Run to Learn Python: see how Python works, line by line

0coins

New here? Start with this 30-second guide

Programming means giving a computer a list of instructions, one line at a time. Python is a popular language for writing them. Every program has to remember things (numbers, words) while it runs, and that remembering happens in the computer’s memory.

You cannot normally see memory. This app lets you watch it. For each lesson: 1. read “In plain words”, 2. look at the next line and predict what will happen, 3. press Next line and check yourself. Win each lesson’s challenge to earn coins, then spend coins to unlock the next lesson. No typing needed, and you cannot break anything.

Already code for a living? Open “Going deeper” under each lesson for how CPython really behaves.

In plain words

Going deeper (for experienced programmers)

Tip: press the right arrow key to run the next line.

Console

How to learn with this app

A computer runs your code one line at a time. This app slows that down and shows you what each line does inside the computer’s memory, so you can see Python instead of guessing.

The learning loop

Pick a lesson on the left and read the idea at the top.
Press Run. The memory picture appears, still empty.
Guess first. Look at the next line of code and ask yourself what will change.
Press Next line. That line runs and lights up in the code.
Check your guess. Read the explanation, the “What changed in memory” list, and the Console. Then look at the picture.
Keep going line by line, or press Execute all to watch it play. Use Speed to slow it down, and Pause to stop.
Take the challenge that appears at the end. Pick one of two code options. It runs the same way. Win and you earn coins; lose and you must replay the whole lesson before trying again.
Reset to replay any time, then try the next lesson.

The one big idea

A variable is never a box holding a value. It is a name pointing at an object. Everything in the picture follows from that.

The three areas

Global frame: Your variable names live here. Each name is a dot with an arrow to the thing it points at. Changing a variable moves the arrow, nothing else.
Heap: Objects that are not small whole numbers: decimals, text and bigger numbers. Arrows from names end here.
Small ints: Python keeps one shared copy of every whole number from −5 to 256. Two names holding 5 point to the exact same object.

Reading a box

Type: The small word at the top left: int (whole number), float (decimal) or str (text).
Address: The 0x number at the top right is where the object sits in memory. Same address means the same object.
refs: How many names point at this object right now.
refs 0: Nothing points at it any more. A heap object disappears, because Python’s garbage collector cleans it up. A small int just fades and says cached, unused, because Python keeps it for later.

Colours and extras

Dot and arrow: A pointer. Follow the arrow from a name to its object.
Orange flash: Whatever changed on the line you just ran.
Highlighted line: The line of code that just ran.
Console: What the program prints, exactly as a real Python would.
Coins: You start with 0. Winning a challenge earns coins. A wrong answer loses 5 (never below 0) and sends you back to replay the lesson. Spend coins to unlock lessons marked with a lock in the sidebar.

This lesson is locked

New here? Start with this 30-second guide

In plain words

You finished the course

Where to go next

Challenge