Python handling specific error codes?

Question

Hey I'm wondering how to handle specific error codes. For example, [Errno 111] Connection refused

I want to catch this specific error in the socket module and print something.

Solution 1

If you want to get the error code, this seems to do the trick;

import errno

try:
    socket_connection()
except socket.error as error:
    if error.errno == errno.ECONNREFUSED:
        print(os.strerror(error.errno))
    else:
        raise

You can look up errno error codes.

Solution 2

On Unix platforms, at least, you can do the following.

import socket, errno
try:
    # Do something...
except socket.error as e:
    if e.errno == errno.ECONNREFUSED:
        # Handle the exception...
    else:
        raise

Before Python 2.6, use e.args[ 0 ] instead of e.errno.

Solution 3

This seems hard to do reliably/portably but perhaps something like:

import socket

try:
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect(('localhost', 4167))
except socket.error, e:
    if 'Connection refused' in e:
        print '*** Connection refused ***'

which yields:

$ python socketexception.py 
*** Connection refused ***

Pretty yucky though.

Comments

[Yavor Angelov]

Why would you say this is unreliable/not portable? What is "yucky" about this?

[Thomas Wouters]

Using e.errno instead of e.args[0] is usually preferred (for exceptions that use errnos).

[Marc Abramowitz]

Because I am not 100% sure that the exception message on Windows would have "Connection refused" in it and not something similar but different like "Could not connect". Not sure if those error messages are standardize (e.g.: in POSIX) and besides not all platforms are necessarily POSIX-compliant.

[jchl]

On OSX, ECONNREFUSED appears to be 61 not 111, so hard-coding the value 111 would be a bad idea for portability.

[Utku Zihnioglu]

You are absolutely right. I am sure that there are more cases like that.

[jchl]

I thought that to begin with, but testing it out on my Mac it seemed that socket.error didn't have an errno member. It turns out that before Python 2.6, socket.error wasn't a subclass of IOError and so didn't have an errno member. But of course, before Python 2.6 the except t as e syntax wasn't valid either... I'll update my code.

[sum1stolemyname]

(-1) WARNING: This will fail completely in non-english locales.

[Marc Abramowitz]

Yeah, that's a good point. This will only work in an English locale. All in all, this is a pretty terrible idea and you should use errno like in the above answers.

[jtpereyda]

It's important to have an else: raise, otherwise all other error codes will be silently ignored!

[byxor]

Thanks. This resolved all visual warnings in PyCharm about errno not being a valid attribute of Exception, even though the code ran perfectly fine.

[Anatoly Alekseev]

@jtpereyda did you mean except: raise ?

[jtpereyda]

@AnatolyAlekseev no, I mean the same code with an else: raise tacked on the end. See @jchl answer for an example. By "all other error codes" I mean all other socket.error objects with errorcode != errno.ECONNREFUSED

[Anatoly Alekseev]

@jtpereyda this is weird then. I thought else: xxx is called when there were no exception inside try.. block. Docs for Python 3 confirm this. Was it different for python 2?

[jtpereyda]

@AnatolyAlekseev I mean an else that pairs with the if. So the else would be at the first indent level, within the except block.

[Jerther]

os.strerror(error.errno) will convert the error code to a message string. i.e.: os.strerror(104) returns 'Connection reset by peer'

[MestreLion]

+1 for the honesty of saying "Pretty yucky, non-portable, locale-dependent, terrible idea, please don't use" :-)