Projects — Python 3 Patterns, Recipes and Idioms (original) (raw)
Note
This chapter has not had any significant translation yet.
A number of more challenging projects for you to solve. [[Some of these may turn into examples in the book, and so at some point might disappear from here]]
Rats & Mazes¶
First, create a Blackboard (cite reference) which is an object on which anyone may record information. This particular blackboard draws a maze, and is used as information comes back about the structure of a maze from the rats that are investigating it.
Now create the maze itself. Like a real maze, this object reveals very little information about itself - given a coordinate, it will tell you whether there are walls or spaces in the four directions immediately surrounding that coordinate, but no more. For starters, read the maze in from a text file but consider hunting on the internet for a maze-generating algorithm. In any event, the result should be an object that, given a maze coordinate, will report walls and spaces around that coordinate. Also, you must be able to ask it for an entry point to the maze.
Finally, create the maze-investigating Rat class. Each rat can communicate with both the blackboard to give the current information and the maze to request new information based on the current position of the rat. However, each time a rat reaches a decision point where the maze branches, it creates a new rat to go down each of the branches. Each rat is driven by its own thread. When a rat reaches a dead end, it terminates itself after reporting the results of its final investigation to the blackboard.
The goal is to completely map the maze, but you must also determine whether the end condition will be naturally found or whether the blackboard must be responsible for the decision.
An example implementation by Jeremy Meyer:
Projects/ratsAndMazes/Maze.py
class Maze(Canvas): lines = [] # a line is a char array width = -1 height = -1 main(self): if (args.length < 1): print("Enter filename") System.exit(0)
Maze m = Maze()
m.load(args[0])
Frame f = Frame()
f.setSize(m.width*20, m.height*20)
f.add(m)
Rat r = Rat(m, 0, 0)
f.setVisible(1)
def __init__(self):
lines = Vector()
setBackground(Color.lightGray)
def isEmptyXY(x, y):
if (x < 0) x += width
if (y < 0) y += height
# Use mod arithmetic to bring rat in line:
byte[] by =
(byte[])(lines.elementAt(y%height))
return by[x%width]==' '
def setXY(x, y, byte newByte):
if (x < 0) x += width
if (y < 0) y += height
byte[] by =
(byte[])(lines.elementAt(y%height))
by[x%width] = newByte
repaint()
def load(String filename):
String currentLine = null
BufferedReader br = BufferedReader(
FileReader(filename))
for(currentLine = br.readLine()
currentLine != null
currentLine = br.readLine()) :
lines.addElement(currentLine.getBytes())
if(width < 0 ||
currentLine.getBytes().length > width)
width = currentLine.getBytes().length
height = len(lines)
br.close()
def update(self, Graphics g): paint(g)
def paint(Graphics g):
canvasHeight = self.getBounds().height
canvasWidth = self.getBounds().width
if (height < 1 || width < 1)
return # nothing to do
width =
((byte[])(lines.elementAt(0))).length
for (int y = 0 y < len(lines) y++):
byte[] b
b = (byte[])(lines.elementAt(y))
for (int x = 0 x < width x++):
switch(b[x]):
case ' ': # empty part of maze
g.setColor(Color.lightGray)
g.fillRect(
x*(canvasWidth/width),
y*(canvasHeight/height),
canvasWidth/width,
canvasHeight/height)
break
case '*': # a wall
g.setColor(Color.darkGray)
g.fillRect(
x*(canvasWidth/width),
y*(canvasHeight/height),
(canvasWidth/width)-1,
(canvasHeight/height)-1)
break
default: # must be rat
g.setColor(Color.red)
g.fillOval(x*(canvasWidth/width),
y*(canvasHeight/height),
canvasWidth/width,
canvasHeight/height)
break::Projects/ratsAndMazes/Rat.py
class Rat: ratCount = 0 prison = Maze() vertDir = 0 horizDir = 0 x = 0, y = 0 myRatNo = 0 def init(self, maze, xStart, yStart): myRatNo = ratCount++ print ("Rat no." + myRatNo + " ready to scurry.") prison = maze x = xStart y = yStart prison.setXY(x,y, (byte)'R')
def run(self): scurry().start()
def scurry(self):
# Try and maintain direction if possible.
# Horizontal backward
boolean ratCanMove = 1
while(ratCanMove):
ratCanMove = 0
# South
if (prison.isEmptyXY(x, y + 1)):
vertDir = 1 horizDir = 0
ratCanMove = 1
# North
if (prison.isEmptyXY(x, y - 1))
if (ratCanMove)
Rat(prison, x, y-1)
# Rat can move already, so give
# this choice to the next rat.
else:
vertDir = -1 horizDir = 0
ratCanMove = 1
# West
if (prison.isEmptyXY(x-1, y))
if (ratCanMove)
Rat(prison, x-1, y)
# Rat can move already, so give
# this choice to the next rat.
else:
vertDir = 0 horizDir = -1
ratCanMove = 1
# East
if (prison.isEmptyXY(x+1, y))
if (ratCanMove)
Rat(prison, x+1, y)
# Rat can move already, so give
# this choice to the next rat.
else:
vertDir = 0 horizDir = 1
ratCanMove = 1
if (ratCanMove): # Move original rat.
x += horizDir
y += vertDir
prison.setXY(x,y,(byte)'R')
# If not then the rat will die.
Thread.sleep(2000)
print ("Rat no." + myRatNo +
" can't move..dying..aarrgggh.")The maze initialization file:
Projects/ratsAndMazes/Amaze.txt
*** ***