Python 3.x 提升值_错误不'；行不通

未打印值错误或raise语句未引发异常。有人能帮我纠正打印时的“值错误…”吗

try:
    if(a>b):
       raise Value_Error;
    else:
       print("no err");
except "Value_Error":
    print("value error ...");
else:
    print("a is smaller");

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下划线和引号是怎么回事？

try:
    if a > b:
       raise ValueError()
    else:
       print("no err")
except ValueError:
    print("value error ...")
else:
    print("a is smaller")