If You have python of the same version with different subversion e.g. 2.6, 3.7,.. 3.9.
Use the below command to open specific python version's terminal in command prompt:

py -2.6
py -3.7
.

for installing modules in command prompt:

py -2.6 -m pip install <modules>
py -3.7 -m pip install <modules>

Hi @EMP_H ,

Yes, you should be able to switch between python versions. As a standard, it is recommended to use the python3 command or python3.7 to select a specific version. The py.exe launcher will automatically select the most recent version of Python you've installed. You can also use commands like py -3.7 to select a particular version, or py --list to see which versions can be used. HOWEVER, the py.exe launcher will only work if you are using a version of Python installed from python.org. When you install Python from the Microsoft Store, the py command is not included. Since you're using the Microsoft Store version of Python, you should use the python3 (or python3.7) command.

Hope that helped. Please let us know if you have further questions.

Best,
Grace

For information please read this FAQ doc for using python on windows.

This is most likely due to the path variable. You can see this in a command window by typing

Path

at your prompt.

To update the settings,

1. Open the 'System' properties.

2. Open Environment Variables

3. Highlight the 'Path' Variable and click edit.

4. Edit the values for the Python entries, to point to the desired python version.

5. OK on all boxes, close any CMD windows open, and open new one. Python command should now reference the correct location.

Solution refers to Windows 10, but is essentially the same on Windows 7 +

This is if you have both the versions installed.

Go to This PC → Right-click → Click on Properties → Advanced System Settings.

You will see the System Properties. From here navigate to the Advanced Tab -> Click on Environment Variables.

You will see a top half for the user variables and the bottom half for System variables.

Check the System Variables and double-click on the Path (to edit the Path).

Check for the path of Python(which you wish to run i.e. Python 2.x or 3.x) and move it to the top of the Path list.

Restart the Command Prompt, and now when you check the version of Python, it should correctly display the required version.

Call py.exe instead of python:

• py -3.6 will open a 3.6 shell;
• py -3.5 will open a 3.5 shell;
• py (check the version of an open shell: it could depend on installation and further settings).

Source: Python Launcher for Windows

New in version 3.3.

The Python launcher for Windows is a utility which aids in locating and executing of different Python versions. It allows scripts (or the command-line) to indicate a preference for a specific Python version, and will locate and execute that version.

Unlike the PATH variable, the launcher will correctly select the most appropriate version of Python. It will prefer per-user installations over system-wide ones, and orders by language version rather than using the most recently installed version.

Here's my discoveries.

Step 1. Go to System Properties. Click on Environment Variables

Step 2. Add new variables, such as PYTHON_27_HOME

• PYTHON_27_HOME:%ProgramFiles%\Python27\
• PYTHON_36_HOME:%ProgramFiles%\Python36\
• PYTHON_HOME:%PYTHON_27_HOME%

In my case, PYTHON_27_HOME(Python 2.7) is pointing to C:\Program Files\Python27\. You can replace this with your own path to python. %PYTHON_HOME% has a default value pointing to %PYTHON_27_HOME%, which is the path for Python 2.7. That's my preference, feel free to adjust accordingly. Be aware that there're 32-bit and 64-bit python. Please use %PROGRAMFILES% for path to C:\Program Files and %PROGRAMFILES(X86)% for path to C:\Program Files (x86).

Step 3. Select PATH and click on Edit. PATH

Step 4. Click on New and add %PYTHON_HOME%. %PYTHON_HOME% will be added to PATH automatically every time you launch a command prompt.

In order to switch between different versions of python in cmd, here's the trick.

Step 5. I created a batch file with

@echo off
:: Switch Python version
DOSKEY python27=SET PATH=%PYTHON_27_HOME%;%PATH%
DOSKEY python36=SET PATH=%PYTHON_36_HOME%;%PATH%

Basically, it disables echo and creates two alias. In batch file any string after :: is the comments. Every time, python27 or python36 is called, it re-exports %PATH% with the new Python path. Save it as profile.bat. You can name it whatever you want.

Step 6. Search for regedit (Registry Editor). Click on Edit > New > String Value. Give AutoRun as the Value name and %USERPROFILE%\profile.bat as the Value data. Here, please put your actual path value to the profile.bat we just created. So, whenever a command prompt is opened, it automatically loads profile.bat, which creates those two alias in the script.

Step 7. Close any command prompt you're using or just open a new command prompt. This is because your changes will not affect opened cmd window. Environment changes only happens to new CMD.

Step 8. Verify your results here.

If you're using different Java versions, same trick applies, too. Find my javac environment setting here.

As you can see, I have both Python2 and Python3 installed.

I hope you know that the path of the python executable has to be added to the PATH environment variable in Windows. As you can see, the path of Python2 is placed above the path of Python3 in my system.

How does the cmd run commands?
It searches for an executable with the same name in the directory the cmd has been opened in, then goes and searches for the command in the locations provided in the Windows PATH variable, from TOP to BOTTOM. Which means, it gets the path to Python2 before it can get to the path of Python3. Therefore, every time you type python in your cmd, it runs Python2.
Both Python2 and Python3 executables have the same name in Windows so it never runs python3.

What seems like an obvious solution?
You might think, changing the name of python.exe to python3.exe for the Python3 executable will solve your problem. You are partially right, it will work. But you have to use python3 file.py or python3 --version, which I think, is understandable. But, pip will no longer work if you change the name of the original python executable. You will no longer be able to install external packages and modules.

How can this problem be solved?
You can create an alias for the Python3 executable called python3.bat.
.exe and .bat files can be called from the cmd directly without using their extension. We always write python filename.py instead of python.exe filename.py although both are correct. The same can be done with .bat files.
Go back to the first image and notice the python3.bat file below python.exe. That is my way of calling python3 without renaming my original python executable.

python3.bat
Create a new file using notepad or something and paste this %~dp0python %*
I don't fully understand how this works except that dp0 basically runs python from inside the same directory and %* passes all the arguments to the python executable. Place this file inside your Python3 installation directory and your problem will hopefully be solved.

python3 basically runs your python3.bat file, which in turn runs the python.exe from its folder and passes the arguments to it.

I hope this solves your problem.