PyLogo is a lightweight library that uses PyGame to provide a LOGO-like drawing interface for Python, to provide a simple and fun way to learn programming in Python.

pylogo.py

"""
PyLogo is a lightweight library that uses PyGame to provide a LOGO-like
drawing interface for Python, to provide a simple and fun way to learn
programming in Python.
"""

import pygame
from math import cos, radians, sin

# Functions that 'from pylogo import *' exports
__all__ = [
        "start",
        "draw",
        "moveto",
        "forward",
        "back",
        "left",
        "right",
        "penup",
        "pendown"
        ]

_pylogo_FPS = 60 # Constant: frames per second of event loop
_pylogo_initialised = False # Whether pylogo has been initialised
_pylogo_colour = 0, 0, 0 # Colour to draw in

def start(w, h):
    """
    Initialise the PyLogo module.
    """
    global _pylogo_screen
    global _pylogo_clock
    global _pylogo_turtle
    global _pylogo_initialised
    pygame.init()
    # Create a window with the given dimensions
    _pylogo_screen = pygame.display.set_mode((w, h))
    # Fill the screen with white
    _pylogo_screen.fill((255, 255, 255))
    # Create the game clock
    _pylogo_clock = pygame.time.Clock()
    # Place the turtle in the centre
    _pylogo_turtle = [ w / 2, h / 2, 0 ]
    # Draw lines by default
    pendown()
    # Set initialised to true
    _pylogo_initialised = True

def draw():
    """
    Draw the lines and listen for events.
    """
    # Make sure pylogo is started first
    if not _pylogo_initialised:
        print "PyLogo not initialised, do so with `start` function first."
        return
    # Display everything in the window
    pygame.display.flip()
    # Keep the window open and listen for input
    _pylogo_eventloop()
    # Quit after the loop is done
    pygame.quit()

def _pylogo_eventloop():
    running = True
    while running:
        # Run through the event queue
        for event in pygame.event.get():
            # If the widnow is closed or the q/Q key is pressed
            if event.type == pygame.QUIT or\
                    (event.type == pygame.KEYDOWN and event.key == pygame.K_q):
                # Do not continue the loop
                running = False
        # Maintain a constant rate
        _pylogo_clock.tick(_pylogo_FPS)

def moveto(x, y):
    """
    Jump directly to a point without drawing.
    """
    # Directly set x;y coordinates
    _pylogo_turtle[0], _pylogo_turtle[1] = x, y

def forward(distance):
    """
    Move a given distance in the direction pointed to,
    drawing if the pen is down.
    """
    # Original position and angle
    start_x = _pylogo_turtle[0]
    start_y = _pylogo_turtle[1]
    angle = _pylogo_turtle[2]
    # Calculate the new position (right-angle triangle trigonometry)
    x = start_x + cos(angle) * distance
    y = start_y + sin(angle) * distance
    # Move the turtle to the new position
    moveto(x, y)
    # Draw the line if pen down
    if _pylogo_drawing:
        pygame.draw.aaline(\
                _pylogo_screen, _pylogo_colour,\
                (start_x, start_y), (x, y))

def back(distance):
    """
    Move a given distance backwards from the direction pointed to,
    drawing if the pen is down.
    """
    left(180)
    forward(distance)
    left(180)

def left(angle):
    """
    Turn a given angle (degrees) to the left of the current direction.
    """
    _pylogo_turtle[2] -= radians(angle)

def right(angle):
    """
    Turn a given angle (degrees) to the right of the current direction.
    """
    left(-angle)

def penup():
    """
    Prevent lines from being drawn.
    """
    global _pylogo_drawing
    _pylogo_drawing = False

def pendown():
    """
    Allow lines to be drawn.
    """
    global _pylogo_drawing
    _pylogo_drawing = True