def lazy(html, src_field='data-lazyload', class_name='lazy'):
u'''
'''
placeholder_src = 'http://static.fuwo.com/static/images/new/common/placeholder.jpg'
soup = BeautifulSoup(html)
img_tags = soup.findAll('img')
if img_tags:
for tag in img_tags:
if tag.has_key('src'):
tag['class'] = class_name
tag[src_field] = tag['src']
tag['src'] = placeholder_src
lazy_html = unicode(soup)
else:
lazy_html = html
soup.clear()
del soup
return lazy_html
