[Python-ideas] PEP-3151 pattern-matching

[Antoine Pitrou]

On Fri, 8 Apr 2011 14:50:02 -0700
Guido van Rossum <guido at python.org> wrote:
> 
> > You still have to type an explicit condition, and
> > remember the appropriate errno mnemonic for the situation. The very
> > notion of "errno" and its plethora of abbreviated error codes is ok for
> > people used to C programming, but certainly alien to other programmers
> > (how long does it take to remember that "file not found" is spelled
> > "ENOENT"?).
> 
> Well, this is a fact of life. There are hundreds of errno codes and
> they vary across systems, both in names and in numbers (even if there
> is a handful that exist nearly everywhere and almost always have the
> same values). This is the nature of system calls, and some operating
> systems just have a richer set of error conditions that they present
> to the (advanced) programmer.

Well, errnos are only the POSIX API side of system calls.
For example, a seasoned Windows C programmer might not have faced a
single EBADF or ENOENT in their entire career, since the Win32 API
doesn't use those.
Similarly, java.io.IOException doesn't seem to have any errno-alike
member, but it does have subclasses named e.g. FileNotFoundException.
(http://download.oracle.com/javase/1.5.0/docs/api/java/io/IOException.html)
Looking at .Net, System.IO.IOException does have a numerical "HResult"
member, but it doesn't seem related to POSIX errnos (“HRESULT is a
32-bit value, divided into three different fields: a severity code, a
facility code, and an error code”). It also has a couple of subclasses
such as FileNotFoundException.
(http://msdn.microsoft.com/en-us/library/system.io.ioexception.aspx)

> > The fact that exception messages typically indicate the errno *number*,
> > not mnemonic, is an additional annoyance (errno numbers are not
> > standardized and can vary from system to system).
> 
> That would seem to be a problem with the str() and/or repr() of
> OSError, and perhaps we can fix it there.

True.

Regards

Antoine.