Is there a particular reason on why update-alternatives is not working with Python since ubuntu comes bundled with different versions?
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2 Answers
The reason is, python versions aren't fully compatibile. If you set 3.3 version as your default, the applications made for 2.7 could not work.
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9Exactly. Python 2 and 3 aren't considered alternatives. They're completely different things.– Oli ♦Apr 22, 2014 at 10:28
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1@Oli Great... except that python 2.x automatically symlinks itself to /usr/bin/python, which IMO is the wrong behavior since they aren't alternatives. May 27, 2014 at 18:29
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2@Powerlord, No, it's not a wrong behaviour, as
pythonis intended to always mean Python 2 and Python 3 expected to always be referred to aspython3. I saw a note about this somewhere in a PEP or in Python mailing list (can't remember exactly what's the source).– Hibou57Aug 21, 2014 at 10:29 -
@Hibou57 I think that in one or two years, they'll change meaning and python will be symbolic link to python3. That is in Arch Linux.– enedilAug 21, 2014 at 10:31
Or maybe you can.
Important Note: The solution presented bellow (with update-alternatives) may break your system.
If things break, try to fix them and keep python 3, report the fix you made to the correct bug-reports sites and help the community to grow; or move back to python 2. At the end of the answer I'll add a simple command to move back in case you need.
If you just want to change python for your own user, not the system, you can use an alias.
Solution 1: alias
Add this line into your ~/.bashrc or ~/.bash_aliases file:
alias python=python3
And that is it. You don't need to read bellow.
Solution 2: update-alternatives
If alias is not what you are looking for, and you really want to change the default of the whole system, then keep reading.
First check your python version:
# python -V
Python 2.7.13
Use this commands to install alternatives (run as root or use sudo)
# update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
# update-alternatives --install /usr/bin/python python /usr/bin/python3.5 2
Then configure the version with (again, run also root or use sudo):
$ 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 2 auto mode
1 /usr/bin/python2.7 1 manual mode
2 /usr/bin/python3.5 2 manual mode
Press <enter> to keep the current choice[*], or type selection number: 2
Now check the version:
# python -V
Python 3.5.3
When you run the above command, you can choose selection number: 0 or 2 to pick python 3. If things break, run again and pick number 1 to go back to python 2.
Good readings:
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Maybe on ubuntu. As long term user of Debian and system admin, I'm experimenting with one of my systems for a month now and nothing breaks. I'll update if something goes wrong.– DrBecoSep 9, 2019 at 20:20
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That breaks Debian too because not everything's undergone a Python 2 -> Python 3 conversion. Keep the Debian specific 'fixes' for this out of Ubuntu, because I can 100% guarantee you that doing this right now in Debian Unstable or in Ubuntu will *explode the system's ability to operate.– Thomas Ward ♦Sep 9, 2019 at 20:21
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Also check: unix.stackexchange.com/questions/177899/… and gist.github.com/patrickmmartin/5b6b2ddecd29d6a1b2ffee2d8eea31ec and of course about the apt break: stackoverflow.com/questions/43062608/… . No need to cause alarm, just discuss what will break instead of trying to add fear to people.– DrBecoSep 9, 2019 at 20:22
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Sorry to disagree, but my current experiment (real server running 24/7 for a month now) is not broken. Maybe other packages that are not so necessary to my own system and are not installed breaks and I don't have them to check. Anyway, it is working fine.– DrBecoSep 9, 2019 at 20:23