How to switch between python 2.7 to python 3 from command line?

Question

I'm trying to find the best way to switch between the two python compilers, 2.7 to 3.3. I ran the python script from the cmd like this:

python ex1.py

Where do I set the "python" environment in the window's environment variable to point to either python 3.3 or 2.7?

I am wondering if there is an easy way to switch between the two versions from the cmd line?

Mark Tolonen · Answer 1 · 2020-09-30T18:10:14.463

173

No need for "tricks". Python 3.3 comes with PyLauncher "py.exe", installs it in the path, and registers it as the ".py" extension handler. With it, a special comment at the top of a script tells the launcher which version of Python to run:

#!python2
print "hello"

Or

#!python3
print("hello")

From the command line:

py -3 hello.py

Or

py -2 hello.py

py hello.py by itself will choose the latest Python installed, or consult the PY_PYTHON environment variable, e.g. set PY_PYTHON=3.6.

See Python Launcher for Windows

edited Sep 30 '20 at 18:10

answered Aug 05 '13 at 13:17

Mark Tolonen

166,664
26
169
251

2

Being compatible with Unix shebang, you can also use `#!/usr/bin/env python2` or `#!/usr/bin/python2`. However, `#!/usr/bin/python` will not make PyLauncher use Python 2. – InQβ Feb 07 '18 at 07:14

Sukrit Kalra · Accepted Answer · 2013-08-05T12:50:33.353

84

For Windows 7, I just rename the python.exe from the Python 3 folder to python3.exe and add the path into the environment variables. Using that, I can execute python test_script.py and the script runs with Python 2.7 and when I do python3 test_script.py, it runs the script in Python 3.

To add Python 3 to the environment variables, follow these steps -

Right Click on My Computer and go to Properties.
Go to Advanced System Settings.
Click on Environment Variables and edit PATH and add the path to your Python 3 installation directory.

For example,

enter image description here

edited Aug 05 '13 at 12:50

answered Aug 05 '13 at 12:45

Sukrit Kalra

33,167
7
69
71

6

it is better to copy python.exe and name the copy python3.exe, this way you avoid errors when you use python 3's IDLE, console and other things that depend on python.exe – mand Nov 15 '15 at 08:47
2

It's better not to use renaming tricks and use the PyLauncher installed with Python. See my answer. – Mark Tolonen Apr 30 '18 at 22:51
life saviour answer – Just code Sep 23 '19 at 11:44

score 3 · Answer 3 · answered Mar 17 '19 at 14:33

3

In case you have both python 2 and 3 in your path, you can move up the Python27 folder in your path, so it search and executes python 2 first.

answered Mar 17 '19 at 14:33

Philip L

339
1
3
14

score 2 · Answer 4 · answered May 18 '17 at 10:35

2

They are 3 ways you can achieve this using the py command (py-launcher) in python 3, virtual environment or configuring your default python system path. For illustration purpose, you may see tutorial https://www.youtube.com/watch?v=ynDlb0n27cw&t=38s

answered May 18 '17 at 10:35

Timothy Mugayi

1,449
17
11

score 1 · Answer 5 · answered Mar 15 '21 at 12:06

1

simply add both to the env variables and then you have to move the version you want to the top

answered Mar 15 '21 at 12:06

MOHAMMED NABAWY

221
3
7

score 0 · Answer 6 · answered Jul 11 '19 at 20:49

0

There is an easier way than all of the above; You can use the PY_PYTHON environment variable. From inside the cmd.exe shell;

For the latest version of Python 2

set PY_PYTHON=2

For the latest version of Python 3

set PY_PYTHON=3

If you want it to be permanent, set it in the control panel. Or use setx instead of set in the cmd.exe shell.

answered Jul 11 '19 at 20:49

ehambright

1,416
19
27

2

this doesn't work for me... both pythons are in path and this would be elegant / the desired solution. What do I do to make this work exactly? – user2305193 Jul 19 '20 at 10:49
@user2305193 This variable is used by PyLauncher. See my answer. – Mark Tolonen Sep 30 '20 at 18:08
and that's called with `py`? – user2305193 Sep 30 '20 at 18:10
@user2305193 Its part of pylauncher which is installed with Python 3. If you installed 3.3 or later as a system wide installation it will be on your path. If you didn't you'll need to add it to your path. – ehambright Sep 30 '20 at 20:03
@ehambright sorry, please allow the silly question, but how is pylauncher called, like, what do I type into console after setting the local variable (`PY_PYTHON`)? – user2305193 Oct 01 '20 at 08:14
1

@user2305193 - you type "py" into the console.The name of the launcher is py.exe. If you installed it for all users it will be in %SYSTEMROOT%. If you didn't, I think it will be wherever you installed python 3 to, although im not sure 100% – ehambright Oct 02 '20 at 21:16

score 0 · Answer 7 · answered Dec 20 '21 at 09:58

0

Are you using Python version 3+?

Go to your project path
Run py -[version_number_here] and hit Enter -> This will open the Python's Terminal (sort of)

Happy Coding

answered Dec 20 '21 at 09:58

Kenn

587
1
5
7

score -5 · Answer 8 · answered Dec 31 '16 at 17:16

-5

You can try to rename the python executable in the python3 folder to python3, that is if it was named python formally... it worked for me

answered Dec 31 '16 at 17:16

ahmadore

13

3

This does not seem to add anything new to the existing answers. – lfurini Feb 26 '17 at 12:57

How to switch between python 2.7 to python 3 from command line?

8 Answers8

Linked

Related