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  lt enter gt  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  lt module gt      from debpython version import SUPPORTED  debsorted  vrepr      File   usr share python debpython version py   line 24  in  lt module gt      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  lt enter gt  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

User · Answer

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 - gt   usr bin python3 6  lrwxrwxrwx 1 root root 25 2019-05-03 02 58 53  usr local bin python3 - gt   etc alternatives python3

User · Answer

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  lt enter gt  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

User · Answer

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

User · Answer

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  lt some install gt  bin is generally recommended to ensure that customizations are found before the default system ones

[python] How to update-alternatives to Python 3 without breaking apt?

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