text: """
      This small module makes it easy to bake PNG images with links to
      Open Badge assertions. It also allows for easy retrieval of the link
      from baked PNGs.
  
      For more information on badge baking, see:
  
          https://github.com/mozilla/openbadges/wiki/Badge-Baking
  
      Note that this module requires the PyPNG module:
  
          http://pythonhosted.org/pypng/
  
      The module can be installed via 'pip install pypng'.
  """
  
  import os
  import png
  
  def get_baked_url(src):
      """
      Return the assertion URL contained in the given baked PNG. If
      the image isn't baked, return None.
  
      Example:
  
          >>> get_baked_url('baked.png')
          'http://f.org/assertion.json'
      """
  
      if isinstance(src, basestring): src = open(src, 'rb')
      reader = png.Reader(file=src)
      for chunktype, content in reader.chunks():
          if chunktype == 'tEXt' and content.startswith('openbadges\x00'):
              return content.split('\x00')[1]
  
  def bake_badge(src, dest, url):
      """
      Bake the given PNG file with the given assertion URL. The source and
      destination can be filenames or file objects.
  
      Example:
  
          >>> bake_badge('unbaked.png', 'baked.png', 'http://f.org/a.json')
      """
  
      if isinstance(src, basestring): src = open(src, 'rb')
      if isinstance(dest, basestring): dest = open(dest, 'wb')
  
      reader = png.Reader(file=src)
      chunks = [
          (chunktype, content)
          for chunktype, content in reader.chunks()
          if not (chunktype == 'tEXt' and content.startswith('openbadges\x00'))
      ]
  
      chunks.insert(1, ('tEXt', '\x00'.join(('openbadges', url))))
      png.write_chunks(dest, chunks)
  
  def test(show_output=False):
      """
      Test suite for this module.
      """
  
      from StringIO import StringIO
  
      def eq(a, b, msg):
          if show_output: print "%-60s" % msg,
          if a != b:
              raise AssertionError('%s (%s != %s)' % (msg, repr(a), repr(b)))
          if show_output: print "OK"
  
      PNG = '\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\x00\x00\x00\x10\x00\x00\x00' \
            '\x10\x08\x02\x00\x00\x00\x90\x91h6\x00\x00\x00\x19IDAT(\x91c' \
            '\xfc\xff\xff?\x03)\x80\x89$\xd5\xa3\x1aF5\x0c)\r\x00Um\x03' \
            '\x1d\xe4FZ\x12\x00\x00\x00\x00IEND\xaeB`\x82'
  
      unbaked = StringIO(PNG)
      eq(get_baked_url(unbaked), None,
         'get_baked_url() on unbaked badge returns None')
  
      unbaked = StringIO(PNG)
      baked = StringIO()
      bake_badge(unbaked, baked, 'http://foo.org/assertion.json')
  
      baked = StringIO(baked.getvalue())
      eq(get_baked_url(baked), 'http://foo.org/assertion.json',
         'baking an unbaked badge works')
  
      baked = StringIO(baked.getvalue())
      rebaked = StringIO()
      bake_badge(baked, rebaked, 'http://another/assertion')
  
      rebaked = StringIO(rebaked.getvalue())
      eq(get_baked_url(rebaked), 'http://another/assertion',
         'baking an already-baked badge re-bakes it')
  
  if __name__ == '__main__':
      test(show_output=True)
  