python–traceback.format_excprint_exc在期待回溯时返回None

我无法弄清楚为什么traceback.format_exc()在以下示例中返回“None”：

#!/usr/bin/env pythonimport sysimport tracebackdef my_excepthook(type,value,tb):    print type.__name__    print value    # the problem: why does this return "None"?    print traceback.format_exc(tb) # see http://docs.python.org/library/traceback.HTML#traceback.format_excsys.excepthook = my_excepthook # see http://docs.python.org/library/sys.HTML#sys.excepthook# some code to generate a naturaListic exceptiona = "text"b = 5error = a + b

使用Python 2.7.1,我得到以下输出：

TypeErrorcannot concatenate 'str' and 'int' objectsNone

而不是第3行的“无”,我希望得到当我注释掉sys.excepthook行时会发生什么：

Traceback (most recent call last):  file "log-test.py",line 17,in 

print "".join(traceback.format_exception(type,tb))