Chapter 10a - Space Invaders

You will build a text Space Invaders: a 2D grid printed each turn. That is the version that runs in this site’s Python sandbox.

Why not pygame?

Piston runs Python without a window. There is no display, mouse, or speaker.

Approach	Works here?	Notes
pygame, turtle window, OpenGL	No	Need a real screen / framebuffer
Pillow drawing PNGs	Possible later	Would need extra packages and a page that shows images
ASCII grid + print	Yes	Same idea as arcade: a 2D board, a player, aliens, a game loop

So the “rendering library” is a handful of functions that fill a list-of-lists and print it. You already have lists and functions from chapters 8 and 9.

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A tiny draw library

A cell is one character. A frame is a rectangle of cells.

EMPTY = "."
PLAYER = "A"
ALIEN = "M"
SHOT = "|"

def make_board(rows, cols):
    return [[EMPTY] * cols for _ in range(rows)]

def put(board, row, col, ch):
    if 0 <= row < len(board) and 0 <= col < len(board[0]):
        board[row][col] = ch

def draw(board):
    for row in board:
        print("".join(row))

🧪 Draw one frame

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Mini quiz — frames and loops

Quiz

Frames and loops

1 / 2

Why can pygame not open a window in this sandbox?

Pick an answer to see if you are right.

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Game loop (one command per frame)

Each line of input is a command: L move left, R move right, S shoot (we will grade movement first).

.....M.....
...........
...........
.....A.....

Player starts at the bottom, middle.

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Challenge: move the player

Given width 11, player column starts at 5. Read a line of commands made of L and R. Print the final column (0–10). Ignore moves that would leave the board.

Commands	Result
LL	3
RRRRRRRRRR	10

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Aliens as a list of columns

Store alien columns in a list, e.g. [2, 5, 8]. A hit removes that column from the list.

🧪 Hit detection idea

Mini quiz — hits

Quiz

Hits

1 / 2

After this, what is aliens?

aliens = [2, 5, 8]
shot_col = 5
if shot_col in aliens:
  aliens.remove(shot_col)

Pick an answer to see if you are right.

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Challenge: did the shot hit?

Read shot_col then a line of alien columns. Print HIT if the shot column is in that list, otherwise MISS.

Input	Output
5 then 2 5 8	HIT
1 then 2 5 8	MISS

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Exercises

Activity: Draw the board

Write draw_line(width, player_col) so it returns a string of length width: . everywhere except A at player_col.

Expected for 11 5:

.....A.....

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Going further (optional)

If you later want pixels, options are:

1. Keep ASCII and add colors with ANSI codes in a real terminal (not required here).
2. Bake Pillow into the Piston Python image (pip install pillow at package build time) and print a base64 PNG — then a small site component could display it.
3. Browser game in JavaScript/canvas, with Python only scoring logic.

For class, stay on the grid: it is the same architecture as the arcade game (state, input, collide, redraw).