How to update-alternatives to Python 3 without breaking apt?
PythonLinuxUbuntuDebianAptPython Problem Overview
The other day I decided that I wanted the command python to default to firing up python3 instead of python2.
So I did this:
$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 2
$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.5 3
$ sudo update-alternatives --config python
$ sudo update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/python3.5 3 auto mode
1 /usr/bin/python2.7 2 manual mode
2 /usr/bin/python3.5 3 manual mode
Press <enter> to keep the current choice[*], or type selection number: 0
And that all worked. Great! :)
$ python -V
Python 3.5.2
But it wasn't long before I realised I had broken apt/aptitude when it comes to installing and removing python packages because apt was expecting python2 it transpired.
This is what happened.
$ sudo apt remove python-samba
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following package was automatically installed and is no longer required:
samba-libs
Use 'sudo apt autoremove' to remove it.
The following packages will be REMOVED:
python-samba
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
After this operation, 5,790 kB disk space will be freed.
Do you want to continue? [Y/n]
(Reading database ... 187285 files and directories currently installed.)
Removing python-samba (2:4.3.11+dfsg-0ubuntu0.16.04.5) ...
File "/usr/bin/pyclean", line 63
except (IOError, OSError), e:
^
SyntaxError: invalid syntax
dpkg: error processing package python-samba (--remove):
subprocess installed pre-removal script returned error exit status 1
Traceback (most recent call last):
File "/usr/bin/pycompile", line 35, in <module>
from debpython.version import SUPPORTED, debsorted, vrepr, \
File "/usr/share/python/debpython/version.py", line 24, in <module>
from ConfigParser import SafeConfigParser
ImportError: No module named 'ConfigParser'
dpkg: error while cleaning up:
subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
python-samba
E: Sub-process /usr/bin/dpkg returned an error code (1)
Eventually I guessed it wanted python2 as the default, so I undid my changes as follows:
$ sudo update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/python3.5 3 auto mode
1 /usr/bin/python2.7 2 manual mode
2 /usr/bin/python3.5 3 manual mode
Press <enter> to keep the current choice[*], or type selection number: 1
$ python -V
Python 2.7.12
And then apt worked again
$ sudo apt remove python-samba
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following package was automatically installed and is no longer required:
samba-libs
Use 'sudo apt autoremove' to remove it.
The following packages will be REMOVED:
python-samba
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
1 not fully installed or removed.
After this operation, 5,790 kB disk space will be freed.
Do you want to continue? [Y/n]
(Reading database ... 187285 files and directories currently installed.)
Removing python-samba (2:4.3.11+dfsg-0ubuntu0.16.04.5) ...
So I have had to leave it as defaulting to python 2 but I develop in python 3 and so would like my system to default to python 3 for when I run python and idle.
Can anyone tell me how I can achieve this without breaking apt?
My system is a Raspberry Pi 3B running Ubuntu:
Linux mymachine 4.4.38-v7+ #938 SMP Thu Dec 15 15:22:21 GMT 2016 armv7l armv7l armv7l GNU/Linux
(It's actually an arm v8)
$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=16.04
DISTRIB_CODENAME=xenial
DISTRIB_DESCRIPTION="Ubuntu 16.04.2 LTS"
Python Solutions
Solution 1 - Python
Per Debian policy, python
refers to Python 2 and python3
refers to Python 3. Don't try to change this system-wide or you are in for the sort of trouble you already discovered.
Virtual environments allow you to run an isolated Python installation with whatever version of Python and whatever libraries you need without messing with the system Python install.
With recent Python 3, venv
is part of the standard library; with older versions, you might need to install python3-venv
or a similar package.
$HOME~$ python --version
Python 2.7.11
$HOME~$ python3 -m venv myenv
... stuff happens ...
$HOME~$ . ./myenv/bin/activate
(myenv) $HOME~$ type python # "type" is preferred over which; see POSIX
python is /home/you/myenv/bin/python
(myenv) $HOME~$ python --version
Python 3.5.1
A common practice is to have a separate environment for each project you work on, anyway; but if you want this to look like it's effectively system-wide for your own login, you could add the activation stanza to your .profile
or similar.
Solution 2 - Python
replace
[bash:~] $ sudo update-alternatives --install /usr/bin/python python \
/usr/bin/python2.7 2
[bash:~] $ sudo update-alternatives --install /usr/bin/python python \
/usr/bin/python3.5 3
with
[bash:~] $ sudo update-alternatives --install /usr/local/bin/python python \
/usr/bin/python2.7 2
[bash:~] $ sudo update-alternatives --install /usr/local/bin/python python \
/usr/bin/python3.5 3
e.g. installing into /usr/local/bin
instead of /usr/bin
.
and ensure the /usr/local/bin
is before /usr/bin
in PATH.
i.e.
[bash:~] $ echo $PATH
/usr/local/bin:/usr/bin:/bin
Ensure this always is the case by adding
export PATH=/usr/local/bin:$PATH
to the end of your ~/.bashrc
file. Prefixing the PATH
environment variable with custom bin folder such as /usr/local/bin
or /opt/<some install>/bin
is generally recommended to ensure that customizations are found before the default system ones.
Solution 3 - Python
For anyone finding this question now in 2021, pretty much all of the earlier answers are obsolete.
It is perfectly fine, and expected now, for /usr/bin/python
to point to Python 3. Python 2 is not being updated with security fixes, so any system still using it should be upgraded to use Python 3 as the system Python. Any modern distribution should already have solved potential incompatibilities in upgrading the system Python to Python 3.
Solution 4 - Python
As I didn't want to break anything, I did this to be able to use newer versions of Python3 than Python v3.4 :
$ sudo update-alternatives --install /usr/local/bin/python3 python3 /usr/bin/python3.6 1
update-alternatives: using /usr/bin/python3.6 to provide /usr/local/bin/python3 (python3) in auto mode
$ sudo update-alternatives --install /usr/local/bin/python3 python3 /usr/bin/python3.7 2
update-alternatives: using /usr/bin/python3.7 to provide /usr/local/bin/python3 (python3) in auto mode
$ update-alternatives --list python3
/usr/bin/python3.6
/usr/bin/python3.7
$ sudo update-alternatives --config python3
There are 2 choices for the alternative python3 (providing /usr/local/bin/python3).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/python3.7 2 auto mode
1 /usr/bin/python3.6 1 manual mode
2 /usr/bin/python3.7 2 manual mode
Press enter to keep the current choice[*], or type selection number: 1
update-alternatives: using /usr/bin/python3.6 to provide /usr/local/bin/python3 (python3) in manual mode
$ ls -l /usr/local/bin/python3 /etc/alternatives/python3
lrwxrwxrwx 1 root root 18 2019-05-03 02:59:03 /etc/alternatives/python3 -> /usr/bin/python3.6*
lrwxrwxrwx 1 root root 25 2019-05-03 02:58:53 /usr/local/bin/python3 -> /etc/alternatives/python3*
Solution 5 - Python
Somehow python 3 came back (after some updates?) and is causing big issues with apt updates, so I've decided to remove python 3 completely from the alternatives:
root:~# python -V
Python 3.5.2
root:~# update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/python3.5 3 auto mode
1 /usr/bin/python2.7 2 manual mode
2 /usr/bin/python3.5 3 manual mode
root:~# update-alternatives --remove python /usr/bin/python3.5
root:~# update-alternatives --config python
There is 1 choice for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
------------------------------------------------------------
0 /usr/bin/python2.7 2 auto mode
* 1 /usr/bin/python2.7 2 manual mode
Press <enter> to keep the current choice[*], or type selection number: 0
root:~# python -V
Python 2.7.12
root:~# update-alternatives --config python
There is only one alternative in link group python (providing /usr/bin/python): /usr/bin/python2.7
Nothing to configure.
Solution 6 - Python
I've edited /home/user/.bashrc with
alias python27=/usr/bin/python2.7
alias python31=/usr/bin/python3.10
on linux ubuntu 20.xx. I've added it to the very end of the document.
this binds python27 to the version you want.
python27 pythonscrypt.py
Test the sample with
python27 --version
but I was wondering if python and python3 should be pointing to the /usr/local/python3.9?
I thought it was customary to have 2.7-2.8 as python and 3.9.x as python3.
Updating the alternatives from above post has it a bit wonky for me so have to backtrack some by setting defaults to normal. Also was trying anaconda and some parts were getting a bit confusing between python and anaconda versions.
But I think editing the .bashrc then exiting terminal and logging back in is the best solution in 2021.
Don't forget to exit terminal and log back in after saving changes!
Another technique would be using envs in python3.