Any problem with accessing files, whether it is the inability to find a file, a problem with reading or writing a file, or trying to open a protected system file or even a directory, leads to an IOError exception. Since problems with file access are quite common and usually at least partly outside the realm of control of the program, it is a good idea to handle IOError exceptions in your programs when you can.

Since many different things can go wrong with files, the args tuple explained above might be used to provide better information on what you have to do to handle the problem. For instance, if your program asked the user to supply a filename, and when you open that file you get an IOError, if the error number (the first element of the tuple) indicates that the file does not exist (2), then an appropriate response might be to simply report this to the user and ask for a new name.

The error numbers are defined in the errno module, which you can import in your program. The module offers constants that you can use instead of the actual numbers, which is the convention. The most common error numbers are:

• errno.ENOENT: No such file or directory. You get this when you try to access a file that does not exist.

• errno.EACCESS: Permission denied. You can get this in varied circumstances, such as when you try to read from a closed file, when you try to open a read-only file for writing, or when you try to open a directory as if it is a file.

• errno.ENOSPC: No more space left on device. You get this when you try to write a file and there is no room for it, for instance when you try to write to a USB-stick that is full.

There is a big list of such error numbers which you can easily find in the reference manuals. You might not understand what all of them refer to, and actually many of them are archaic and no longer occur on modern computer systems. The best thing to do when you develop your program, is to try to capture IOErrors, and when you do encounter an IOError, print the arguments so that you know the number and the error message. You can then look up which errno constant that message belongs to, and respond to it in your program if you can do that in a sensible way.

However, just as with other kinds of exceptions, it is better to avoid them than to capture them. There is no reason that you should ever encounter a “file does not exist” error, as you can test whether a file exists with the exists() and isfile() functions from the os.path module.

import errno

try:
    fp = open( "NotAFile" )
    fp.close()
except IOError as ex:
    if ex.args[0] == errno.ENOENT:
        print( "File not found!" )
    else:
        print( ex.args[0], ex.args[1] )

Note

Exception FileNotFoundError is a “subclass” (see Chapter 23¹) of IOError, which, in Python 3, is actually an alias for yet another exception, namely OSError. This means that capturing FileNotFoundError is equivalent with capturing IOError (or OSError) as ex and testing whether ex.args[0] holds errno.ENOENT.