Socket exceptions aren't in the standard exception hierarchy

[John Nagle]

Steve Holden wrote:
> John Nagle wrote:
> [socket.error bug report]
> 
>>>
>>> Where did you get this information? If true it would certainly need 
>>> to be logged as a bug, but under Windows on 2,4 I see
>>>
>>>  >>> issubclass(socket.gaierror, Exception)
>>> True
>>>  >>>
>>>
>>> and the same under Cygwin 2.5. I am presuming most other users will 
>>> see the same thing.
>>>
>>> regards
>>>  Steve
>>
>>
>>      Ah.  "socket.error" is a subclass of "Exception", but not
>> of "StandardError".
>>
>>      issubclass(socket.error,StandardError)
>>
>> is False.
>>
> Right, so this *is* a bug, as long as Brett Cannon's upcoming (further) 
> reorganization of the standard exception hierarchy doesn't stamp on it. 
> It probably *was* overlooked in the reorganization of the hierarchy, and 
> this implies there may be other extensions that also make the same error.
> 
> It should be logged as a bug in the tracker - the fix is probably pretty 
> simple, but it'll need some consideration of the forward compatibility 
> considerations.
> 
> regards
>  Steve

   The bug tracker shows quite a number of problems related to
socket exceptions.  Searching for "socket AND exception" returns
45 results.  These seem to be the most relevant ones:

[ 805194 ] Inappropriate error received using socket timeout
[ 1019808 ] wrong socket error returned
[ 1571878 ] Improvements to socket module exceptions
[ 708927 ] socket timeouts produce wrong errors in win32

    None of them quite cover this new issue, but it's clear
that the area needs some work.  Should this new issue
be added as a new bug or as an added comment to one of the
above?

    Just figuring out what exceptions can be raised from
the socket module is tough.  I've seen exceptions
derived from "socket.error", exceptions from IOError,
and exceptions from the SSL layer, which patches the
sockets module when loaded.  These are non-bug
exceptions; that is, the problem is out in the network,
external to the program.  I'm still not sure I have all
the possibilities covered.

    Retrying on unknown exceptions isn't the answer;
that leads to exception loops if there's a program bug.
The most important distinction with sockets is "external
network problem" vs. "local program program".  I'd like
to see a "NetworkException" in the exception hierarchy,
with all the things that can go wrong due to conditions
external to the local machine under that exception.

    I'd suggest the following:

    1.  Add "NetworkError" under "IOError" in
the exception hierarchy.

    2.  Put the existing "socket.error" under "NetworkError".
Since "socket.error" needs to be reparented anyway (it's currently
a direct descendant of "Exception") this provides a good place for it.

    3.  Find any places where the socket module can raise IOError
or OSError due to an external network condition, and make them raise
something under NetworkError instead.  Code that catches IOError
will still work.

    4.  Put all errors in the various SSL modules (SSLError, etc.)
which can be raised due to external network conditions under "NetworkError".

    5.  Move "urllib2.URLError", which is currently under IOError, to be
under NetworkError.

    6.  Move the misc. errors from "urllib", like "ContentTooShortError",
which are currently under IOError, down a level under NetworkError.

    Then, programs that catch NetworkError could be sure of catching
all network trouble conditions, but not local code bugs.

				John Nagle