May my 'particular' Py2 coexists with Py3 in Windows OS?

[samsonite]

My abrupt answer in _no_, because pip2 installation interfers (as far as I know) with pip of Py3, but I'm confident in forum's drifts to overcome the problem.

The main Python installation in my pc, Win-10 equipped, is as follows:

Output:
C:\Users\Alfabeta>python -V
Python 3.7.2

C:\Users\Alfabeta>pip -V
pip 19.0.3 from c:\users\alfabeta\appdata\local\programs\python\python37\lib\site-packages\pip (python 3.7)

C:\Users\Alfabeta>pip list
Package        Version
-------------- --------
2to3           1.0       <---- converter py2/py3, I won't use it!
altgraph       0.16.1
astropy        3.1.2
de421          2008.1
future         0.17.1
jplephem       2.9
julian         0.14
macholib       1.11
mpmath         1.1.0
numpy          1.16.2
pefile         2018.8.8
pip            19.0.3
PyInstaller    3.4
pywin32-ctypes 0.2.0
setuptools     40.6.2
sympy          1.3

I reach the Py2 version by means of this batch file:

Output:
@echo off
cls
cd\0_BackUP-29-Feb-2016\Temp\Python26
python2 -V
python2 --help
python2 my-newton.py  
pause

So, the python release name is shown below and runs properly next script

# --------- my-newton.py ------------------ 
# --  (derivative computed through finite interval dx=h) ----
def derivative(f, x, h):
      return (f(x+h) - f(x-h)) / (2.0*h)  # via finite interval h

def funzione(x):
    return x**3 - x -1     # Given function 

def solve(f, x0, h):
    lastX = x0
    nextX = lastX + 10* h                   # different than lastX so loop starts OK
    while (abs(lastX - nextX) > h):         
        newY = f(nextX)                     
        print "f(", nextX, ") = ", newY     # printout progress
        lastX = nextX
        nextX = lastX - newY / derivative(f, lastX, h)  # update estimate using Newton method
    return nextX

xFound = solve(funzione, 2, 10**(-7))   # call the solver given starting value, tolerance
print "solution: x = ", xFound          # print the result
# ------------------------------------------------------

Output:
Python 2.6.5
usage: python2 [option] ... [-c cmd | -m mod | file | -] [arg] ...
Options and arguments (and corresponding environment variables):
-B     : don't write .py[co] files on import; also PYTHONDONTWRITEBYTECODE=x
-c cmd : program passed in as string (terminates option list)
-d     : debug output from parser; also PYTHONDEBUG=x
-E     : ignore PYTHON* environment variables (such as PYTHONPATH)
-h     : print this help message and exit (also --help)
...........
...........
Running my-newton.py
-------------
f( 2.000001 ) =  5.00001100001
f( 1.54545504037 ) =  1.14575812265
f( 1.35961505435 ) =  0.153705563753
f( 1.32580135343 ) =  0.00462495304571
f( 1.32471904943 ) =  4.65779447723e-06
f( 1.32471795725 ) =  4.74331685041e-12
solution: x =  1.32471795724

Unfortunately, such a 'particular' installation allows a strong limitative use of Py2, due to the absence of libraries, commonly coming from (pip).

Thx in advance, and cheers

[snippsat]

When install Python 3.7 it also put py.exe in Path of Windows.
This py is what you use to access other versions.
Example:

# Main version set in Path
E:\div_code
λ python -V
Python 3.7.2

# Main pip set in Path
E:\div_code
λ pip -V
pip 19.0.3 from c:\python37\lib\site-packages\pip (python 3.7)

# Access Python 3.4  
E:\div_code
λ py -3.4 -V
Python 3.4.2

# Access Python 2.7
E:\div_code
λ py -2.7 -V
Python 2.7.9

# Install with pip to 2.7
E:\div_code
λ py -2.7 -m pip install jsonlines
Collecting jsonlines
  Downloading https://files.pythonhosted.org/......any.whl
Requirement already satisfied: six in c:\python27\lib\site-packages (from jsonlines) (1.9.0)
Installing collected packages: jsonlines
Successfully installed jsonlines-1.2.0

# Running 2.7 code would be
E:\div_code
λ py -2.7 my_script.py

[samsonite]

Good drift, snippsat.
Will perform back to home and then refer here. Thanks a lot

[samsonite]

Oh, yes! Now all is clear.

This command said me how to query my system:

Output:
C:\>py --help
Python Launcher for Windows Version 3.7.2150.1013

usage:
py [launcher-args] [python-args] script [script-args]

Launcher arguments:

-2     : Launch the latest Python 2.x version
-3     : Launch the latest Python 3.x version
-X.Y   : Launch the specified Python version
     The above all default to 64 bit if a matching 64 bit python is present.
-X.Y-32: Launch the specified 32bit Python version
-X-32  : Launch the latest 32bit Python X version
-X.Y-64: Launch the specified 64bit Python version
-X-64  : Launch the latest 64bit Python X version
-0  --list       : List the available pythons
-0p --list-paths : List with paths

The following help text is from Python:

usage: C:\Users\Alfabeta\AppData\Local\Programs\Python\Python37\python.exe [option] ... [-c cmd | -m mod | file | -]
[arg] ...
..........
.......... and so on


My trials
---------
C:\>py -2
Python 2 not found!
Installed Pythons found by py Launcher for Windows
 -3.7-64 *

Requested Python version (2) not installed, use -0 for available pythons

C:\>py -0
Installed Pythons found by py Launcher for Windows
 -3.7-64 *


My comments: the answer is correct, because my 'particular' Py2 has been unlinked
-----------  from the environment variables. 

Now my next steps are easy to carry out, as detailed in this screenshot http://imgbox.com/ewyV3XM2

Thank you very much, wonderful forum!

[samsonite]

Some helps more, please! Py2 version been installed

Output:
C:\Training>py -2 -V
Python 2.7.15

alongside astropy library, which properly performs the script through the command (py -2 marte.py)

# --------- marte.py ----------  (mars coords) --------
from astropy.time import Time
from astropy.coordinates import solar_system_ephemeris, EarthLocation
from astropy.coordinates import get_body_barycentric, get_body, get_moon
t = Time("2017-08-21 17:28:11.284")
loc = EarthLocation.of_site('greenwich') 
marte = get_body('mars', t, loc) 
#
print(marte)  
print(solar_system_ephemeris.bodies)
# ---------- OUTPUT -----------
# C:\Training>py -2 marte.py
# <SkyCoord (GCRS: obstime=2017-08-21 17:28:11.284, obsgeoloc=(-2439719.30740335, -3138944.00436915, 4970891.10618061
# ) m, obsgeovel=(228.88127335, -178.52172321, -0.39499058) m / s): (ra, dec, distance) in (deg, deg, AU)
#     (143.19625021, 15.77320554, 2.64972634)>
# (u'earth', u'sun', u'moon', u'mercury', u'venus', u'earth-moon-barycenter', u'mars', u'jupiter', u'saturn', u'uranu
# s', u'neptune')
# -------------------

My problem is: how to catch the list of libraries in Py2? Some errors arose, Py3 pip was called.

Error:
C:\Users\Alfabeta>py pip -2 freeze
C:\Users\Alfabeta\AppData\Local\Programs\Python\Python37\python.exe: can't find '__main__' module in 'pip'
C:\Users\Alfabeta>py pip -2 list
C:\Users\Alfabeta\AppData\Local\Programs\Python\Python37\python.exe: can't find '__main__' module in 'pip'

C:\Training>py -2 pip freeze
C:\Python27\python.exe: can't open file 'pip': [Errno 2] No such file or directory
C:\Training>py pip -2 list
C:\Users\Alfabeta\AppData\Local\Programs\Python\Python37\python.exe: can't open file 'pip': [Errno 2] No such file or
 directory

Thx in advance

[snippsat]

You need -m in command.

py -2.7 -m pip freeze
py -2.7 -m pip list

[samsonite]

Oh yes! Problem solved, thank you.

Output:
C:\Users\Alfabeta>py -2.7 -m pip freeze
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7.
astropy==2.0.12
atomicwrites==1.3.0
attrs==19.1.0
colorama==0.4.1
funcsigs==1.0.2
more-itertools==5.0.0
numpy==1.16.2
pluggy==0.7.1
py==1.8.0
pytest==3.6.4
six==1.12.0