Python broken after upgrading some aspects to 3.x

Question

I think the last thing I did was use update-alternatives but I had made many little changes to python so am not entirely sure. However, I was then unable to apt-get update my system. I received message like the following:

sh: 1: /usr/lib/cnf-update-db: Permission denied E: Problem executing scripts APT::Update::Post-Invoke-Success ....

I have tried all the online fixes that involve dpkg / -configure etc... but they also result in failure due to permission failures.

I have learned that if you try to force uninstall all python related in an attempt to solve this you will most likely hose your entire Ubuntu installation

What sayeth the group?

score 0 · Answer 1 · answered Jan 24 '22 at 22:40

Partial Credit: How to update-alternatives to Python 3 without breaking apt?

Here is the road I went down to solve this problem :-)

sudo update-alternatives --query python

Name: python
Link: /usr/bin/python
Status: auto
Best: /usr/bin/python3
Value: /usr/bin/python3

Alternative: /usr/bin/Python-3.7.0/python
Priority: 1

Alternative: /usr/bin/python3
Priority: 10

Alternative: /usr/bin/python3.8
Priority: 2

By default, the HIGHEST number (10) as the default python distro for the OS (but this can be overridden)

Then Ran:

# to get rid of one than I last installed that started my problems 
command: sudo update-alternatives --remove python /usr/bin/Python-3.7.0/python
reply  : update-alternatives: warning: forcing reinstallation of alternative /usr/bin/python3 because link group python is broken

Then ran this again to get a new view of what is contained in update-alternatives

sudo update-alternatives --query python
Name: python
Link: /usr/bin/python
Status: auto
Best: /usr/bin/python3
Value: /usr/bin/python3

Alternative: /usr/bin/python3
Priority: 10

Alternative: /usr/bin/python3.8
Priority: 2

Wonderful - problem python removed. But I noticed 2.7 was not in the mix and I think that's what used to be the default in my 20.04 xfce distro so ran

You have to supply a priority number so I gave it new 'high' of 12 which should make the OS priority

Command: sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 12
Replied: update-alternatives: using /usr/bin/python2.7 to provide /usr/bin/python (python) in auto mode

The above seems to indicate it made 2.7 the default. Let's have a look sudo update-alternatives --query python

Indeed it did !

sudo update-alternatives --query python
Name: python
Link: /usr/bin/python
Status: auto
Best: /usr/bin/python2.7
Value: /usr/bin/python2.7

Alternative: /usr/bin/python2.7
Priority: 12

Alternative: /usr/bin/python3
Priority: 10

Alternative: /usr/bin/python3.8
Priority: 2

Ran: python -V
Replied: Python 2.7.18

Command: sudo update-alternatives --config python
Reply: 
There are 3 choices for the alternative python (providing /usr/bin/python).

  Selection    Path                Priority   Status
------------------------------------------------------------
* 0            /usr/bin/python2.7   12        auto mode
  1            /usr/bin/python2.7   12        manual mode
  2            /usr/bin/python3     10        manual mode
  3            /usr/bin/python3.8   2         manual mode

Press <enter> to keep the current choice[*], or type selection number:

This is great. Allows me to switch between them and test the results

At this point I have a better understanding of Python configs but apt-get update still hosed...

So so more reading led me here.

cd /etc/alternatives
ls -l python*
lrwxrwxrwx 1 root root 18 Jan 24 13:54 python -> /usr/bin/python2.7
lrwxrwxrwx 1 root root 18 Jan 24 13:59 python3 -> /usr/local/bin/python3.7

That second one looked wonky. Was pointing to /usr/local and not /usr/bin so wasn't global. So I linked it to the 3.7 in /usr/bin with the following

sudo rm /etc/alternatives/python3
sudo ln -s /usr/bin/python3.7 /etc/alternatives/python3

Then had another go at it

sudo apt-get update 

Hit:1 http://us.archive.ubuntu.com/ubuntu focal InRelease
Get:2 https://pkgs.tailscale.com/stable/ubuntu focal InRelease
Hit:3 http://us.archive.ubuntu.com/ubuntu focal-updates InRelease
Hit:4 http://ppa.launchpad.net/ansible/ansible/ubuntu focal InRelease
Get:5 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]
Get:6 http://us.archive.ubuntu.com/ubuntu focal-backports InRelease [108 kB]
Hit:7 http://ppa.launchpad.net/ondrej/php/ubuntu focal InRelease
Fetched 228 kB in 2s (112 kB/s)
Reading package lists... Done

RESOLVED !!!!!

This was such a critical problem and resulting solution that I created this post so others could find this needle in the haystack

Python broken after upgrading some aspects to 3.x

1 Answers1