ran” breaks English grammar. The rules for how code must be written are the language’s syntax. Indentation means the spaces at the start of a line, and in Python indentation is part of the syntax: it shows which lines belong together in a group. You will build those groups on Day 4. Until then, every statement belongs at the left edge, so any spaces before a statement break the rules.

There is one more fact you need: before running anything, the interpreter reads the whole file to check its syntax. If it finds a broken rule anywhere, it reports the error and runs nothing, not even the lines above the mistake.

print("Line one")
    print("Line two")

  File ".../day01/unexpected_indent.py", line 2
    print("Line two")
IndentationError: unexpected indent

Now vary where the extra spaces go. Blank lines and spaces inside a line do not count as indentation:

print("Start")

print(2+3)
print(2   +   3)

Start
5
5

The blank line is skipped, and 2+3 and 2 + 3 mean the same as 2 + 3. Only spaces at the start of a line are part of the syntax. (Most Python programmers write one space on each side of +, as in 2 + 3, because it is easiest to read.)

print("Welcome")
print("Today we learn Python")
  print("Let's begin")
print("Goodbye")

Everyone who writes code makes mistakes, every day. The difference between a beginner who gets stuck and one who keeps going is reading the error message instead of panicking. Python’s messages tell you where to look and what is wrong.

An error message is the report the interpreter prints when it cannot continue. A SyntaxError is the general name for a broken syntax rule: a missing parenthesis, a missing quote, a missing comma. The IndentationError from 1.7 is one special kind of syntax error. Like it, every SyntaxError is found before the program runs, so nothing prints.

print("Hello")
print("World"
print("Bye")

  File ".../day01/never_closed.py", line 2
    print("World"
         ^
SyntaxError: '(' was never closed

Now vary the mistake: leave off a closing quote instead.

print("Hello)
print("Bye")

  File ".../day01/unclosed_quote.py", line 1
    print("Hello)
          ^
SyntaxError: unterminated string literal (detected at line 1)

A string literal is a string typed directly into the code, like "Hello". Unterminated means it was never ended: the opening " has no closing " on that line, so Python thinks the ) is part of the text. The caret points at the quote that was opened. The fix is print("Hello").

Older versions of Python may word these messages differently, but they always give the line number and the kind of error.

print("Quiz time!")
print("Question 1)
print("Good luck!")

SyntaxError: unterminated string literal (detected at line 2)

print("Quiz time!")
print("Question 1")
print("Good luck!")

Quiz time!
Question 1
Good luck!

Some mistakes follow every syntax rule, so Python happily starts running the program, and only discovers the problem when it reaches the broken line.

Python knows a set of built-in names, such as print. When a line uses a name Python does not know, it stops at that line with a NameError. Because the file’s syntax was fine, the program really did start, so every line before the mistake has already run.

print("Ready")
pront("Go")
print("Done")

Ready
Traceback (most recent call last):
  File ".../day01/misspelled_print.py", line 2, in <module>
    pront("Go")
    ^^^^^
NameError: name 'pront' is not defined. Did you mean: 'print'?

Now vary the mistake: spell print correctly but forget the quotes around a word.

print("Ready")
print(Hello)

Ready
Traceback (most recent call last):
  File ".../day01/missing_quotes.py", line 2, in <module>
    print(Hello)
          ^^^^^
NameError: name 'Hello' is not defined

Without quotes, Hello is not text; Python treats it as a name and looks it up. It knows no name Hello, so it stops. The fix is print("Hello"). Python is also case-sensitive, meaning capital and small letters count as different: Print is a different name from print, and causes the same kind of error.

As with syntax errors, older versions of Python may word these messages a little differently, and some print no “Did you mean” suggestion.

print("Step 1")
print("Step 2:", 5 * 2)
print(Finished)
print("Step 3")

Step 1
Step 2: 10
Traceback (most recent call last):
  ...
NameError: name 'Finished' is not defined

Maya is making a sign for her school’s bake sale. Follow her through three runs; each stage uses a different part of today’s lesson.

Attempt 1 (1.6, 1.8). Maya starts with a comment saying what the program is for, then the sign’s lines:

# Bake sale sign
print("BAKE SALE TODAY")
print("Muffins left:", 30 - 12
prnt("Cookies left:", 48 - 20)
print("All money goes to the library")

  File ".../day01/bake_sale_attempt1.py", line 3
    print("Muffins left:", 30 - 12
         ^
SyntaxError: '(' was never closed

Nothing prints, not even BAKE SALE TODAY, because a syntax error stops the whole file before it runs. The caret points at line 3’s (, which has no ). Maya adds the ).

Attempt 2 (1.5, 1.9). Now the syntax is fine, so the program starts running from the top:

# Bake sale sign
print("BAKE SALE TODAY")
print("Muffins left:", 30 - 12)
prnt("Cookies left:", 48 - 20)
print("All money goes to the library")

BAKE SALE TODAY
Muffins left: 18
Traceback (most recent call last):
  File ".../day01/bake_sale_attempt2.py", line 4, in <module>
    prnt("Cookies left:", 48 - 20)
    ^^^^
NameError: name 'prnt' is not defined. Did you mean: 'print'?

Lines 2 and 3 ran (line 1 is a comment), then line 4 stopped the program. Reading the message from the bottom up, Maya sees the misspelling and Python’s suggestion.

Attempt 3 (1.4). With print spelled correctly, every line runs:

# Bake sale sign
print("BAKE SALE TODAY")
print("Muffins left:", 30 - 12)
print("Cookies left:", 48 - 20)
print("All money goes to the library")

BAKE SALE TODAY
Muffins left: 18
Cookies left: 28
All money goes to the library

• The Python interpreter runs Python code. python --version in a terminal checks that it is installed.
• The REPL (>>>) shows the value of each expression at once; a script is a saved .py file run with python name.py, and it shows only what you print.
• print displays its arguments separated by spaces; text in quotes is copied exactly, and expressions without quotes are worked out first. Comments start with # and are ignored.
• Statements run one at a time, top to bottom, in the order written. In Python, indentation is part of the syntax, so for now every statement starts at the left edge.
• A SyntaxError (including an IndentationError) is found before anything runs, so nothing prints. A NameError happens when Python reaches an unknown name, after earlier lines have run. Read error messages from the bottom up.

Do these in your day01 folder. Type each program yourself, run it, and only then open the answer.

print("Jordan Lee")
print("27 Willow Street")
print("Apartment 3")
print("Brookfield")

Jordan Lee
27 Willow Street
Apartment 3
Brookfield

>>> 12 + 30 + 7
49
>>> 60 * 24
1440
>>> 100 - 37
63
>>> 45 / 9
5.0
>>> (8 + 6 + 10) / 3
8.0

print("Jordan Lee")
print("27 Willow Street)
print("Apartment 3")
print("Brookfield")

SyntaxError: unterminated string literal (detected at line 2)

print("Jordan Lee")
print("27 Willow Street")
Print("Apartment 3")
print("Brookfield")

Jordan Lee
27 Willow Street
Traceback (most recent call last):
  ...
NameError: name 'Print' is not defined. Did you mean: 'print'?

print("Jordan Lee")
print("27 Willow Street")
print("Apartment 3")
    print("Brookfield")

IndentationError: unexpected indent

# Movie night plan
print("Movie night")
print("Snacks:", 3 + 4)  # popcorn and juice
# print("Cancelled")
print()
print("Tickets:", 2 * 9)
print("2 * 9")

Movie night
Snacks: 7

Tickets: 18
2 * 9

print("Total:" 4 + 5)

  File ".../day01/review_missing_comma.py", line 1
    print("Total:" 4 + 5)
          ^^^^^^^^^^^^^^
SyntaxError: invalid syntax. Perhaps you forgot a comma?

print("Total:", 4 + 5)

Total: 9

print("A")
print("B")
pritn("C")
print("D")

print("A")
print("B")
print("C"
print("D")

The official Python Tutorial’s first chapters cover why Python is worth learning and how to start the interpreter and the REPL: Whetting Your Appetite and Using the Python Interpreter.

Check each statement. If the answer is “not yet,” reread the linked subsection.

• I can check which version of Python is installed from the terminal. Reread
• I can use the REPL as a calculator and leave it. Reread
• I can create a .py file and run it from the terminal. Reread
• I can predict what print shows with and without quotes. Reread
• I can trace a program from top to bottom. Reread
• I can write a comment and comment out a line. Reread
• I can explain why a stray indent stops a whole program. Reread
• I can find the mistake a SyntaxError is pointing at. Reread
• I can read a NameError and say which lines ran before it. Reread