posix.error is a tuple, not an object or a string as docs say
Michael P. Reilly
arcege at shore.net
Tue Aug 10 21:08:26 CEST 1999
Ben Gertzfield <che at debian.org> wrote:
: Today I was trying to use the posix.fdopen() function (through
: os.fdopen() as recommended in the documentation) so that I could open
: a pre-existing file descriptor passed in through sys.argv to a normal
: Python file object.
: Obviously I want to be able to catch errors in case the file
: descriptor passed through sys.argv isn't legal, so I read the nice
: documentation for posix.error:
: This exception is raised when a POSIX function returns a
: POSIX-related error (e.g., not for illegal argument types). The
: accompanying value is a pair containing the numeric error code
: from errno and the corresponding string, as would be printed by
: the C function perror(). See the module errno , which contains
: names for the error codes defined by the underlying operating
: When exceptions are classes, this exception carries two
: attributes, errno and strerror. The first holds the value of
: the C errno variable, and the latter holds the corresponding
: error message from strerror().
: When exceptions are strings, the string for the exception is
: 'os.error'; this reflects the more portable access to the
: exception through the os module.
: This doesn't seem to be quite true, however:
: Python 1.5.1 (#1, Dec 17 1998, 20:58:15) [GCC 188.8.131.52] on linux2
: Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
:>>> import os
: ... input = os.fdopen(42)
: ... except os.error, error:
: ... print error
: (9, 'Bad file descriptor')
: 'error', in this case, is a tuple, not an object. Am I incorrect in
: thinking this? If so, how do I access the aforementioned attributes
: errno and strerror?
: Obviously it's no problem to just say:
: except os.error, error:
: errnum, errstr = error
: but this contradicts the documentation for posix.error -- and there is
: no documentation for a separate os.error, so I assume they are one and
: the same.
: I guess I'm missing something major, but it's odd that the docs don't
: match reality at all.
It sounds like you are using a release of either Python 1.5 or 1.5.1.
If you are using Python 1.4 or earlier, then yes, it is a tuple. If
Python 1.5 or later, then it is an instance of an exception class.
And this is polymorphism at it's best. From the appearance, it looks
like a tuple, but if you perform some interspection on the "error"
object, you will see it is an instance of IOError and inherits the
"__str__" method from the Exception class, which converts the arguments
to the constructor into a string.
The long and short of this is that you can use it like a tuple if more
than one argument was given, and as the object otherwise.
For some of your own reassurance, print the type of "error" in the
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