[python] How to install both Python 2.x and Python 3.x in Windows

I do most of my programming in Python 3.x on Windows 7, but now I need to use the Python Imaging Library (PIL), ImageMagick, and wxPython, all of which require Python 2.x.

Can I have both Python 2.x and Python 3.x installed in Windows 7? When I run a script, how would I "choose" which version of Python should run it? Will the aforementioned programs be able to handle multiple versions of Python installed at once? I have searched for hours and hours for how to do this to no avail.

Thanks.

I did this in three steps by following the instructions here: This is all taken directly from here: http://ipython.readthedocs.io/en/stable/install/kernel_install.html. I'm currently running Python 2.x on Windows 8 and have Anaconda 4.2.13 installed.

1) First install the latest version of python:

conda create -n python3 python=3 ipykernel

2) Next activate python3

activate python3

3) Install the kernel:

python -m ipykernel install --user

If you have Python 3 installed and want to install 2, switch the 2 and the 3 above. When you open a new notebook, you can now choose between Python 2 or 3.

I have installed both python 2.7.13 and python 3.6.1 on windows 10pro and I was getting the same "Fatal error" when I tried pip2 or pip3.

What I did to correct this was to go to the location of python.exe for python 2 and python 3 files and create a copy of each, I then renamed each copy to python2.exe and python3.exe depending on the python version in the installation folder. I therefore had in each python installation folder both a python.exe file and a python2.exe or python3.exe depending on the python version.

This resolved my problem when I typed either pip2 or pip3.

To install and run any version of Python in the same system follow my guide below.

For example say you want to install Python 2.x and Python 3.x on the same Windows system.

1. Install both of their binary releases anywhere you want.

   • When prompted do not register their file extensions and
   • do not add them automatically to the PATH environment variable

2. Running simply the command python the executable that is first met in PATH will be chosen for launch. In other words, add the Python directories manually. The one you add first will be selected when you type python. Consecutive python programs (increasing order that their directories are placed in PATH) will be chosen like so:

   • py -2 for the second python
   • py -3 for the third python etc..

3. No matter the order of "pythons" you can:

   • run Python 2.x scripts using the command: py -2 (Python 3.x functionality) (ie. the first Python 2.x installation program found in your PATH will be selected)
   • run Python 3.x scripts using the command: or py -3 (ie. the first Python 3.x installation program found in your PATH will be selected)

In my example I have Python 2.7.14 installed first and Python 3.5.3. This is how my PATH variable starts with:

PATH=C:\Program Files\Microsoft MPI\Bin\;C:\Python27;C:\Program Files\Python_3.6\Scripts\;C:\Program Files\Python_3.6\;C:\ProgramData\Oracle\Java\javapath;C:\Program Files (x86)\Common Files\Intel\Shared

...

Note that Python 2.7 is first and Python 3.5 second.

• So running python command will launch python 2.7 (if Python 3.5 the same command would launch Python 3.5).
• Running py -2 launches Python 2.7 (because it happens that the second Python is Python 3.5 which is incompatible with py -2). Running py -3 launches Python 3.5 (because it's Python 3.x)
• If you had another python later in your path you would launch like so: py -4. This may change if/when Python version 4 is released.

Now py -4 or py -5 etc. on my system outputs: Requested Python version (4) not installed or Requested Python version (5) not installed etc.

Hopefully this is clear enough.

You can install multiple versions of Python one machine, and during setup, you can choose to have one of them associate itself with Python file extensions. If you install modules, there will be different setup packages for different versions, or you can choose which version you want to target. Since they generally install themselves into the site-packages directory of the interpreter version, there shouldn't be any conflicts (but I haven't tested this). To choose which version of python, you would have to manually specify the path to the interpreter if it is not the default one. As far as I know, they would share the same PATH and PYTHONPATH variables, which may be a problem.

Note: I run Windows XP. I have no idea if any of this changes for other versions, but I don't see any reason that it would.

Starting version 3.3 Windows version has Python launcher, please take a look at section 3.4. Python Launcher for Windows

I actually just thought of an interesting solution. While Windows will not allow you to easily alias programs, you can instead create renamed batch files that will call the current program.

Instead of renaming the executable which will break a lot of thing including pip, create the file python2.bat in the same directory as the python2.exe. Then add the following line:

%~dp0python %*

What does this archaic syntax mean? Well, it's a batch script, (Windows version of bash). %~dp0 gets the current directory and %* will just pass all the arguments to python that were passed to the script.

Repeat for python3.bat

You can also do the same for pip and other utilities, just replace the word python in the file with pip or whathever the filename. The alias will be whatever the file is named.

Best of all, when added to the PATH, Windows ignores the extension so running

python3

Will launch the python3 version and and the command python2 will launch the python2 version.

BTW, this is the same technique Spyder uses to add itself to the path on Windows. :)

I have multiple versions in windows. I just change the exe name of the version I'm not defaulting to.

python.exe --> python26.exe

pythonw.exe --> pythonw26.exe

As for package installers, most exe installers allow you to choose the python install to add the package too. For manual installation check out the --prefix option to define where the package should be installed:

http://docs.python.org/install/index.html#alternate-installation-windows-the-prefix-scheme

Here's what you can do:

Install cmder. Open and use Cmder as you would with you cmd terminal. Use the command alias to create command aliases.

I did the following:

alias python2 = c:\python27\python.exe
alias python3 = c:\python34\python.exe

And that's it! ;-)

What I did was download both 2.7.6 and 3.3.4. Python 3.3.4 has the option to add the path to it in the environment variable so that was done. So basically I just manually added Python 2.7.6.

How to...

1. Start > in the search type in environment select "Edit environment variables to your account"1

2. Scroll down to Path, select path, click edit.

3. Add C:\Python27; so you should have paths to both versions of Python there, but if you don't this you can easily edit it so that you do..... C:\Python27;C:\Python33;

4. Navigate to the Python27 folder in C:\ and rename a copy of python.exe to python2.exe

5. Navigate to the Python34 folder in C:\ and rename a copy of python.exe to python3.exe

6. Test: open up commmand prompt and type python2 ....BOOM! Python 2.7.6. exit out.

7. Test: open up commmand prompt and type python3 ....BOOM! Python 3.4.3. exit out.

Note: (so as not to break pip commands in step 4 and 5, keep copy of python.exe in the same directory as the renamed file)

I just had to install them. Then I used the free (and portable) soft at http://defaultprogramseditor.com/ under "File type settings"/"Context menu"/search:"py", chose .py file and added an 'open' command for the 2 IDLE by copying the existant command named 'open with IDLE, changing names to IDLE 3.4.1/2.7.8, and remplacing the files numbers of their respective versions in the program path. Now I have just to right click the .py file and chose which IDLE I want to use. Can do the same with direct interpreters if you prefer.

I found that the formal way to do this is as follows:

Just install two (or more, using their installers) versions of Python on Windows 7 (for me work with 3.3 and 2.7).

Follow the instuctions below, changing the parameters for your needs.

Create the following environment variable (to default on double click):

Name:  PY_PYTHON
Value: 3

To launch a script in a particular interpreter, add the following shebang (beginning of script):

#! python2

To execute a script using a specific interpreter, use the following prompt command:

> py -2 MyScript.py

To launch a specific interpreter:

> py -2

To launch the default interpreter (defined by the PY_PYTHON variable):

> py

Resources

Documentation: Using Python on Windows

PEP 397 - Python launcher for Windows

I use a simple solution to switch from a version to another version of python, you can install all version you want. All you have to do is creating some variable environment. In my case, I have installed python 2.7 and python 3.8.1, so I have created this environment variables:

PYTHON_HOME_2.7=<path_python_2.7> PYTHON_HOME_3.8.1=<path_python_3.8.1> PYTHON_HOME=%PYTHON_HOME_2.7%

then in my PATH environment variable I put only %PYTHON_HOME% and %PYTHON_HOME%\Scripts. In the example above I'm using the version 2.7, when I want to switch to the other version I have only to set the PYTHON_HOME=%PYTHON_HOME_3.8.1%. I use this method to switch quickly from a version to another also for JAVA, MAVEN, GRADLE,ANT, and so on.

I have encountered that problem myself and I made my launchers in a .bat so you could choose the version you want to launch.

The only problem is your .py must be in the python folder, but anyway here is the code:

For Python2

@echo off
title Python2 Launcher by KinDa
cls
echo Type the exact version of Python you use (eg. 23, 24, 25, 26)
set/p version=
cls
echo Type the file you want to launch without .py (eg. hello world, calculator)
set/p launch=
path = %PATH%;C:\Python%version%
cd C:\Python%version%
python %launch%.py
pause

For Python3

@echo off
title Python3 Launcher by KinDa
cls
echo Type the exact version of Python you use (eg. 31, 32, 33, 34)
set/p version=
cls
echo Type the file you want to launch without .py (eg. hello world, calculator)
set/p launch=
cls
set path = %PATH%:C:\Python%version%
cd C:\Python%version%
python %launch%.py
pause

Save them as .bat and follow the instructions inside.

What I have done on my own windows computer where I have Python 2.7 and Python 3.4 installed is I wrote a simple .bat file in the same directory as my Python.exe files. They look something like,

cmd /k "c:\python27\python.exe" %*

The %* allows you to add arguments (Python files) afterwards. I believe /k keeps the prompt open after it finishes running the script. Then I save that as python27.bat Then I go to my Python 3 directory and make a bat file there. Now in my command line I can write

Python27 helloworld.py

Or

Python34 helloworld.py

And they will run in their respective versions of Python. Make sure that c:\python27 and c:\python34 are in your environment variables.

I got my answer from here

Install the one you use most (3.3 in my case) over the top of the other. That'll force IDLE to use the one you want.

Alternatively (from the python3.3 README):

Installing multiple versions

On Unix and Mac systems if you intend to install multiple versions of Python using the same installation prefix (--prefix argument to the configure script) you must take care that your primary python executable is not overwritten by the installation of a different version. All files and directories installed using "make altinstall" contain the major and minor version and can thus live side-by-side. "make install" also creates ${prefix}/bin/python3 which refers to ${prefix}/bin/pythonX.Y. If you intend to install multiple versions using the same prefix you must decide which version (if any) is your "primary" version. Install that version using "make install". Install all other versions using "make altinstall".

For example, if you want to install Python 2.6, 2.7 and 3.3 with 2.7 being the primary version, you would execute "make install" in your 2.7 build directory and "make altinstall" in the others.

If you can't get anything else to work, open an interpreter in whichever version you choose (I prefer using iPython) and:

import subprocess

subprocess.call('python script.py -flags')

This uses whichever python version you are currently operating under. Works fine for a single script, but will quickly get out of hand if there are lots of scripts you run, in which case you can always make a batch file with all of these calls inside. Not the most elegant answer, but it works.

Is there a way to make aliases for different python version a la Linux?

If you use Anaconda Python, you can easily install various environments.

Say you had Anaconda Python 2.7 installed and you wanted a python 3.4 environment:

conda create -n py34 python=3.4 anaconda

Then to activate the environment:

activate py34

And to deactive:

deactivate py34

(With Linux, you should use source activate py34.)

Links:

Download Anaconda Python

Instructions for environments

Only Works if your running your code in your Python IDE

I have both Python 2.7 and Python 3.3 installed on my windows operating system. If I try to launch a file, it will usually open up on the python 2.7 IDE. How I solved this issue, was when I choose to run my code on python 3.3, I open up python 3.3 IDLE(Python GUI), select file, open my file with the IDLE and save it. Then when I run my code, it runs to the IDLE that I currently opened it with. It works vice versa with 2.7.

Check your system environment variables after installing Python, python 3's directories should be first in your PATH variable, then python 2.

Whichever path variable matches first is the one Windows uses.

As always py -2 will launch python2 in this scenario.