code: def lookup(label):
      
      # Only strip ASCII whitespace: U+0009, U+000A, U+000C, U+000D, and U+0020.
      label = ascii_lower(label.strip('\t\n\f\r '))
      name = LABELS.get(label)
      if name is None:
          return None
      encoding = CACHE.get(name)
      if encoding is None:
          if name == 'x-user-defined':
              from .x_user_defined import codec_info
          else:
              python_name = PYTHON_NAMES.get(name, name)
              # Any python_name value that gets to here should be valid.
              codec_info = codecs.lookup(python_name)
          encoding = Encoding(name, codec_info)
          CACHE[name] = encoding
      return encoding
docs: Look for an encoding by its label.
      This is the spec’s `get an encoding
      <http://encoding.spec.whatwg.org/#concept-encoding-get>`_ algorithm.
      Supported labels are listed there.
  
      :param label: A string.
      :returns:
          An :class:`Encoding` object, or :obj:`None` for an unknown label.