[Bug 1718035] [NEW] python 3.5 and 2.7 don't provide update-alternatives

[Christ Schlacta]

Public bug reported:

Python 3.5 and python 2.7 don't currently install themselves as
alternatives.  Furthermore, install packages that depend on python2
don't call python2 during their install process and instead call python.
When architecting solutions that depend on python pointing to python3,
this can cause problems installing updates and packages through apt as
well.  Python system should be updated to always call python2 or python3
and not generic python, and python3 and python2 should provide
alternatives by default.

lsb_release -rd
Description:    Ubuntu 16.04.3 LTS
Release:        16.04

apt-cache policy python python3
python:
  Installed: 2.7.11-1
  Candidate: 2.7.11-1
  Version table:
 *** 2.7.11-1 500
        500 http://us.archive.ubuntu.com/ubuntu xenial/main amd64 Packages
        100 /var/lib/dpkg/status
python3:
  Installed: 3.5.1-3
  Candidate: 3.5.1-3
  Version table:
 *** 3.5.1-3 500
        500 http://us.archive.ubuntu.com/ubuntu xenial/main amd64 Packages
        100 /var/lib/dpkg/status


Expected behaviour:
Install Python, Install Python 3.
sudo update-alternatives --config python
There are 3 choices for the alternative python (providing /usr/bin/python).

  Selection    Path                 Priority   Status
------------------------------------------------------------
  0            /usr/bin/python2.7    40        auto mode
* 1            /usr/bin/python2.7    40        manual mode
  2            /usr/bin/python3.5    40        manual mode
  3            /usr/bin/python3.5m   30        manual mode

Press <enter> to keep the current choice[*], or type selection number: 2

sudo apt-get install python-somepackagewhichusespython2;
apt-get installs successfully here

What actually happens:
Install Python, Install Python 3.
sudo update-alternatives --config python
Some absurd message about python not being an alternative.

sudo update-alternatives --install python /usr/bin/python /usr/bin/python2.7 40
sudo update-alternatives --install python /usr/bin/python /usr/bin/python3.5 40
sudo update-alternatives --install python /usr/bin/python /usr/bin/python3.5m 30
sudo update-alternatives --config python
There are 3 choices for the alternative python (providing /usr/bin/python).

  Selection    Path                 Priority   Status
------------------------------------------------------------
  0            /usr/bin/python2.7    40        auto mode
* 1            /usr/bin/python2.7    40        manual mode
  2            /usr/bin/python3.5    40        manual mode
  3            /usr/bin/python3.5m   30        manual mode

Press <enter> to keep the current choice[*], or type selection number: 2

sudo apt-get install python-somepackagewhichusespython2;
apt-get just blatantly fails here because python points to python 3 now and someone didn't think to make them use python2

** Affects: python2.7 (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: python3.5 (Ubuntu)
     Importance: Undecided
         Status: New

** Also affects: python2.7 (Ubuntu)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1718035

Title:
  python 3.5 and 2.7 don't provide update-alternatives

Status in python2.7 package in Ubuntu:
  New
Status in python3.5 package in Ubuntu:
  New

Bug description:
  Python 3.5 and python 2.7 don't currently install themselves as
  alternatives.  Furthermore, install packages that depend on python2
  don't call python2 during their install process and instead call
  python.  When architecting solutions that depend on python pointing to
  python3, this can cause problems installing updates and packages
  through apt as well.  Python system should be updated to always call
  python2 or python3 and not generic python, and python3 and python2
  should provide alternatives by default.

  lsb_release -rd
  Description:    Ubuntu 16.04.3 LTS
  Release:        16.04

  apt-cache policy python python3
  python:
    Installed: 2.7.11-1
    Candidate: 2.7.11-1
    Version table:
   *** 2.7.11-1 500
          500 http://us.archive.ubuntu.com/ubuntu xenial/main amd64 Packages
          100 /var/lib/dpkg/status
  python3:
    Installed: 3.5.1-3
    Candidate: 3.5.1-3
    Version table:
   *** 3.5.1-3 500
          500 http://us.archive.ubuntu.com/ubuntu xenial/main amd64 Packages
          100 /var/lib/dpkg/status

  
  Expected behaviour:
  Install Python, Install Python 3.
  sudo update-alternatives --config python
  There are 3 choices for the alternative python (providing /usr/bin/python).

    Selection    Path                 Priority   Status
  ------------------------------------------------------------
    0            /usr/bin/python2.7    40        auto mode
  * 1            /usr/bin/python2.7    40        manual mode
    2            /usr/bin/python3.5    40        manual mode
    3            /usr/bin/python3.5m   30        manual mode

  Press <enter> to keep the current choice[*], or type selection number:
  2

  sudo apt-get install python-somepackagewhichusespython2;
  apt-get installs successfully here

  What actually happens:
  Install Python, Install Python 3.
  sudo update-alternatives --config python
  Some absurd message about python not being an alternative.

  sudo update-alternatives --install python /usr/bin/python /usr/bin/python2.7 40
  sudo update-alternatives --install python /usr/bin/python /usr/bin/python3.5 40
  sudo update-alternatives --install python /usr/bin/python /usr/bin/python3.5m 30
  sudo update-alternatives --config python
  There are 3 choices for the alternative python (providing /usr/bin/python).

    Selection    Path                 Priority   Status
  ------------------------------------------------------------
    0            /usr/bin/python2.7    40        auto mode
  * 1            /usr/bin/python2.7    40        manual mode
    2            /usr/bin/python3.5    40        manual mode
    3            /usr/bin/python3.5m   30        manual mode

  Press <enter> to keep the current choice[*], or type selection number:
  2

  sudo apt-get install python-somepackagewhichusespython2;
  apt-get just blatantly fails here because python points to python 3 now and someone didn't think to make them use python2

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