How to update-alternatives to Python 3 without breaking apt?

Question

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" 

Answer 1

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.

Answer 2

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.