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

Question

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

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 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

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

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.

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! ;-)