The errno module defines a number of symbolic error codes, such as ENOENT (“no such directory entry”) and EPERM (“permission denied”). It also provides a dictionary mapping from platform-dependent numerical error codes to symbolic names. Example 2-21 shows how to use errno.

In most cases, the IOError exception provides a 2-tuple with the numerical error code and an explanatory string. If you need to distinguish between different error codes, use the symbolic names where possible.

File: errno-example-1.py

import errno

try:
    fp = open("no.such.file")
except IOError, (error, message):
    if error == errno.ENOENT:
        print "no such file"
    elif error == errno.EPERM:
        print "permission denied"
    else:
        print message

no such file

Example 2-22 is a bit contrived, but it shows how to use the errorcode dictionary to map from a numerical error code to the symbolic name.

File: errno-example-2.py

import errno

try:
    fp = open("no.such.file")
except IOError, (error, message):
    print error, repr(message)
    print errno.errorcode[error]

# 2 'No such file or directory'
# ENOENT