为什么我不能在Python中腌制错误的Traceback？

import sys, traceback

def print_exc_plus():
    \"\"\"
    Print the usual traceback information, followed by a listing of all the
    local variables in each frame.
    \"\"\"
    tb = sys.exc_info()[2]
    while 1:
        if not tb.tb_next:
            break
        tb = tb.tb_next
    stack = []
    f = tb.tb_frame
    while f:
        stack.append(f)
        f = f.f_back
    stack.reverse()
    traceback.print_exc()
    print \"Locals by frame, innermost last\"
    for frame in stack:
        print
        print \"Frame %s in %s at line %s\" % (frame.f_code.co_name,
                                             frame.f_code.co_filename,
                                             frame.f_lineno)
        for key, value in frame.f_locals.items():
            print \"\\t%20s = \" % key,
            #We have to be careful not to cause a new error in our error
            #printer! Calling str() on an unknown object could cause an
            #error we don\'t want.
            try:                   
                print value
            except:
                print \"<ERROR WHILE PRINTING VALUE>\"