<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def to_header_str(self):
 """ Convert to a tracing header string that can be injected to outgoing http request headers. """

        h_parts = []
        if self.root:
            h_parts.append(ROOT + '=' + self.root)
        if self.parent:
            h_parts.append(PARENT + '=' + self.parent)
        if self.sampled is not None:
            h_parts.append(SAMPLE + '=' + str(self.sampled))
        if self.data:
            for key in self.data:
                h_parts.append(key + '=' + self.data[key])

        return HEADER_DELIMITER.join(h_parts)