At line 237 of string.py:
    def convert_field(self, value, conversion):
        # do any conversion on the resulting object
        if conversion == 'r':
            return repr(value)
        elif conversion == 's':
            return str(value)
        elif conversion is None:
            return value
add
        elif conversion == 'a':
            return ascii(value)

The alternatives should be ordered by decreasing frequency. I suspect that should be None, 's', 'r', 'a'.