Comment 0 for bug 1825655

do-release-upgrade from 18.04 to 18.10 fails:

"
Reading cache

Checking package manager

Can not upgrade

Your python3 install is corrupted. Please fix the '/usr/bin/python3'
symlink.
=== Command terminated with exit status 1 (Sat Apr 20 17:19:49 2019) ===
"

There is nothing wrong with /usr/bin/python3

➜ ~ ls -al /usr/bin/python3
lrwxrwxrwx 1 root root 9 окт 25 14:11 /usr/bin/python3 -> python3.6

I also have python2 installed for which Debian alternatives are configured which causes the problem as far as I can see:

➜ ~ update-alternatives --display python
python - auto mode
  link best version is /usr/bin/python2.7
  link currently points to /usr/bin/python2.7
  link python is /usr/bin/python
/usr/bin/python2.7 - priority 100

https://git.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/tree/DistUpgrade/DistUpgradeController.py?h=ubuntu/bionic-updates#n361

(Pdb) l
 11 debian_defaults = '/usr/share/%s/debian_defaults' % dirname
 12 if os.path.exists(debian_defaults):
 13 config = SafeConfigParser()
 14 with open(debian_defaults) as f:
 15 config.readfp(f)
 16 -> try:
 17 expected_default = config.get('DEFAULT', 'default-version')
 18 except NoOptionError:
 19 logging.debug("no default version for %s found in '%s'" %
 20 (binary, config))
 21 return False
(Pdb) n
> <ipython-input-13-93c4105e7796>(17)_pythonSymlinkCheck()
-> expected_default = config.get('DEFAULT', 'default-version')
(Pdb) n
> <ipython-input-13-93c4105e7796>(22)_pythonSymlinkCheck()
-> try:
(Pdb) expected_default
'python2.7'

> <ipython-input-13-93c4105e7796>(23)_pythonSymlinkCheck()
-> fs_default_version = os.readlink('/usr/bin/%s' % binary)
(Pdb) n
> <ipython-input-13-93c4105e7796>(27)_pythonSymlinkCheck()
-> if not fs_default_version in (expected_default, os.path.join('/usr/bin', expected_default)) and not (binary == 'python' and fs_default_version in ('python2', '/usr/bin/python2')):
(Pdb) fs_default_version
'/etc/alternatives/python'
(Pdb) os.path.join('/usr/bin', expected_default)
'/usr/bin/python2.7'

(Pdb) not fs_default_version in (expected_default, os.path.join('/usr/bin', expected_default))
True

(Pdb) print("%s symlink points to: '%s', but expected is '%s' or '%s'" % (binary, fs_default_version, expected_default, os.path.join('/usr/bin', expected_default)))
python symlink points to: '/etc/alternatives/python', but expected is 'python2.7' or '/usr/bin/python2.7'