code: def _get_conversion(cls, conv_chr, modifier):
          
  
          # Do we need to look up the conversion?
          if conv_chr not in cls._conversion_cache:
              for ep in pkg_resources.iter_entry_points('bark.conversion',
                                                        conv_chr):
                  try:
                      # Load the conversion class
                      cls._conversion_cache[conv_chr] = ep.load()
                      break
                  except (ImportError, pkg_resources.UnknownExtra):
                      # Couldn't load it; odd...
                      continue
              else:
                  # Cache the negative result
                  cls._conversion_cache[conv_chr] = None
  
          # Handle negative caching
          if cls._conversion_cache[conv_chr] is None:
              return conversions.StringConversion("(Unknown conversion '%%%s')" %
                                                  conv_chr)
  
          # Instantiate the conversion
          return cls._conversion_cache[conv_chr](conv_chr, modifier)
docs: Return a conversion given its character.
  
          :param conv_chr: The letter of the conversion, e.g., "a" for
                           AddressConversion, etc.
          :param modifier: The format modifier applied to this
                           conversion.
  
          :returns: An instance of bark.conversions.Conversion.