How to make iPython use Python 2 instead of Python 3 - python-2.7

Question

I have both Python 2.7 and 3.5 installed. If I run a script from the command line using python, it uses Python 2.7, but if I launch iPython, it uses Python 3:
kurt#kurt-ThinkPad:~$ python -V
Python 2.7.12
kurt#kurt-ThinkPad:~$ ipython
Python 3.5.2 (default, Sep 10 2016, 08:21:44)
Type "copyright", "credits" or "license" for more information.
IPython 5.1.0 -- An enhanced Interactive Python.
? -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help -> Python's own help system.
object? -> Details about 'object', use 'object??' for extra details.
In [1]:
Is there a way to launch iPython so that it uses Python 2? (I'm using Ubuntu LTS 16.04).

Answer 1

Less intrusive solution(as my solution below does not need changing any library files) to this problem is
python2.7 -m IPython notebook
so the general command is
{{python-you-want-ipython-to-use}} -m IPython notebook
Why will this work ?
Because if you see the ipython script (/usr/local/bin/ipython) it seems to be a python script by itself and it has the shebang (#!/usr/bin/python3), so the ipython is not a standalone binary, but it gets life because of some python. So as that ipython script itself needs some python to run it, so you run the ipython module directly using some python of your choice instead of letting that /usr/local/bin/ipython to decide it for you, and that is the fix for the problem of ‘what python ipython uses’.

Answer 2

Following ipython reads wrong python version, in /usr/local/bin/ipython, I simply changed
#!/usr/bin/python3
in the first line to
#!/usr/bin/python
and Python 2 has become the default version used by iPython:
kurt#kurt-ThinkPad:~$ ipython
Python 2.7.12 (default, Jul 1 2016, 15:12:24)
Type "copyright", "credits" or "license" for more information.
IPython 2.4.1 -- An enhanced Interactive Python.
? -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help -> Python's own help system.
object? -> Details about 'object', use 'object??' for extra details.

Answer 3

now IPython 6.0+ does not support Python 2.6, 2.7, 3.0, 3.1, or 3.2.
When using Python 2.7, please install IPython 5.x LTS Long Term Support version.
Beginning with IPython 6.0, Python 3.3 and above is required.

Answer 4

I select what python version execute by means of py, like this:
py -2.7 -m IPython
where 2.7 is the version I need.

Answer 5

Following cel's second solution (for non-Anaconda users) on Using both Python 2.x and Python 3.x in IPython Notebook, I set up two virtual environments for Python 2 and Python 3, and installed iPython separately on each.