[Tutor] Guidance on using custom exceptions please

[Steven D'Aprano]

On Mon, Oct 12, 2015 at 02:55:43PM +0000, David Aldrich wrote:
> Hi
> 
> Consider a 'send' method that sends a message to another system via a 
> socket.  This method will wait for a response before returning.  There 
> are two possible error conditions:
[...]
> So, my question is, what's the pythonic way of doing this?  Should I 
> subclass RuntimeError for each possible error condition?  E.g.:
> 
>                class MessageTimeoutError(RuntimeError): pass
>                class IllegalResponseError(RuntimeError): pass

I don't think you should be subclassing RuntimeError at all. I'm not 
quite sure what exception you should subclass, but I am confident it 
shouldn't be RuntimeError.

Help on class RuntimeError in module exceptions:

class RuntimeError(StandardError)
    Unspecified run-time error.


Since you are working with sockets, I think a socket error might be most 
useful:

import socket  # which I expect you are already doing

class MessageTimeoutError(socket.error): pass
class IllegalResponseError(socket.error): pass

Or possibly inherit from the same exception class that socket.error 
inherits from: IOError.

I'm not certain that you actually need MessageTimeoutError since the 
socket module itself already defines a socket.timeout error that will be 
raised on a timeout. Just re-use that.


-- 
Steve