How to Install Python 3.X and 2.X on the Same Windows Computer

install python 2.x and 3.x on the same windows computer

If you have installed WinPython for Python 3.3 it also installs Python 2.7 but with a different name py.exe instead of the default python.exe.

Python 2.x and 3.x together on Windows

So would I have to manually change the PYTHONPATH and replace Python27 with Python32?

Yes.

How to I get Python 2.x and 3.x to co-exist?

I'm not sure I understand your question, but I'll take a shot. I'm also assuming you're on Windows.

It's simple -- just install both. They will install to different directories, create different start menu folders, etc. I'd also reccomend PyWin32 for the PythonWin editor installed in both 2.7 and 3.2,

http://sourceforge.net/projects/pywin32/files/pywin32/Build216/pywin32-216.1.win32-py3.2.exe/download &
http://sourceforge.net/projects/pywin32/files/pywin32/Build216/pywin32-216.win32-py2.7.exe/download

If you mean how do you write one script that works with either Python 2 or Python 3, look at http://docs.python.org/library/2to3.html

How can I install Python 3x and 2x into same machine?

Just install them normally. By default each version of Python is installed to a different path, and won't conflict. Python 3 and Python 2 don't even use the same executable name (python vs python3), so there's not a lot to worry about.

How to run multiple Python versions on Windows

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.

How to use pip with Python 3.x alongside Python 2.x

The approach you should take is to install pip for Python 3.2.

You do this in the following way:

$ curl -O https://bootstrap.pypa.io/get-pip.py
$ sudo python3.2 get-pip.py

Then, you can install things for Python 3.2 with pip-3.2, and install things for Python 2-7 with pip-2.7. The pip command will end up pointing to one of these, but I'm not sure which, so you will have to check.

How to install multiple versions of Python in Windows?

Your questions depend a bit on "all the other software". For example, as @leiyang indicated, the answer will be different if you use conda vs just pip on vanilla CPython (the standard Windows Python).

I'm also going to assume you're actually on Windows, because on Linux I would recommend looking at pyenv. There is a pyenv-win, which may be worth looking into, but I don't use it myself because it doesn't play as nice if you also want (mini)conda environments.

1. (a) How do I install multiple Python versions?

Simply download the various installers and install them in sensible locations. E.g. "C:\Program Files\Python39" for Python 3.9, or some other location where you're allowed to install software.

Don't have Python add itself to the PATH though, since that'll only find the last version to do so and can really confuse things.

Also, you probably want to use virtual environments consistently, as this ties a specific project very clearly to a specific Python version, avoiding future confusion or problems.

1. (b) "3.8.1, 3.8.2 till 3.8.13" which should I pick?

Always pick the latest 3.x.y, so if there's a 3.8.13 for Windows, but no 3.8.14, pick that. Check if the version is actually available for your operating system, sometimes there are later versions for one OS, but not for another.

The reason is that between a verion like 3.6 and 3.7, there may be major changes that change how Python works. Generally, there will be backwards compatibility, but some changes may break how some of your packages work. However, when going up a minor version, there won't be any such breaking changes, just fixes and additions that don't get in the way of what was already there. A change from 2.x to 3.x only happens if the language itself goes through a major change, and rarely happens (and perhaps never will again, depending on who you ask).

An exception to the "no minor version change problems" is of course if you run some script that very specifically relies on something that was broken in 3.8.6, but no fixed in 3.8.7+ (as an example). However, that's very bad coding, to rely on what's broken and not fixing it later, so only go along with that if you have no other recourse. Otherwise, just the latest minor version of any version you're after.

Also: make sure you pick the correct architecture. If there's no specific requirement, just pick 64-bit, but if your script needs to interact with other installed software at the binary level, it may require you to install 32-bit Python (and 32-bit packages as well). If you have no such requirement, 64-bit allows more memory access and has some other benefits on modern computers.

2. Why does Python have such heavy dependency on the exact versions of libraries and packages etc?

It's not just Python, this is true for many languages. It's just more visible to the end user for Python, because you run it as an interpreted language. It's only compiled at the very last moment, on the computer it's running on.

This has the advantage that the code can run on a variety of computers and operating systems, but the downside that you need the right environment where you're running it. For people who code in languages like C++, they have to deal with this problem when they're coding, but target a much smaller number of environments (although there's still runtimes to contend with, and DirectX versions, etc.). Other languages just roll everything up into the program that's being distributed, while a Python script by itself can be tiny. It's a design choice.

There are a lot of tools to help you automate the process though and well-written packages will make the process quite painless. If you feel Python is very shakey when it comes to this, that's probable to blame on the packages or scripts you're using, not really the language. The only fault of the language is that it makes it very easy for developers to make such a mess for you and make your life hard with getting specific requirements.

Look for alternatives, but if you can't avoid using a specific script or package, once you figure out how to install or use it, document it or better yet, automate it so you don't have to think about it again.

3. Do virtual environments copy all the files from the main Python installation to create a virtual environment and then install specific packages inside it? Isn't that a lot of wasted resources in duplication because almost all projects require there own virtual environment.

Not all of them, but quite a few of them. However, you still need the original installation to be present on the system. Also, you can't pick up a virtual environment and put it somewhere else, not even on the same PC without some careful changes (often better to just recreate it).

You're right that this is a bit wasteful - but this is a difficult choice.

Either Python would be even more complicated, having to manage many different version of packages in a single environment (Java developers will be able to tell you war stories about this, with their dependency management - or wax lyrically about it, once they get it themselves).

Or you get what we have: a bit wasteful, but in the end diskspace is a lot cheaper than your time. And unlike your time, diskspace is almost infinitely expandable.

You can share virtual environments between very similar projects though, but especially if you get your code from someone else, it's best to not have to worry and just give up a few dozen MB for the project. On the upside: you can just delete a virtual environment directory and that pretty much gets rid of the whole things. Some applications like PyCharm may remember that it was once there, but other than that, that's the virtual environment gone.