Filling Spaces & Spawning Mobs

STEP 1 Warm-up: Which Tool Would You Choose?

You now have three tools for shaping the Minecraft world: the Agent, the Builder, and — new today — blocks.fill(). Each one is right for different jobs.

Look at each task below and decide which tool fits best before reading the answer.

Today's Vocabulary

STEP 2 Open MakeCode (Python Only)

You should already be in your fresh flat world from Lesson 0. If not, go back and complete Lesson 0 first.

1. In Minecraft, press C to open Code Builder.
2. Choose MakeCode.
3. Click New Project and name it Lesson 4 - Fill.
4. Click Code options, then select Python Only, then click Create.

RECAP Bridge from Lesson 3: The Same Wall, Even Shorter

In Lesson 3 you built a 10-long, 5-tall stone wall with a loop, mark(), and line(). Here's the same wall written with blocks.fill():

def on_chat():
    blocks.fill(STONE,
                pos(0, 0, 0),
                pos(10, 5, 0),
                FillOperation.REPLACE)
player.on_chat("wall", on_chat)

STEP 4 The Big New Idea: Opposite Corners

blocks.fill() does not move like the Agent or Builder. It needs two opposite corners of a rectangular box. The world fills every block between them.

def on_chat():
    blocks.fill(GLASS,
                pos(0, 0, 0),
                pos(10, 5, 10),
                FillOperation.REPLACE)
player.on_chat("solid", on_chat)

That fills a solid glass brick — 11 wide, 6 tall, 11 deep. Now change one word and look at what happens:

def on_chat():
    blocks.fill(GLASS,
                pos(0, 0, 0),
                pos(10, 5, 10),
                FillOperation.HOLLOW)
player.on_chat("room", on_chat)

Same two corners. Same block type. But HOLLOW gives you a room you can stand in instead of a solid cube.

Fill operations — pick the one you want

STEP 5 Starter Build: A Habitat

Now we'll combine two fills — a hollow glass-pane shell with a grass floor inside. This is the base for almost any habitat, arena, or scene.

def on_chat():
    # Hollow glass shell
    blocks.fill(GLASS_PANE,
                pos(5, 0, -10),
                pos(12, 7, 10),
                FillOperation.HOLLOW)

    # Grass floor inside the shell
    blocks.fill(GRASS,
                pos(6, 0, -9),
                pos(11, 0, 9),
                FillOperation.REPLACE)
player.on_chat("habitat", on_chat)

Read the coordinates before running

• The first fill goes from (5, 0, -10) to (12, 7, 10). That's 8 wide (12 - 5 + 1), 8 tall, 21 deep.
• The second fill is smaller — it starts at 6 and ends at 11 on the X axis, and -9 to 9 on Z. It sits inside the shell.
• Both fills share the same Y axis (0), so the grass becomes the floor of the habitat.

STEP 6 Add a Builder Accent

The habitat looks plain. blocks.fill() is great for big regions — whole walls, whole floors. But for a single line — say, a stone path running down the middle of the habitat — the Builder from Lesson 3 is still the right tool. Use the right tool for the size of the job.

def on_chat():
    # Walls and floor (big regions — fill is the right tool)
    blocks.fill(GLASS_PANE, pos(5, 0, -10), pos(12, 7, 10), FillOperation.HOLLOW)
    blocks.fill(GRASS,      pos(6, 0, -9),  pos(11, 0, 9),  FillOperation.REPLACE)

    # Walkway down the middle (a single line — Builder is the right tool)
    builder.teleport_to(pos(8, 1, -9))
    builder.mark()
    builder.teleport_to(pos(8, 1, 9))
    builder.line(COBBLESTONE)
player.on_chat("habitat", on_chat)

What the Builder is doing

• builder.teleport_to(pos(8, 1, -9)) — warp the Builder to the north end of the habitat, one block above the grass.
• builder.mark() — remember this position.
• builder.teleport_to(pos(8, 1, 9)) — warp to the south end.
• builder.line(COBBLESTONE) — stamp a cobblestone line between the mark and here.

STEP 7 Bring It to Life: mobs.spawn()

An empty habitat is just architecture. Add a single mob first, then turn it into a loop.

One mob, exact position

def on_chat():
    blocks.fill(GLASS_PANE, pos(5, 0, -10), pos(12, 7, 10), FillOperation.HOLLOW)
    blocks.fill(GRASS,      pos(6, 0, -9),  pos(11, 0, 9),  FillOperation.REPLACE)

    mobs.spawn(SHEEP, pos(8, 1, 0))
player.on_chat("habitat", on_chat)

The sheep appears at (8, 1, 0) — one block above the grass floor, centered between the walls. Run it. You should see exactly one sheep standing inside the habitat.

Many mobs, random positions (callback to Lesson 2's loop)

Now bring the for loop from Lesson 2 back into the program. Each iteration spawns one sheep at a random position inside the habitat.

def on_chat():
    blocks.fill(GLASS_PANE, pos(5, 0, -10), pos(12, 7, 10), FillOperation.HOLLOW)
    blocks.fill(GRASS,      pos(6, 0, -9),  pos(11, 0, 9),  FillOperation.REPLACE)

    for i in range(5):
        mobs.spawn(SHEEP, randpos(pos(6, 1, -9), pos(11, 1, 9)))
player.on_chat("habitat", on_chat)

What's happening

• range(5) — the loop runs 5 times. Same pattern as Lesson 2.
• randpos(corner1, corner2) — pick a random position inside the box between those two corners.
• The corners here match the grass floor's footprint with Y bumped up to 1 so the sheep spawn on the grass, not inside it.

STEP 8 Debugging: Two Bugs to Recognize

Two bugs come up in nearly every Lesson 4 program. Recognize them once and you'll spot them instantly from now on.

Bug 1 — Indentation (callback to Lesson 2)

This program looks right but spawns only one sheep, not five. Can you spot why?

def on_chat():
    blocks.fill(GLASS_PANE, pos(5, 0, -10), pos(12, 7, 10), FillOperation.HOLLOW)
    blocks.fill(GRASS,      pos(6, 0, -9),  pos(11, 0, 9),  FillOperation.REPLACE)

    for i in range(5):
    mobs.spawn(SHEEP, randpos(pos(6, 1, -9), pos(11, 1, 9)))
player.on_chat("broken", on_chat)

Bug 2 — Wrong Y value (callback to Lesson 3)

This one runs without errors — but the sheep fall from the sky every time. Can you see why?

def on_chat():
    blocks.fill(GLASS_PANE, pos(5, 0, -10), pos(12, 7, 10), FillOperation.HOLLOW)
    blocks.fill(GRASS,      pos(6, 0, -9),  pos(11, 0, 9),  FillOperation.REPLACE)

    for i in range(5):
        mobs.spawn(SHEEP, randpos(pos(6, 10, -9), pos(11, 10, 9)))
player.on_chat("falling", on_chat)

CHALLENGE Build Your Own Scene

Create an original program that uses blocks.fill() and mobs.spawn() to build a habitat, arena, pond, aquarium, or scene of your own design. Pick one of the options below or invent your own.

Option C full program — the Trident Aquarium

def trident_summon_aquarium():
    blocks.fill(GLASS, pos(5, 0, -10), pos(12, 7, 10), FillOperation.REPLACE)
    blocks.fill(WATER, pos(6, 1, -9),  pos(11, 7, 9),  FillOperation.REPLACE)
    for i in range(4):
        mobs.spawn(TROPICAL_FISH, randpos(pos(6, 1, -9), pos(11, 6, 9)))
        blocks.place(BUBBLE_CORAL, randpos(pos(6, 1, -9), pos(11, 6, 9)))
        mobs.spawn(PUFFERFISH,    randpos(pos(6, 1, -9), pos(11, 6, 9)))
        mobs.spawn(DOLPHIN,       randpos(pos(6, 1, -9), pos(11, 6, 9)))

        # blocks.place(SEAGRASS, randpos(pos(6, 1, -9), pos(11, 1, 9)))
        # Doesn't work because seagrass can't be placed on glass.
        # Fix it later by filling a dirt layer at the bottom first.

player.on_item_interacted(TRIDENT, trident_summon_aquarium)

What's different about this program

• Solid then carved: the first fill makes a solid glass cube with REPLACE. The second fill replaces the inside with water. Result: glass shell with water inside — a different way to get the same effect as HOLLOW.
• Three different mobs in one loop: each iteration spawns a fish, a pufferfish, and a dolphin — so range(4) gives you 4 of each, 12 total.
• blocks.place() for single blocks: bubble coral is placed one block at a time inside the loop. Use place for one-offs; use fill for regions.
• The commented bug: the seagrass line is intentionally commented out — seagrass needs dirt under it. Real programs include notes like this to remember what to fix later.

How to actually summon the aquarium (Trident 101)

The program only runs when you use a trident — so first you need one in your hand. If you've never opened a Minecraft inventory before, follow these steps exactly:

1. Run your code in MakeCode (green play button). Nothing will happen yet — the program is now waiting for you to use a trident.
2. Switch back to the Minecraft world.
3. Press the E key. This opens your inventory.
4. In the search bar at the top, type trident. A trident icon appears.
5. Click and drag the trident into one of the squares along the bottom row — that's your hotbar, the items you can hold.
6. Press Esc (or E again) to close the inventory.
7. Scroll your mouse wheel until the trident is selected in your hotbar (or press the number key matching its slot — 1 through 9). You should now be holding the trident.
8. Right-click the mouse to use the trident. The aquarium appears!

Right-click does nothing? Make sure: (1) the trident is actually selected in your hotbar (the slot has a white border), (2) you ran the MakeCode program first, and (3) you're aiming at empty air or a block in front of you, not at the sky or out of bounds.

Minimum requirements

• Your program starts with an event — either a chat command (player.on_chat) or an item interaction (player.on_item_interacted).
• Uses blocks.fill() at least twice.
• Uses at least two different block types.
• Uses mobs.spawn() at least once.
• Uses a for loop to spawn multiple mobs.
• Uses randpos(...) or carefully chosen coordinates so the mobs end up inside the structure.
• Final result is a habitat, arena, pond, aquarium, or other enclosed scene.

Stretch goals

• Use a different mob type per loop iteration with an if statement.
• Add a second floor or interior wall with a third blocks.fill().
• Use FillOperation.OUTLINE for the walls and decorate the inside separately.
• Add an opening (a door or window) by running another fill of AIR in a small region.
• Add # comments above each section to explain what it does.

When You're Done

• Trade computers and run a classmate's program. Did their habitat surprise you?
• Look up FillOperation.KEEP — how is it different from REPLACE?
• Try chaining three or four habitats side-by-side using one chat command and a loop around the fills.
• Re-write your Lesson 3 wall using a single blocks.fill(). How short can the program get?