why was the file where I want to write a test_report into alway closed

[huangri212]

Traceback (most recent call last):
File "C:\Python27\lib\multiprocessing\process.py", line 232, in _bootstrap
self.run()
File "C:\Python27\lib\multiprocessing\process.py", line 88, in run
self._target(*self._args, **self._kwargs)
File "C:\Python27\lib\HTMLTestRunner.py", line 630, in run
self.generateReport(test, result)
File "C:\Python27\lib\HTMLTestRunner.py", line 687, in generateReport
self.stream.write(output.encode('utf8'))
ValueError: I/O operation on closed file