## 17.1.3 An example

Let us say that we want a slightly more relaxed policy than the standard `RExec` class. For example, if we're willing to allow files in /tmp to be written, we can subclass the `RExec` class:

    class TmpWriterRExec(rexec.RExec):
        def r_open(self, file, mode='r', buf=-1):
            if mode in ('r', 'rb'):
                pass
            elif mode in ('w', 'wb', 'a', 'ab'):
                # check filename : must begin with /tmp/
                if file[:5]!='/tmp/':
                    raise IOError, "can't write outside /tmp"
                elif (string.find(file, '/../') >= 0 or
                     file[:3] == '../' or file[-3:] == '/..'):
                    raise IOError, "'..' in filename forbidden"
            else: raise IOError, "Illegal open() mode"
            return open(file, mode, buf)

Notice that the above code will occasionally forbid a perfectly valid filename; for example, code in the restricted environment won't be able to open a file called /tmp/foo/../bar. To fix this, the `r_open()` method would have to simplify the filename to /tmp/bar, which would require splitting apart the filename and performing various operations on it. In cases where security is at stake, it may be preferable to write simple code which is sometimes overly restrictive, instead of more general code that is also more complex and may harbor a subtle security hole.