引发Python内置异常时如何设置它的message属性

Question

I know I can specify an arbitrary number of arguments to an Exception (or any subclass thereof) when I raise it, but I see that Exceptions have a message attribute which isn't populated.

def throw():
    raise Exception("This is my message", 10)

try:
    throw()
except Exception as E:
    print "message = ", E.message
    print E.args

produces

message = 
("This is my message", 10)

What I would like to do is raise a variety ValueErrors depending on how or why a value in my function isn't valid. I would also like to attach a code, so something like

raise ValueError(1, "You need to supply a string value")
# or
raise ValueError(2, "Not a valid place name") #but it is a valid string

I'm writing a code that runs callbacks supplied by the user, and I expect the user's functions to raise ValueErrors in certain situations, but their callback can also call into some library functions I provide, and I want to be able distinguish between the user's ValueErrors and Mine.

I know I can subclass the ValueError and check for that exception type explicitly, but that seems like overkill. These are all errors with user supplied values after all, so I would like to use the first argument to the exception as a code, and then somehow set the message attribute explicitly

raise ValueError(10, message = "Error 10 is something quite specific")

The above doesn't work, because exceptions don't accept keyword arguments on init .

Answer 1

You can pass multiple argument when you raise an exception:

try:
    raise ValueError("message", 1, 2, 3)
except ValueError as e:
    print e
    print e[1]

The exception object is simply a tuple, and you can access any item in te tuple in array[index] manner.

OUTPUT:

('message', 1, 2, 3)
1

Answer 2

The message attribute is set when the Exception instance is created with a single argument. Otherwise it is empty:

In [1]: Exception(42).message
Out[1]: 42

In [2]: Exception('foo').message
Out[2]: 'foo'

In [3]: Exception('foo', 42).message
Out[3]: ''

Of course you can create an instance of Exception and manually set its .message attribute before raising:

e = Exception()
e.message = 'test'
raise e

But it would probably make a lot of sense to use a custom exception class instead.

Also note that Exception does not have a message attribute in Python 3, only args .