following instructions in Wiki to install PothosSDR-2021.07.25-vc16-x64.exe fails to install a working GNU Radio Companion

[csylvain]

the latest Pothos referenced in the wiki is PothosSDR-2021.07.25-vc16-x64.exe
the instructions refer to an installer for Python 3.9 -- there is no installer for this version any more at python.org

while similar to Issue #90, the problem is now unique to the 2021-07-25 release.

errors encountered: no installer for Python 3.9 at python.org, "failed to find amd64 python.exe", and "not amd64"

TL;DR

skip past the debugging and detailed technical info to the "To Fix" section below

Debugging

a long debug which included delving into the source of gnuradio-companion.cpp ensued. after installing Python 3.9 from Microsoft Store (the apparently only viable place to obtain this version of python presently), i find the InstallPath key was not created by PothosSDR-2021.07.25-vc16-x64.exe.

i did manage to remedy the missing InstallPath key by creating HKEY_LOCAL_MACHINE\SOFTWARE\Python\PythonCore\3.9\InstallPath and then filling in the (Default) REG_SZ value with C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.9_3.9.3568.0_x64__qbz5n2kfra8p0, which is where Microsoft Store places the files for the Python 3.9 package.

this further lead me to a cul-de-sac, since the Microsoft Store install of Python 3.9 in C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.9_3.9.3568.0_x64__qbz5n2kfra8p0 creates files not openable/readable by gnuradio-companion executable, which leads to the second "not amd64" error listed in the "failed to find amd64 python.exe" error message box.

To Fix

1. run command shell as Administrator, and copy the entire contents at C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.9_3.9.3568.0_x64__qbz5n2kfra8p0 to C:\Users<your username>\AppData\Local\Programs\Python\Python39 (use something like "xcopy dir-a dir-b /s/e" at CLI). you did it right if more than 7000 files are copied.
2. create Scripts directory in the ..\Python\Python39 directory
3. copy pip39.exe and python39.all to ..\Python\Python39\Scripts
4. rename pip39.exe to pip.exe in ..\Python\Python39\Scripts
5. run GNU Radio Companion. it will fail, but will also offer to fix the install with a helper. Say "Yes"
6. let the installs run to completion. ignore warning messages
7. GNU Radio Companion start up can take a while - be patient when it seems nothing further is happening

Phew!

this took an entire morning to sort out.

are there no new PothosSDR releases?

instructions for installing old releases cannot easily be made complete and correct since the Python Org is vigorous in leaving behind old versions. the consequence of Python Org policy is the Pothos Wiki is now well behind the times. and the Pothos 2021-07-25 release is therefore now behind the times.