Python errno

Introduction

You probably won’t come across errno until you have been using Python for a little while. When you do, it may not be immediately clear why you need it.

Here’s a code snippet to view the full range of errors:

import errno
import os

print {i: os.strerror(i) for i in sorted(errno.errorcode)}

The docs give the following intro:

This module makes available standard errno system symbols. The value of each symbol is the corresponding integer value. The names and descriptions are borrowed from linux/include/errno.h, which should be pretty all-inclusive.

OK, but why do we need these guys? We can already raise exceptions, such as IOError or TypeError…

Well, what you need to know is that most of the standard exceptions cater for a “bucket” of different errors, and there will be times when you need to be able to distinguish between different errors which are from the same “bucket”. You could also create a custom exception, but often times this is overkill.

(Bad) Example

Both ‘invalid path’ and ‘file does not exist’ fall under IOError.

Here’s how we would distinguish between them:

file does not exist

import errno
import os

try:
	my_file = open('no.txt')

except IOError, e:
    # 'No such file or directory'
    if e.errno == 2:
        print(e.strerror)
        print("this will print")
        # handle one way
    elif e.errno == 9:
        print(e.strerror)
        print("this will not print")
        # handle another way
        

Avoiding Confusion

The above code will run fine, but we should ensure that our code is portable. If we compare against specific errno values (which may be different on other OS/platforms) then we may run into problems

Instead, we compare against the names of the errors, which the errno library provides. Let’s re-write the above code to take this into consideration:

import errno
import os

try:
	my_file = open('no.txt')

except IOError, e:
    # 'No such file or directory'
    if e.errno == errno.ENOENT:
        print(e.strerror)
        print("this will print")
        # handle one way
    elif e.errno == errno.EBADF:
        print(e.strerror)
        print("this will not print")
        # handle another way
        

If we ever need to get the human readable form of an error, then we use os.strerror

os.strerror(errno.EBADF)`
# >>> 'Bad file descriptor'

Now we have achieved cross-platform clarity with our error handling.