Python Forum

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

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

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

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!

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

You need -m in command.

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

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