On 13Oct2015 10:37, Steven D'Aprano <st...@pearwood.info> wrote:
On Mon, Oct 12, 2015 at 02:55:43PM +0000, David Aldrich wrote:
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.
I tend to use RuntimeError for program logic errors (code paths that shoudn't
happen, like not handling an option combination). I would not use it for this.
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 have mixed feeling about this; I feel that socket.error or IOError should
reflect actual OS level or IO subsystem errors, not higher level program
discrepancies such as invalid packet data. I would be included to subclass
StandardError.
Antipattern example: I discovered yesterday that PILLOW raises OSError when it
doesn't recognise an image file's content. I consider that a bad choice; I'd
prefer ValueError probably - there's been no OS level failure like lack of
permission to open the file.
On that basis, I recommend either just raising a ValueError for an invalid
packet or subclassing it.
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.
That seems reasonable, if he's using the recv timeout facility.
Cheers,
Cameron Simpson <c...@zip.com.au>
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