To answer your first question
Why update-alternatives has not created the link in /usr/local/bin for maintaining for proper searching of application ?
This is simply because alternatives for python3 is configured at the moment to create the symbolic link in /usr/bin. From your output above:
sudo update-alternatives --config python3
There are 3 choices for the alternative python3 (providing /usr/bin/python3).
...
It pretty normal for any Linux distribution to search /usr/local/bin before /usr/bin for executables, as this gives you the freedom to put any binaries and libraries into /usr/local without interfering with the dpkg packages.
For you specific problem and from the output of sudo update-alternatives --config python3 I guess /usr/local/bin/python3 is a symbolic link to /usr/local/bin/python3.7
root@host:~# ls -l /usr/local/bin/python3
lrwxrwxrwx 1 root root 25 Sep 22 15:36 /usr/local/bin/python3 -> /usr/local/bin/python3.7
If that is the case, just remove the symbolic link /usr/local/bin/python3 and you should be fine, since when calling python3 only /usr/bin/python3 is found, which should point to /etc/alternatives/python3 which in turn should point to the selected binary.
If /usr/local/bin/python3 is not a symbolic link, rename it and configure update-alternatives appropriately.
While this might save you for now, the next update might render your configuration useless again. The symbolic link /usr/bin/python3 is shipped with the package python3-minimal and by managing this link with update-alternatives the symbolic link is broken from the view of the python3-minimal package. The next update of this package could fix this problem, but then the link is broken from the view of update-alternatives.
To come around both problems, it would be better to configure update-alternatives to use /usr/local/bin/python3 as the symbolic link providing the selected version.
To do so, the steps as follows should do the trick, including fixing the /usr/bin/python3 link.
sudo update-alternatives --install /usr/local/bin/python3 python3 /usr/bin/python3.6 2
apt-get install --reinstall python3-minimal
After many attempts,
I found the cause of issue. Python upgraded to version 3.7 and lost its version 3.5
sudo apt-get install python3.5
Above command installed python 3.5 which is used to run terminal by Ubuntu 18.04 by default.
Note:
Uninstalling Python 3.5 causes removal of terminal, desktop, firefox
and some applications in Ubuntu.
Better don't do it.
Carefully upgrade/install python versions.
Best Answer
This way the upgrade to 3.7 is useless. You should edit the
gnome-terminalfile:Open a terminal and issue the following commands:
Change
#!/usr/bin/python3to#!/usr/bin/python3.6.This way you can still use the power of 3.7 and solve the issue with the terminal.