"Your python3 install is corrupted"

I just ran into this problem on Pop!_OS 18.04, trying to upgrade to 18.10, and it turns out that the problem lay in the symlink for /usr/bin/python and not for /usr/bin/python3. I had had /usr/bin/python3.6 configured as an alternative for python (not python3), and when I changed this, then I could run do-release-upgrade as expected.

I wish the error message pointed to python and not python3.

Before, with the problem:

$ update-alternatives --display python
python - manual mode
  link best version is /usr/bin/python3.6
  link currently points to /usr/bin/python2.7
  link python is /usr/bin/python
/usr/bin/python2.7 - priority 1
/usr/bin/python3.6 - priority 2 

I fixed it this way:

$ sudo update-alternatives --remove-all python
$ sudo ln -sf /usr/bin/python2.7 /usr/bin/python

Also see this comment below which describes a more precise solution that also better explains what is going on and how to fix it.

You need to use the default Python 3 version for 16.04. That's 3.5, not 3.6. So run:

sudo ln -sf /usr/bin/python3.5 /usr/bin/python3

If that doesn't work, try reinstalling the python3 package.

sudo apt-get install --reinstall python3

By the way, update-alternatives --display python3 should give you update-alternatives: error: no alternatives for python3. Different versions of Python are not alternatives in Ubuntu.

None of the answers here seem to explain how you can get to the solution yourself, so I took on a journey, in my case inspecting do-release-upgrade in KDE Neon on Ubuntu 18.04 LTS.

First, I ran it with tracefile -w and discovered that the actual release-upgrade-scripts were downloaded into a /tmp/ubuntu-release-upgrader-xxxxxxxx directory.

Using grep in that directory, I found the error message in DistUpgradeController.py:

❯ grep --line-number --recursive --binary-files=without-match "python3 install is corrupted"
DistUpgradeController.py:426:                             _("Your python3 install is corrupted. "

So I inspected the surrounding code, which used the function _pythonSymlinkCheck, jumped to that and discovered the root of the problem: The script expected the symlink /usr/bin/python3 to resolve to exactly /usr/bin/<debian_default_python>:

binaries_and_dirnames = [("python3", "python3")]
for binary, dirname in binaries_and_dirnames:
    debian_defaults = '/usr/share/%s/debian_defaults' % dirname
    if os.path.exists(debian_defaults):
        config = SafeConfigParser()
        with open(debian_defaults) as f:
            config.readfp(f)
        try:
            expected_default = config.get('DEFAULT', 'default-version')
        except NoOptionError:
            logging.debug("no default version for %s found in '%s'" %
                          (binary, config))
            return False
        try:
            fs_default_version = os.readlink('/usr/bin/%s' % binary)
        except OSError as e:
            logging.error("os.readlink failed (%s)" % e)
            return False
        if not fs_default_version in (expected_default, os.path.join('/usr/bin', expected_default)):

As visible from the script, <debian_default_python> is the default-version key in the DEFAULT section from /usr/share/python3/debian_defaults:

❯ cat /usr/share/python3/debian_defaults
[DEFAULT]
# the default python3 version
default-version = python3.6

My link did point to /usr/bin/python3.6, but via an extra indirection from update-alternatives, which the script doesn't resolve:

❯ python
Python 3.6.9 (default, Jul 17 2020, 12:50:27)
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.readlink("/usr/bin/python3")
'/etc/alternatives/python3'
>>> os.readlink("/etc/alternatives/python3")
'/usr/bin/python3.6'

So in the end I also resorted to the nuclear option, but now with full knowledge of what was going on :)

sudo ln -sf /usr/bin/python3.6 /usr/bin/python3

I observed this error message on Windows 10 1903 running WSL Ubuntu when I wanted to upgrade from 16.04 LTS to 18.04 LTS.

After do-release-upgrade had failed, I switched python alternatives to every choice offered by update-alternatives --config python and ran the upgrade command again. That did not help.

Then I checked the log file /var/log/dist-upgrade/main.log which contained the lines

2019-09-02 20:58:08,686 DEBUG _pythonSymlinkCheck run
2019-09-02 20:58:08,687 DEBUG python symlink points to: '/etc/alternatives/python', but expected is 'python2.7' or
'/usr/bin/python2.7'
2019-09-02 20:58:08,688 ERROR pythonSymlinkCheck() failed, aborting

So although the error message mentions python3, the issue is about python2.

The upgrade script checks for /usr/bin/python linking to /usr/bin/python2, see the source code of DistUpgrade/DistUpgradeController.py here: ubuntu launchpad

So one solution is to completely remove python from the alternative system and add the link manually, as described in the most popular answer.

If you don't want to remove python from the alternative system, just change the link only for the time during the upgrade process:

# rm /usr/bin/python 
# ln -sf /usr/bin/python2.7 /usr/bin/python
# do-release-upgrade

This worked for me.

During the upgrade process, the link is automatically repaired. So when the upgrade is finished, it points to the python entry in the alternatives directory:

$ ls -l /usr/bin/python
lrwxrwxrwx 1 root root 24 Sep  2 22:01 /usr/bin/python -> /etc/alternatives/python

Edit: for thorough information, the issue might also appear if you upgrade from 18.04 LTS to 19.04 and the anwser applies to this situation, too.

Basically the solution to this problem consists of making /usr/bin/python point to the right version of Python your Ubuntu release expects (for instance, in 16.04 was Python2.7 and in 18.04 was Python3.6).

If you have several versions of Python installed in your system, you might be using update-alternatives to manage them. It doesn't matter much your default alternative for Python is the right version your system expects (3.6 in Ubuntu 18.04), it won't work.

The reason why this doesn't work is that, when using update-alternatives, /usr/bin/python3 points to /etc/alternatives/python3, and it seems that's not exactly the same as making /usr/bin/python3 point to /usr/bin/python3.6.

That's why the solution to this problem often consists of stop managing your Python3 versions with update-alternatives and make /usr/bin/python3 point to the right version of Python3 your system expects.

The following commands should work:

sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.5 2
sudo rm /usr/bin/python
sudo ln -s /usr/bin/python2.7 /usr/bin/python
sudo do-release-upgrade

I just tried upgrading to jammy (22.04) and hit the same error. The error comes down to an incorrect symlink check in the upgrader package. I've submitted a bug report with a fix in here.

What you can do to work around the problem is have your python3 link directly to the final executable (without the detour via /etc/alternatives/python3):

sudo ln -s /usr/bin/python3.9 /usr/bin/python3

Here is the fix when I tried to upgrade my xenial 16.04 box to 18.04

mkdir /usr/share/python2/
cp /usr/share/python/debian_defaults /usr/share/python2/debian_defaults
cd /usr/bin/python2 /usr/bin/python.27

otherwise, the ubuntu-release-upgrader will complain with messages like

/usr/lib/python3$ sudo do-release-upgrade 
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [819 B]                                                                                                                                                                               
Get:2 Upgrade tool [1,245 kB]                                                                                                                                                                                      
Fetched 1,245 kB in 0s (0 B/s)                                                                                                                                                                                     
authenticate 'bionic.tar.gz' against 'bionic.tar.gz.gpg' 
extracting 'bionic.tar.gz'

Reading cache

Checking package manager

Can not upgrade 

Your python3 install is corrupted. Please fix the '/usr/bin/python3' 
symlink. 

The major log file is located at

/var/log/dist-upgrade/main.log

and there the release updater complained. Also, the other file you may want to check out is actually in /tmp/ubuntu-release-upgrader/*

less /tmp/ubuntu-release-upgrader/*/DistUpgradeController.py