In Python 3.5.3, a socket with type AF_CAN returns a tuple in the form (interface, ) from getsockname(). In Python 3.7.3, getsockname() returns a string (the name of the interface). The documentation states "a tuple is used for the AF_CAN address family". The string will break code that worked in 3.5.3 by raising errors such as "Value Error: too many values to unpack (expected 2)".

Example:

#3.5.3
import socket
s = socket.socket(socket.AF_CAN, socket.SOCK_RAW, socket.CAN_RAW)
s.bind(('vcan0',)) # requires tuple
s.getsockname() # returns tuple: ('vcan0', 29)

#3.7.3
import socket
s = socket.socket(socket.AF_CAN, socket.SOCK_RAW, socket.CAN_RAW)
s.bind(('vcan0',)) # requires tuple
s.getsockname() # returns string: 'vcan0'

Changed caused by commit effc12f to cpython/Modules/socketmodule.c on Sep 6, 2017.

Correction, change caused by a30f6d4 to cpython/Modules/socketmodule.c on Aug 28, 2017.

New changeset 954900a by Benjamin Peterson (bggardner) in branch 'master':
closes bpo-37405: Make socket.getsockname() always return a tuple for AF_CAN. (GH-14392)
954900a

New changeset f60fd95 by Benjamin Peterson (Miss Islington (bot)) in branch '3.8':
closes bpo-37405: Make socket.getsockname() always return a tuple for AF_CAN. (GH-14392) (GH-16018)
f60fd95

$ make test TESTOPTS="-v test_socket"

ERROR: testSendFrame (main.CANTest)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/s/opt/cpython/debug/test_socket.py", line 2052, in testSendFrame
    self.assertEqual(addr[1], socket.AF_CAN)
IndexError: tuple index out of range

$ cat ../.git/config
https://github.com/bennett78/cpython.git

$ uname -r
5.4.0-67-genericg$ cat /etc/issue
Ubuntu 20.04.2 LTS \n \l

$ /s/opt/cpython/debug$ ./python -V
Python 3.10.0a6+

This test was overlooked, and line 2052 should now be removed per the discussion here:

#14392 (comment)

In short, getsockname() used to return (interface, socket.AF_CAN), in which the socket.AF_CAN element was superfluous (also undocumented), so it was removed in the fix, and the tuple format was reinstated.