How to clear the Python environment on Windows

Question

Olav Queseth 2022-10-4

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评论： Olav Queseth 2022-10-17

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Hi!

When running Python with MATLAB there is the command

pyenv

to set or change the python interpreter that MATLAB uses.

I'm developing code that should work with and without Python installed (but with reduced functionallity) and for testing purposes I need to unset/clear the environment so that it looks like Python is not installed. I.e. so that the PythonEnvironment is not populated. Uninstalling Python does not help.

So. Is there any good way to clear the fields of the PythonEnvironment returned by pyenv ?

BTW: I'm running MATLAB on Windows 10

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Answer 1

Vijay 2022-10-14

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https://ww2.mathworks.cn/matlabcentral/answers/1817315-how-to-clear-the-python-environment-on-windows#answer_1074853

编辑：Vijay 2022-10-14

Hello @Olav Queseth

you can edit the PATH environment variable of windows and remove the path that point to python executable.

python - How to add to the PYTHONPATH in Windows, so it finds my modules/packages? - Stack Overflow

This link demonstrates the addition of python to environment. You have to remove it. You can remove it just by clearing it.

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Olav Queseth 2022-10-17

在 MATLAB Online 中打开

Hello @Vijay

Thanks for looking into this, but this does not answer my question. I'll try to explain a bit better:

On a clean MATLAB installation I can do:

>> pyenv
ans = 
PythonEnvironment with properties:
Version: ""
Executable: ""
Library: ""
Home: ""
Status: NotLoaded
ExecutionMode: InProcess

Then I install Python on the computer and click all the defaults. The python.exe gets added to the path and then I can do:

>> pyenv
ans = 
  PythonEnvironment with properties:
          Version: "3.7"
       Executable: "C:\Users\xxxx\AppData\Local\Programs\Python\Python37\python.EXE"
          Library: "C:\Users\xxxx\AppData\Local\Programs\Python\Python37\python37.dll"
             Home: "C:\Users\xxxx\AppData\Local\Programs\Python\Python37"
           Status: NotLoaded
    ExecutionMode: InProcess
    

Next I uninstall Python (obviously python.exe is removed from the path)

>> pyenv
ans = 
PythonEnvironment with properties:
Version: ""
Executable: ""
Library: ""
Home: ""
Status: NotLoaded
ExecutionMode: InProcess

This is all fine. But here is the problem: Suppose I installed Python in a non-standard location which is also not on the path. I can still execute Python like this:

>> pyenv('Version','C:\Python37\python.exe')
>> pyenv
ans = 
PythonEnvironment with properties:
Version: "3.7"
Executable: "C:\Python37\python.exe"
Library: "C:\Python37\python37.dll"
Home: "C:\Python37"
Status: NotLoaded
ExecutionMode: InProcess
>> py.print("Hello")
Hello

But now if I uninstall Python MATLAB still thinks that Python is there, like this:

>> pyenv
ans = 
PythonEnvironment with properties:
Version: "3.7"
Executable: "C:\Python37\python.exe"
Library: "C:\Python37\python37.dll"
Home: "C:\Python37"
Status: NotLoaded
ExecutionMode: InProcess

(BTW I have checked that there is no trace of Python on the path.)

The question is: Is there a way to make MATLAB forget that there was a Python interpreter installed?

The intuitive way to reset the flag does not work:

>> pyenv('Version','')
Error using pyenv
Path argument does not specify a valid executable.

I have tried looking all over for the file or registry value where the path to the Python interpreter is stored, but I'm not able to find it.

A second, but related, question is if there is a way to disable Python even if it is installed on the machine. For testing purposes it is much nicer to set a simple flag than to install/uninstall the whole interpreter to run the test.

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