Returns the number of '$' characters right in front of s[i].

def _count_dollars_before_index(self, s, i):
        """Returns the number of '$' characters right in front of s[i]."""
        dollar_count = 0
        dollar_index = i - 1
        while dollar_index > 0 and s[dollar_index] == '$':
            dollar_count += 1
            dollar_index -= 1
        return dollar_count