FindExternalEvaluators["Python"] gives me an uninstalled version of Python

Question

Long time ago, I have two versions of python installed in my pc, and I registered them all in mathematica. Later I uninstalled one of them, but FindExternalEvaluators["Python"] still gived me two results, and then when I wanted to run StartExternalSession["Python"],it tried to launch the uninstalled one first and gived me a lot of warning. The uninstalled Python is neither in the path of my system nor in the path of mathematica, and UnregisterExternalEvaluator doesn't work either. So how can I do if I want to get one result when I run FindExternalEvaluators["Python"]? My system is windows 10, version of mathematica is 11.2.

Answer 1

Update: As of version 12, one should use

FindExternalEvaluators["Python", "ResetCache" -> True]

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To turn my comment into an answer, ExternalEvaluate keeps a cache of evaluators it has found or used previously, and sometimes it can make invalid data persist.

In such cases, it is often useful to start with a clean slate, using the following command

ExternalEvaluate; (* trigger autoloading *)

ExternalEvaluate`Private`resetCache[]

which will clear the cached data.

Answer 2

Just as additional information. I found the above not to work at all. I received a (not wolfram message) "The code execution cannot proceed because python37.dll was not found". I don't really understand how the external evaluator registration works it's dependency's etc. Would be nice to have WRI document this (like the "ResetCache" option) Anyway I had the Registered status for Python 3.7 as MissingDependencies. I also had an old Anaconda installation with a Python version. But both versions where de-installed through windows add/remove programs

Unregister anaconda version The path shown in the Target was pointing to a path that did not exist. Recreating the path and copying a python.exe in there resolved the issue. This version was successfully unregistered.

Unregister the python 3.7 version What helped at the end was to re-install the same version of python in that directory. Re-Install PyZMQ. Then I could unregister the Python registration. Then I de-installed the Python version again.

After this I installed the latest Python version (3.10), upgraded Pip and installed pyZMQ after that the FindExternalEvaluators["Python"] found the installation and the status "Registered" was False. I registered the version manually and now everything works fine.