test1.txt

position 50 50
line 50 0
line -25 -50
line -25 50
move 100 0
line 40 0
line -20 -40
line -20 40
position 100 100
circle 20

Unit13.py

import Canvas
import string
import re


def pos_cmd(state, x, y):
    state['pos'] = [int(x), int(y)]
  
def move_cmd(state, x, y):
    pos = state['pos']
    newpos = [pos[0] + int(x), pos[1] + int(y)]
    state['pos'] = newpos
    
def line_cmd(state, x, y):
    pos = state['pos']
    newpos = [pos[0] + int(x), pos[1] + int(y)]
    
    Canvas.create_line(pos[0], pos[1], newpos[0], newpos[1])

    state['pos'] = newpos

def circle_cmd(state, r):
    centre = state['pos']
    Canvas.create_oval(centre[0] - r, centre[1] - r, centre[0] + r, centre[1] + r)


def parse_line(line):
    line      = re.sub(r'[_\s]+', ' ', line).strip().lower().split()
    command   = line[0]
    arguments = line[1:]
    p_args = []
    for argument in arguments:
        p_args.append(int(argument))
    return [command, p_args]
    

def execute_file(path):
    state = {
        'pos': [0, 0],
        'line': 1
    }

    commands = {
        'position': pos_cmd,
        'move': move_cmd,
        'line': line_cmd,
        'circle': circle_cmd
    }
    
    input_fh = open(path, 'r')
    input_file = input_fh.readlines()
    input_fh.close()

    input_lines = map(parse_line, input_file)

    while state['line']  <= len(input_lines):        
        command, arguments = input_lines[state['line'] - 1]
        print state['line'], command, arguments
        
        if commands.has_key(command):
            commands[command](state, *arguments)
        elif command == 'define':
            func_lines = []
            


            
            pass
        elif command == 'end':
            print '[Line '+str(state['line'])+'] Unexpected "end".' 
            return
        else:
            print '[Line '+str(state['line'])+'] Command not recognized: "' + command + '".' 
            return
   
        state['line'] += 1
    Canvas.complete()

def main():
    execute_file('test1.txt')
main()