[Tutor] Subclassing Exceptions

Steven D'Aprano steve at pearwood.info
Sat Jan 7 05:56:32 CET 2012

Chris Fuller wrote:

>>>> class Foo(SyntaxError):
> ...  def __init__(self, a,b,c):
> ...   self.args = (a,b,c)
> ... 
>>>> raise Foo(1,2,3)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> __main__.Foo: None
> Inheriting from SyntaxError doesn't work!  When I create a new exception, I 
> generally subclass from the built-in exception it most resembles, in case 
> there was some reason to also catch it via an ancestor.  But I'm not sure if 
> that is really all that useful an idea in practice.  How do you folk do it?

What do you mean, "doesn't work"? It looks like it works to me. You get a Foo 
exception, exactly as expected. The error message isn't what you expect, 
because you're making unwarranted assumptions about SyntaxError and how it works.

In general, when you override a method, you take full responsibility to 
perform everything that the superclass method was supposed to do. In this 
case, you fail to assign to msg as well as args. It is safer to overload a 
message rather than override it:

 >>> class Spam(SyntaxError):
...     def __init__(self, *args):
...             if args:
...                     args  = ("I pity the fool who made a mistake",) + args[1:]
...             super(Spam, self).__init__(*args)
 >>> raise Spam('you made a mistake', 1, 2)
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
__main__.Spam: I pity the fool who made a mistake

Unfortunately, there's no real consistency in what arguments exceptions are 
expected to take. The best thing is to read the docs, if they have any, or use 
introspection and trial and error to work out what they do.

 >>> try:
...     raise SyntaxError("you made a mistake")
... except SyntaxError, err:
...     pass
 >>> err.msg
'you made a mistake'

See dir(err) for more; you can use help(SyntaxError) but unfortunately it 
isn't very useful.

You probably shouldn't inherit from SyntaxError, since it represents syntax 
errors in the Python code being interpreted or compiled. Any syntax error in 
your own data structures should be independent of SyntaxError.


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