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

user_319821 0
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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.

build-environment compatibility backwards-compatibility windows python
19 Answers

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.

user_47432750

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.

user_26673520
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