I had two versions of Python installed on my machine (versions 2.6 and 2.5). I want to run 2.6 for one project and 2.5 for another.

How can I specify which I want to use?

I am working on Windows XP SP2.

Running a different copy of Python is as easy as starting the correct executable. You mention that you’ve started a python instance, from the command line, by simply typing python.

What this does under Windows, is to trawl the %PATH% environment variable, checking for an executable, either batch file (.bat), command file (.cmd) or some other executable to run (this is controlled by the PATHEXT environment variable), that matches the name given. When it finds the correct file to run the file is being run.

Now, if you’ve installed two python versions 2.5 and 2.6, the path will have both of their directories in it, something like PATH=c:\python\2.5;c:\python\2.6 but Windows will stop examining the path when it finds a match.

What you really need to do is to explicitly call one or both of the applications, such as c:\python\2.5\python.exe or c:\python\2.6\python.exe.

The other alternative is to create a shortcut to the respective python.exe calling one of them python25 and the other python26; you can then simply run python25 on your command line.

Adding two more solutions to the problem:

• Use pylauncher (if you have Python 3.3 or newer there’s no need to install it as it comes with Python already) and either add shebang lines to your scripts;

#! c:\[path to Python 2.5]\python.exe – for scripts you want to be run with Python 2.5
#! c:\[path to Python 2.6]\python.exe – for scripts you want to be run with Python 2.6

or instead of running python command run pylauncher command (py) specyfing which version of Python you want;

py -2.6 – version 2.6
py -2 – latest installed version 2.x
py -3.4 – version 3.4
py -3 – latest installed version 3.x

• Install virtualenv and create two virtualenvs;

virtualenv -p c:\[path to Python 2.5]\python.exe [path where you want to have virtualenv using Python 2.5 created]\[name of virtualenv]

virtualenv -p c:\[path to Python 2.6]\python.exe [path where you want to have virtualenv using Python 2.6 created]\[name of virtualenv]

for example

virtualenv -p c:\python2.5\python.exe c:\venvs\2.5

virtualenv -p c:\python2.6\python.exe c:\venvs\2.6

then you can activate the first and work with Python 2.5 like this
c:\venvs\2.5\activate
and when you want to switch to Python 2.6 you do

deactivate  
c:\venvs\2.6\activate

From Python 3.3 on, there is the official Python launcher for Windows (http://www.python.org/dev/peps/pep-0397/). Now, you can use the #!pythonX to determine the wanted version of the interpreter also on Windows. See more details in my another comment or read the PEP 397.

Summary: The py script.py launches the Python version stated in #! or Python 2 if #! is missing. The py -3 script.py launches the Python 3.

As per @alexander you can make a set of symbolic links like below. Put them somewhere which is included in your path so they can be easily invoked

> cd c:\bin
> mklink python25.exe c:\python25\python.exe
> mklink python26.exe c:\python26\python.exe

As long as c:\bin or where ever you placed them in is in your path you can now go

> python25

1. install python

   • C:\Python27
   • C:\Python36

2. environment variable

   • PYTHON2_HOME: C:\Python27
   • PYTHON3_HOME: C:\Python36
   • Path: %PYTHON2_HOME%;%PYTHON2_HOME%\Scripts;%PYTHON3_HOME%;%PYTHON3_HOME%\Scripts;

3. file rename

   • C:\Python27\python.exe → C:\Python27\python2.exe
   • C:\Python36\python.exe → C:\Python36\python3.exe

4. pip

   • python2 -m pip install package
   • python3 -m pip install package

For example for 3.6 version type py -3.6. If you have also 32bit and 64bit versions, you can just type py -3.6-64 or py -3.6-32.

When you install Python, it will not overwrite other installs of other major versions. So installing Python 2.5.x will not overwrite Python 2.6.x, although installing 2.6.6 will overwrite 2.6.5.

So you can just install it. Then you call the Python version you want. For example:

C:\Python2.5\Python.exe

for Python 2.5 on windows and

C:\Python2.6\Python.exe

for Python 2.6 on windows, or

/usr/local/bin/python-2.5

or

/usr/local/bin/python-2.6

on Windows Unix (including Linux and OS X).

When you install on Unix (including Linux and OS X) you will get a generic python command installed, which will be the last one you installed. This is mostly not a problem as most scripts will explicitly call /usr/local/bin/python2.5 or something just to protect against that. But if you don’t want to do that, and you probably don’t you can install it like this:

./configure
make
sudo make altinstall

Note the “altinstall” that means it will install it, but it will not replace the python command.

On Windows you don’t get a global python command as far as I know so that’s not an issue.

I strongly recommend the pyenv-win project.

Thanks to kirankotari‘s work, now we have a Windows version of pyenv.

Here’s a quick hack:

1. Go to the directory of the version of python you want to run
2. Right click on python.exe
3. Select ‘Create Shortcut‘
4. Give that shortcut a name to call by( I use p27, p33 etc.)
5. Move that shortcut to your home directory(C:\Users\Your name)
6. Open a command prompt and enter name_of_your_shortcut.lnk(I use p27.lnk)

cp c:\python27\bin\python.exe as python2.7.exe

cp c:\python34\bin\python.exe as python3.4.exe

they are all in the system path, choose the version you want to run

C:\Users\username>python2.7
Python 2.7.8 (default, Jun 30 2014, 16:03:49) [MSC v.1500 32 bit (Intel)] on win
32
Type "help", "copyright", "credits" or "license" for more information.
>>>

C:\Users\username>python3.4
Python 3.4.1 (v3.4.1:c0e311e010fc, May 18 2014, 10:38:22) [MSC v.1600 32 bit Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>

Using a batch file to switch, easy and efficient on windows 7. I use this:

In the environment variable dialog (C:\Windows\System32\SystemPropertiesAdvanced.exe),

In the section user variables

1. added %pathpython% to the path environment variable

2. removed any references to python pathes

In the section system variables

1. removed any references to python pathes

I created batch files for every python installation (exmple for 3.4 x64

Name = SetPathPython34x64 !!! ToExecuteAsAdmin.bat ;-) just to remember.

Content of the file =

     Set PathPython=C:\Python36AMD64\Scripts\;C:\Python36AMD64\;C:\Tcl\bin

     setx PathPython %PathPython%

To switch between versions, I execute the batch file in admin mode.

!!!!! The changes are effective for the SUBSEQUENT command prompt windows OPENED. !!!

So I have exact control on it.

The easiest way to run multiple versions of python on windows is described below as follows:-

1)Download the latest versions of python from python.org/downloads by selecting the relevant version for your system.

2)Run the installer and select Add python 3.x to the path to set path automatically in python 3 (you just have to click the checkbox). For python 2 open up your python 2 installer, select whatever preferences you want but just remember to set Add python.exe to path to Will be installed on local hard drive, Now just click next and wait for the installer to finish.

3)When both the installations are complete. Right click on my computer–Go to properties–Select advanced system settings–Go to environment variables–Click on new under System variables and add a new system variable with variable name as PY_PYTHON and set this variable value to 3. Now click on OK and you should be done.

4)Now to test this open the command prompt. Once you are in there type python or py, It should open up python3.

5)Now exit out of python3 by typing exit(). Now type py -2 it should open python 2.

If none of this works then restart the computer and if the problem still persists then uninstall everything and repeat the steps.

Thanks.

You can create different python development environments graphically from Anaconda Navigator. I had same problem while working with different python versions so I used anaconda navigator to create different python development environments and used different python versions in each environments.

Here is the help documentation for this.

https://docs.anaconda.com/anaconda/navigator/tutorials/manage-environments/

Using the Rapid Environment Editor you can push to the top the directory of the desired Python installation. For example, to start python from the c:\Python27 directory, ensure that c:\Python27 directory is before or on top of the c:\Python36 directory in the Path environment variable. From my experience, the first python executable found in the Path environment is being executed. For example, I have MSYS2 installed with Python27 and since I’ve added C:\MSYS2 to the path before C:\Python36, the python.exe from the C:\MSYS2…. folder is being executed.

Just call the correct executable

I’m running Windows and the shell/OS automatically runs Python based on the registry settings when you run a program on the command line. Will this break if I install a 2.x and 3.x version of Python on the same machine?

I want to play with Python 3 while still being able to run 2.x scripts on the same machine.

The official solution for coexistence seems to be the Python Launcher for Windows, PEP 397 which was included in Python 3.3.0. Installing the release dumps py.exe and pyw.exe launchers into %SYSTEMROOT% (C:\Windows) which is then associated with py and pyw scripts, respectively.

In order to use the new launcher (without manually setting up your own associations to it), leave the “Register Extensions” option enabled. I’m not quite sure why, but on my machine it left Py 2.7 as the “default” (of the launcher).

Running scripts by calling them directly from the command line will route them through the launcher and parse the shebang (if it exists). You can also explicitly call the launcher and use switches: py -3 mypy2script.py.

All manner of shebangs seem to work

• #!C:\Python33\python.exe
• #!python3
• #!/usr/bin/env python3

as well as wanton abuses

• #! notepad.exe

Here’s my setup:

1. Install both Python 2.7 and 3.4 with the windows installers.
2. Go to C:\Python34 (the default install path) and change python.exe to python3.exe
3. Edit your environment variables to include C:\Python27\;C:\Python27\Scripts\;C:\Python34\;C:\Python34\Scripts\;

Now in command line you can use python for 2.7 and python3 for 3.4.

You can have both installed.

You should write this in front of your script:

#!/bin/env python2.7

or, eventually…

#!/bin/env python3.6

Update

My solution works perfectly with Unix, after a quick search on Google, here is the Windows solution:

#!c:/Python/python3_6.exe -u

Same thing: in front of your script.

From version 3.3 Python introduced Launcher for Windows utility https://docs.python.org/3/using/windows.html#python-launcher-for-windows.

So to be able to use multiple versions of Python:

1. install Python 2.x (x is any version you need)
2. install Python 3.x (x is any version you need also you have to have one version 3.x >= 3.3)
3. open Command Prompt
4. type py -2.x to launch Python 2.x
5. type py -3.x to launch Python 3.x

I’m using 2.5, 2.6, and 3.0 from the shell with one line batch scripts of the form:

:: The @ symbol at the start turns off the prompt from displaying the command.
:: The % represents an argument, while the * means all of them.
@c:\programs\pythonX.Y\python.exe %*

Name them pythonX.Y.bat and put them somewhere in your PATH. Copy the file for the preferred minor version (i.e. the latest) to pythonX.bat. (E.g. copy python2.6.bat python2.bat.) Then you can use python2 file.py from anywhere.

However, this doesn’t help or even affect the Windows file association situation. For that you’ll need a launcher program that reads the #! line, and then associate that with .py and .pyw files.

When you add both to environment variables there will a be a conflict because the two executable have the same name: python.exe.

Just rename one of them. In my case I renamed it to python3.exe.

So when I run python it will execute python.exe which is 2.7 and when I run python3 it will execute python3.exe which is 3.6

Here you go…

winpylaunch.py

#
# Looks for a directive in the form: #! C:\Python30\python.exe
# The directive must start with #! and contain ".exe".
# This will be assumed to be the correct python interpreter to
# use to run the script ON WINDOWS. If no interpreter is
# found then the script will be run with 'python.exe'.
# ie: whatever one is found on the path.
# For example, in a script which is saved as utf-8 and which
# runs on Linux and Windows and uses the Python 2.6 interpreter...
#
#    #!/usr/bin/python
#    #!C:\Python26\python.exe
#    # -*- coding: utf-8 -*-
#
# When run on Linux, Linux uses the /usr/bin/python. When run
# on Windows using winpylaunch.py it uses C:\Python26\python.exe.
#
# To set up the association add this to the registry...
#
#    HKEY_CLASSES_ROOT\Python.File\shell\open\command
#    (Default) REG_SZ = "C:\Python30\python.exe" S:\usr\bin\winpylaunch.py "%1" %*
#
# NOTE: winpylaunch.py itself works with either 2.6 and 3.0. Once
# this entry has been added python files can be run on the
# commandline and the use of winpylaunch.py will be transparent.
#

import subprocess
import sys

USAGE = """
USAGE: winpylaunch.py <script.py> [arg1] [arg2...]
"""

if __name__ == "__main__":
  if len(sys.argv) > 1:
    script = sys.argv[1]
    args   = sys.argv[2:]
    if script.endswith(".py"):
      interpreter = "python.exe" # Default to wherever it is found on the path.
      lines = open(script).readlines()
      for line in lines:
        if line.startswith("#!") and line.find(".exe") != -1:
          interpreter = line[2:].strip()
          break
      process = subprocess.Popen([interpreter] + [script] + args)
      process.wait()
      sys.exit()
  print(USAGE)

I’ve just knocked this up on reading this thread (because it’s what I was needing too). I have Pythons 2.6.1 and 3.0.1 on both Ubuntu and Windows. If it doesn’t work for you post fixes here.

As far as I know Python runs off of the commandline using the PATH variable as opposed to a registry setting.

So if you point to the correct version on your PATH you will use that. Remember to restart your command prompt to use the new PATH settings.

The Python installation normally associates .py, .pyw and .pyc files with the Python interpreter. So you can run a Python script either by double-clicking it in Explorer or by typing its name in a command-line window (so no need to type python scriptname.py, just scriptname.py will do).

If you want to manually change this association, you can edit these keys in the Windows registry:

HKEY_CLASSES_ROOT\Python.File\shell\open\command
HKEY_CLASSES_ROOT\Python.NoConFile\shell\open\command
HKEY_CLASSES_ROOT\Python.CompiledFile\shell\open\command

Python Launcher

People have been working on a Python launcher for Windows: a lightweight program associated with .py and .pyw files which would look for a “shebang” line (similar to Linux et al) on the first line, and launch Python 2.x or 3.x as required. See “A Python Launcher for Windows” blog post for details.

Try using Anaconda.

Using the concept of Anaconda environments, let’s say you need Python 3 to learn programming, but you don’t want to wipe out your Python 2.7 environment by updating Python. You can create and activate a new environment named “snakes” (or whatever you want), and install the latest version of Python 3 as follows:

conda create --name snakes python=3

Its simpler than it sounds, take a look at the intro page here: Getting Started with Anaconda

And then to handle your specific problem of having version 2.x and 3.x running side by side, see:

• Managing Python Versions with Anaconda
• Switching between Python 2 and Python 3 environments

Here is how to run Python 2 and 3 on the same machine

1. install Python 2.x
2. install Python 3.x
3. Start Powershell
4. Type Python -2 to launch Python 2.x
5. Type Python -3 to launch Python 2.x

The Python Launcher for Windows was embedded into Python since Version 3.3, as promised in 2011 when the Stand alone first made its debut:

Python Launcher for Windows

Here is a neat and clean way to install Python2 & Python3 on windows.

https://datascience.com.co/how-to-install-python-2-7-and-3-6-in-windows-10-add-python-path-281e7eae62a

My case: I had to install Apache cassandra. I already had Python3 installed in my D: drive. With loads of development work under process i didn’t wanted to mess my Python3 installation. And, i needed Python2 only for Apache cassandra.

So i took following steps:

1. Downloaded & Installed Python2.
2. Added Python2 entries to classpath (C:\Python27;C:\Python27\Scripts)
3. Modified python.exe to python2.exe (as shown in image below)

4. Now i am able to run both. For Python 2(python2 --version) & Python 3 (python --version).

So, my Python3 installation remained intact.

I think there is an option to setup the windows file association for .py files in the installer. Uncheck it and you should be fine.

If not, you can easily re-associate .py files with the previous version. The simplest way is to right click on a .py file, select “open with” / “choose program”. On the dialog that appears, select or browse to the version of python you want to use by default, and check the “always use this program to open this kind of file” checkbox.

You should make sure that the PATH environment variable doesn’t contain both python.exe files ( add the one you’re currently using to run scripts on a day to day basis ) , or do as Kniht suggested with the batch files . Aside from that , I don’t see why not .

P.S : I have 2.6 installed as my “primary” python and 3.0 as my “play” python . The 2.6 is included in the PATH . Everything works fine .

Before I courageously installed both simultaneously, I had so many questions. If I give python will it go to py3 when i want py2? pip/virtualenv will happen under py2/3?

It seems to be very simple now.

Just blindly install both of them. Make sure you get the right type(x64/x32). While/after installing make sure you add to the path to your environment variables.

[ENVIRONMENT]::SETENVIRONMENTVARIABLE("PATH", "$ENV:PATH;C:\PYTHONx", "USER")

Replace the x in the command above to set the path.

Then go to both the folders.

Navigate to

python3.6/Scripts/

and rename pip to pip3.

If pip3 already exists delete the pip. This will make sure that just pip will run under python2. You can verify by:

pip --version

In case you want to use pip with python3 then just use

pip3 install 

You can similarly do the same to python file and others.

Cheers!

Easy-peasy ,after installing both the python versions add the paths to the environment variables ;see. Then go to python 2 and python 3 folders and rename them to python2 and python3 respectively as shown and . Now in cmd type python2 or python3 to use your required version see .

I would assume so, I have Python 2.4, 2.5 and 2.6 installed side-by-side on the same computer.

I am just starting out with python now. I’m reading Zed Shaw’s book “Learn Python the Hard Way” which requires python version 2.x but am also taking a class that requires python 3.x

So here is what I did.

1. Download python 2.7
2. run power shell (should already be installed on windows)
3. run python IN POWERSHELL (if it doesn’t recognize then go to step 4)
4. Only if powershell doesn’t recognize python 2.7 type in the following:

“[ENVIRONMENT]::SETENVIRONMENTVARIABLE(“PATH”, “$ENV:PATH;C:\PYTHON27”, “USER”)” (no outside quotes)

5. Now type python and you should see it say python 2.7 blah blah blah

NOW for python 3.x

Simple, python 3.x download comes with python for windows app. SO simply pin the Python for Windows app to your task bar, or create shortcut to the desktop and you are done!

Open Python for Windows for 3.x

Open Powershell for python 2.x

I hope this helps!

Hmm..I did this right now by just downloading Python 3.6.5 for Windows at https://www.python.org/downloads/release/python-365/ and made sure that the launcher would be installed. Then, I followed the instructions for using python 2 and python 3. Restart the command prompt and then use py -2.7 to use Python 2 and py or py -3.6 to use Python 3. You can also use pip2 for Python 2’s pip and pip for Python 3’s pip.

I had the same problem where I wanted to use python3 for most work but IDA pro required python2. SO, here’s what I did.

I first created 3 variables in the user environment variable as follows:

1. PYTHON_ACTIVE : This is initially empty
2. HOME_PYTHON27 : Has a path to a folder where Python 2 is installed. Eg. “;/scripts;”
3. HOME_PYTHON38 : Similar to python 2, this variable contains a path to python 3 folders.

Now I added

%PYTHON_ACTIVE%

to PATH variable. So, basically saying that whatever this “PYTHON_ACTIVE” contains is the active python. We programmatically change the contains of “PYTHON_ACTIVE” to switch python version.

Here is the example script:

:: This batch file is used to switch between python 2 and 3.
@ECHO OFF

set /p choice= "Please enter '27' for python 2.7 , '38' for python 3.8 : "

IF %choice%==27 (
setx PYTHON_ACTIVE %HOME_PYTHON27%
)

IF %choice%==38 (
setx PYTHON_ACTIVE %HOME_PYTHON38%
)


PAUSE

This script takes python version as input and accordingly copies HOME_PYTHON27 or HOME_PYTHON38 to PYTHON_ACTIVE. Thus changing the global Python version.