Example explanation of Python generating verification code

#!/usr/bin/env python
#coding=utf-8
import random
import Image, ImageDraw, ImageFont, ImageFilter
  
_letter_cases = "abcdefghjkmnpqrstuvwxy" # 小写字母，去除可能干扰的i，l，o，z
_upper_cases = _letter_cases.upper() # 大写字母
_numbers = ''.join(map(str, range(3, 10))) # 数字
init_chars = ''.join((_letter_cases, _upper_cases, _numbers))
fontType="/usr/share/fonts/truetype/freefont/FreeSans.ttf"
  
def create_validate_code(size=(120, 30),
                             chars=init_chars,
                             img_type="GIF",
                             mode="RGB",
                             bg_color=(255, 255, 255),
                             fg_color=(0, 0, 255),
                             font_size=18,
                             font_type=fontType,
                             length=4,
                             draw_lines=True,
                             n_line=(1, 2),
                             draw_points=True,
                             point_chance = 2):
  '''
  @todo: 生成验证码图片
  @param size: 图片的大小，格式（宽，高），默认为(120, 30)
  @param chars: 允许的字符集合，格式字符串
  @param img_type: 图片保存的格式，默认为GIF，可选的为GIF，JPEG，TIFF，PNG
  @param mode: 图片模式，默认为RGB
  @param bg_color: 背景颜色，默认为白色
  @param fg_color: 前景色，验证码字符颜色，默认为蓝色#0000FF
  @param font_size: 验证码字体大小
  @param font_type: 验证码字体，默认为 ae_AlArabiya.ttf
  @param length: 验证码字符个数
  @param draw_lines: 是否划干扰线
  @param n_lines: 干扰线的条数范围，格式元组，默认为(1, 2)，只有draw_lines为True时有效
  @param draw_points: 是否画干扰点
  @param point_chance: 干扰点出现的概率，大小范围[0, 100]
  @return: [0]: PIL Image实例
  @return: [1]: 验证码图片中的字符串
  '''
  
  width, height = size # 宽， 高
  img = Image.new(mode, size, bg_color) # 创建图形
  draw = ImageDraw.Draw(img) # 创建画笔
  if draw_lines:
    create_lines(draw,n_line,width,height)
  if draw_points:
    create_points(draw,point_chance,width,height)
  strs = create_strs(draw,chars,length,font_type, font_size,width,height,fg_color)
  
  # 图形扭曲参数
  params = [1 - float(random.randint(1, 2)) / 100,
            0,
            0,
            0,
            1 - float(random.randint(1, 10)) / 100,
            float(random.randint(1, 2)) / 500,
            0.001,
            float(random.randint(1, 2)) / 500
            ]
  img = img.transform(size, Image.PERSPECTIVE, params) # 创建扭曲
  
  img = img.filter(ImageFilter.EDGE_ENHANCE_MORE) # 滤镜，边界加强（阈值更大）
  
  return img, strs
  
  
def create_lines(draw,n_line,width,height):
  '''绘制干扰线'''
  line_num = random.randint(n_line[0],n_line[1]) # 干扰线条数
  for i in range(line_num):
    # 起始点
    begin = (random.randint(0, width), random.randint(0, height))
    #结束点
    end = (random.randint(0, width), random.randint(0, height))
    draw.line([begin, end], fill=(0, 0, 0))
  
def create_points(draw,point_chance,width,height):
  '''绘制干扰点'''
  chance = min(100, max(0, int(point_chance))) # 大小限制在[0, 100]
   
  for w in xrange(width):
    for h in xrange(height):
      tmp = random.randint(0, 100)
      if tmp > 100 - chance:
        draw.point((w, h), fill=(0, 0, 0))
  
def create_strs(draw,chars,length,font_type, font_size,width,height,fg_color):
  '''绘制验证码字符'''
  '''生成给定长度的字符串，返回列表格式'''
  c_chars = random.sample(chars, length)
  strs = ' %s ' % ' '.join(c_chars) # 每个字符前后以空格隔开
   
  font = ImageFont.truetype(font_type, font_size)
  font_width, font_height = font.getsize(strs)
  
  draw.text(((width - font_width) / 3, (height - font_height) / 3),strs, font=font, fill=fg_color)
   
  return ''.join(c_chars)
  
  
if __name__ == "__main__":
    code_img = create_validate_code()
    code_img[0].save("validate.gif", "GIF")
    print code_img[1]