[Tutor] Subclassing Exceptions

Steven D'Aprano steve at pearwood.info
Sat Jan 7 06:06:06 CET 2012

Devin Jeanpierre wrote:
>> 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?
> If somebody wanted to catch a ValueError, would they also mean to
> catch my error? (The same with SyntaxError and so on). Probably not.

If your error represents a value error, then yes they would.

But in that case, why should I subclass ValueError instead of just using 
ValueError itself? Writing my own exceptions is only useful if I intend to 
give the caller the *choice* of either treating my exception the same or 
different from ValueError: that is, if my error is a more specific kind of 
ValueError. If it is not a specific kind of value error, then just use the 
generic built-in ValueError exception instead.

     x = some_function()
except MyValueError:
     # treat my errors specially
except ValueError:
     # any other kind of ValueError caught here


     x = some_function()
except ValueError:
     # any kind of ValueError caught here, including MyValueError

The same goes for all exception types, although in practice I find that I 
generally only subclass ValueError.


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