Sunday, December 26, 2004

wxPython woes

I've been trying to understand better how to use wxPython but I still have problems with doing "custom" double-buffering displays other than for simple programs. For example, the techniques used in the program below could be used as the basis for a simple pac-man like game. It works well, using the built-in buffered device contexts but I'd like to understand how to do my own buffered drawings on scrolled windows (It's "easy" to get things working with fixed-sized windows). For those interested, have a look at the onPaint() and drawImage() methods.

One of the problems that I am encountering is the incompleteness (for non-expert like me) of the available documentation on wxPython. So, I resolved to write my wxPython based programs with lots of comments in the hope that they will be useful to other people

""" This wxPython-based program illustrates:
* the use of foreground and background program-generated images;
* the use of cursor keys to move an image;
* the automatic scrolling of windows;
* double-buffering of images using "built-in" bufferedDC's.
Using the arrow-keys, we move a simple image (circle) on a checkered
background inside a scrolled-window, scrolling automatically as needed to
keep the circle always visible.

import wx

custom_buffer = False

class CheckeredBackground(object):
"""Checkered-like image pattern defined so that we can better
visualise the moving canvas."""
# Note: there is no real need to define this as a class; it could
# have been simply defined as a function of MyCanvas below.
# Note 2: default values are provided but not used in this program.

def __init__(self, width=600, height=480, side=40):

#-- prepare to create bitmap image
self.image = wx.EmptyBitmap(width, height)

# Before we can "draw", we need to specify what the context will be.
#-- Device context (DC) will be computer memory
offDC = wx.MemoryDC()

#-- prepare to work with the image

#-- select a background colour

#-- "paint" over the entire object with the background colour

#-- create the pattern in "separate" memory
nb_col = width//side
nb_row = height//side
squares = []
for i in range(0, nb_col, 2):
for j in range(0, nb_row, 2):
x = i*side
y = j*side
squares.append( (x, y, side, side) )
for i in range(1, nb_col, 2):
for j in range(1, nb_row, 2):
x = i*side
y = j*side
squares.append( (x, y, side, side) )

#-- Choose square outline colour
offDC.SetPen(wx.Pen("PALE GREEN", side/8))

#-- Choose square interior (fill) colour
offDC.SetBrush(wx.Brush("LIGHT STEEL BLUE"))

#-- "draw" the squares in DC memory

#-- release bitmap image from drawing context. This amounts,
# as I understand, to undoing SelectObject().
del offDC

class MovingCircle(object):
"""Creates a simple circle on transparent background"""
# Note: there is no real need to define this as a class.
# It is done here for later re-use in other examples.
# Note 2: default values are provided but not used in this program.

def __init__(self, radius = 10, string_colour = "RED"):

#-- prepare to create bitmap image
self.image = wx.EmptyBitmap(2*radius, 2*radius)

#-- Device context (DC) will be computer memory
offDC = wx.MemoryDC()

#-- prepare to work with the image

""" Preparing to draw a shape with a transparent background.
I thought that choosing a wx.Brush with a TRANSPARENT_BRUSH
colour, and clearing the DC with it would work - but it
does not. What does work is to select an "unused" colour
for the background and set up a mask with it."""
#-- choose an "unusual" background colour ...
offDC.SetBackground(wx.Brush(wx.Colour(2,2,2), wx.SOLID))

#-- ... and set it everywhere

#-- different colour for object
offDC.SetPen(wx.Pen(string_colour, 1)) # outline
offDC.SetBrush(wx.Brush(string_colour)) # interior

offDC.DrawCircle(radius, radius, radius)# center: (x, y) and radius.

#-- release bitmap image from drawing context to process it further
del offDC

#-- set up a mask with our "unusual" colour
mask = wx.Mask(self.image, wx.Colour(2,2,2))
#-- only regions where colour != wx.Colour(2,2,2) survives.

class MyCanvas(wx.ScrolledWindow):
def __init__(self, parent, id = -1, size = wx.DefaultSize):
wx.ScrolledWindow.__init__(self, parent, id, (0, 0), size=size,
# sets dimensions so that image will be larger than window,
# and scrolling can occur.
self.maxWidth = 1200
self.maxHeight = 800

# Set the size of the total window, of which only a small part
# will be displayed; apparently SetVirtualSize needs
# a single (tuple) argument, which explains the double (( )).
self.SetVirtualSize((self.maxWidth, self.maxHeight))

# Set the scrolling rate; use same value in both horizontal and
scrollRate = 20 # vertical directions.
self.SetScrollRate(scrollRate, scrollRate)

# Create the background image
side = 40
self.background = CheckeredBackground(self.maxWidth,
self.maxHeight, side)

# Create small foreground images; normally, such an image might
# be imported from a file
self.radius = 40
self.red_circle = MovingCircle(self.radius, "RED")

# sets its position (used later)
self.circle_x = 0 # position of top left corner of enclosing box
self.circle_y = 0

# bind the key events that will be used to move the small image

# Initialize the buffer bitmap. No real DC is needed at this point.
self.buffer = wx.EmptyBitmap(self.maxWidth, self.maxHeight)

def bindEvents(self):
# use the old style [still works!] instead of self.Bind(evt, fn)
wx.EVT_PAINT(self, self.OnPaint)
wx.EVT_CHAR(self, self.MyArrowKeys)

##-- onPaint() and drawImage() are the two methods I'd like to know how
##-- to do with custom-based buffered DCs.

def OnPaint(self, event):
if custom_buffer:
# I haven't been able to figure out how to do this
dc = wx.BufferedPaintDC(self, self.buffer)

def drawImage(self):
if custom_buffer:
# I haven't been able to figure out how to do this.
dc = wx.BufferedDC(None, self.buffer)
# First copy the background image onto the buffer
dc.DrawBitmap(self.background.image, 0, 0, True)
# Next, superimpose the foreground image
dc.DrawBitmap(self.red_circle.image, self.circle_x,
self.circle_y, True)
del dc

def MoveCircle(self, x, y):
self.circle_x += x
self.circle_y += y

#-- Prevent the circle from moving out of bounds
# (of the full background image, not the visible part.)
if self.circle_x < circle_x =" 0" circle_y =" 0"> self.maxWidth - 2*self.radius:
self.circle_x = self.maxWidth - 2*self.radius
if self.circle_y > self.maxHeight - 2*self.radius:
self.circle_y = self.maxHeight - 2*self.radius

hidden = 40 # approximate space hidden under scrollbars

# determine the position of top left visible window in
# "scrollrate" units
xView, yView = self.GetViewStart()

# corresponding amount of pixel per "scroll"
xDelta, yDelta = self.GetScrollPixelsPerUnit()

# size of wisible window
width, height = self.GetSizeTuple()

#-- Determine if window needs to be scrolled so that object
# remains visible. Assume that the object fits entirely
# in the visible view.
if self.circle_x < xview =" max(0,"> xView*xDelta + width:
xView = (self.circle_x + 2*self.radius + hidden - width)/xDelta

if self.circle_y < yview =" max(0,"> yView*yDelta + height:
yView = (self.circle_y + 2*self.radius + hidden - height)/yDelta

self.Scroll(xView, yView)


def MyArrowKeys(self, event):
code = event.KeyCode()
if code == wx.WXK_UP:
self.MoveCircle(0, -10) # up on screen is negative y-direction
elif code == wx.WXK_LEFT:
self.MoveCircle(-10, 0)
elif code == wx.WXK_RIGHT:
self.MoveCircle(10, 0)
elif code == wx.WXK_DOWN:
self.MoveCircle(0, 10)
pass # ignore all other keys pressed

class AnimationFrame(wx.Frame):
def __init__(self, parent):
wx.Frame.__init__(self, parent, -1, "Animation Frame", size=(400, 300),
doodle = MyCanvas(self, -1)


if __name__ == '__main__':
app = wx.PySimpleApp()
frame = AnimationFrame(None)

Wednesday, December 22, 2004

Subclassing my own classes

I thought I understood reasonably well objects and classes ... and yet, I had never based one of my classes on one that I had previously created. In fact, most of my classes were not only "terminal" but they were instantiated only once in my program (with the exception of a custom dialog class). Today, it finally dawned on me how I could subclass my own classes. In RUR, I had a robot class and a world class, both of which represented only the logical part; the display was handled separately as I understood it should be. Yet, it lead to enormously interlinked code which didn't feel right.

What I will do, is subclass both the robot and world classes into something like visual_robot and visual_world, which will include all the visual information required to draw themselves. I will also include the robot object in the complete world (eventually creating a list of such objects), since the robot(s) should be part of the world just as the beepers and walls are. This should simplify tremendously the links between the window display and the objects in it.

This approach should also allow for much greater flexibility in giving different appearance, as one can concentrate all the relevant visual parameters in once place, and change them at will.

Monday, December 13, 2004

Settled on a name for my program

I've finally settled for a name for my program. It is RUR: a Python Learning Environment, or RUR-PLE for short.

Pattis's Karel the Robot was named after the author Karel Capek, who popularized the word robot in his play Rossum's Universal Robots (RUR). While RUR-PLE shares the basic RUR acronym, in this case it stands for Roberge's Used Robot. However, through the magic of Rossum's Python, you can learn how to fix it and design a better one, worthy of the name Rossum's Universal Robot.

Saturday, December 11, 2004 | Klein urges same-sex marriage referendum | Klein urges same-sex marriage referendum

Well, the right wing extremists never get it. You can't let the majority vote on minority rights, pretending that this is what democracy is about. Thankfully, the Canadian Supreme Court judges are well ahead of some dinosaurs that still roam the political landscape.

Saturday, December 04, 2004

Teaching computer programming

In 1981, Richard Pattis wrote a delightful little book titled Karel the Robot, a Gentle Introduction to the Art of Programming. In this book, Pattis introduces the main concepts of sequential programming (including loops and decisions, but not variable assignments) using the paradigm of instructing a robot capable of only four basic actions (turning left, moving one step forward, picking up and putting down beepers). Through the "magic" of programming, the robot "learns" to combine those four basic actions in order to accomplish tasks of increasing complexity. Pattis used Pascal, the preferred language of the day, as a means of "teaching" the robot new tricks. Since then, many new versions of Karel the Robot have surfaced, used to introduce students to various computer languages, with a preference for Java and C++, which are both based on the modern Object-oriented programming (OOP) approach.

However, the complexity of Java and C++ contrasts with the simplicity of the robot world; both these languages seem at odds with the idea of providing a Gentle Introduction to the Art of Programming.

Enter Python... Python, like Java and C++, is an OOP language. However Python also allows a non-OOP programming style which is more suitable for interacting with Pattis's robot. A first implementation of Karel the Robot in Python was called PyKarel. The current implementation is called Guido van Robot (GvR for short), and is available at

I am currently working on a "new and improved" version of GvR which extends Pattis's ideas and allow a smooth transition to the use of variables as well as functions/methods and objects. This version, like all Python-based versions of Karel the Robot, will be made available for free to all those interested.

I have been pondering, for over a month, what to call my new version. Pattis's Karel the Robot was named after the author Karel Capek, who popularized the word robot in his play Rossum's Universal Robots. The computer language Python, was named after the famous Monty Python's Flying Circus by its creator, Guido van Rossum. I find it difficult to think of a better name than Guido van Robot to name a programming environment in which one uses Python to teach a robot new tricks!

Any suggestion would be welcome.