Core Python Programming (2nd Edition)

The entire purpose of the updArgs() function is to "update" the exception arguments. What we mean
here is that the original exception is going to provide us a set of arguments. We want to take these
arguments and make them part of our new exception, perhaps embellishing or adding a third argument
(which is not added if nothing is givenNone is a default argument, which we will study in the next
chapter). Our goal is to provide the more informative details to the user so that if and when errors
occur, the problems can be tracked down as quickly as possible.
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The fileArgs() function is used only by myopen() (see below). In particular, we are seeking error EACCES,
which represents "permission denied." We pass all other IOError exceptions along without modification
(lines 54 - 55). If you are curious about ENXIO, EACCES, and other system error numbers, you can hunt
them down by starting at file /usr/include/sys/errno.h on a Unix system, or C:\Msdev\include\Errno.h if
you are using Visual C++ on Windows.
In line 27, we are also checking to make sure that the machine we are using supports the os.access()
function, which helps you check what kind of file permissions you have for any particular file. We do not
proceed unless we receive both a permission error as well as the ability to check what kind of
permissions we have. If all checks out, we set up a dictionary to help us build a string indicating the
permissions we have on our file.
The Unix file system uses explicit file permissions for the user, group (more than one user can belong to
a group), and other (any user other than the owner or someone in the same group as the owner) in
read, write, and execute (`r', `w', `x') order. Windows supports some of these permissions.
Now it is time to build the permission string. If the file has a permission, its corresponding letter shows
up in the string, otherwise a dash ( - ) appears. For example, a string of "rw-" means that you have read
and write access to it. If the string reads "r-x", you have only read and execute access; "---" means no
permission at all.
After the permission string has been constructed, we create a temporary argument list. We then alter
the error string to contain the permission string, something that standard IOError exception does not
provide. "Permission denied" sometimes seems silly if the system does not tell you what permissions
you have to correct the problem. The reason, of course, is security. When intruders do not have
permission to access something, the last thing you want them to see is what the file permissions are,
hence the dilemma. However, our example here is merely an exercise, so we allow for the temporary
"breach of security." The point is to verify whether or not the os.chmod()functions call affected file
permissions the way they are supposed to.
The final thing we do is to add the filename to our argument list and return the set of arguments as a
tuple.
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Our new myconnect() function simply wraps the standard socket method connect()to provide an IOError -
type exception if the network connection fails. Unlike the general socket.error exception, we also
provide the hostname and port number as an added value to the programmer.
For those new to network programming, a hostname and port number pair are analogous to an area
code and telephone number when you are trying to contact someone. In this case, we are trying to
contact a program running on the remote host, presumably a server of some sort; therefore, we require