Recently installed Python 3.8 (from 2.7) on Windows 10, and all my scripts didn't work anymore. For some strange reason 

> python -s script.py args

would put the args into sys.argv[], while

> script.py args

would only show the script in sys.argv[]. I found this report which fixed the problem: 

https://stackoverflow.com/questions/15281951/sys-argv-contents-when-calling-python-script-implicitly-on-windows

(Basically, the .py(w) association in the registry need an additional '%*' parameter to pass the script parameters.)

So I assume the Python 3 Windows installer overwrote my earlier registry associations. 

The other way the problem happened could be that the Python 3 installer did not change the registry assocation to 3.8, and I did this manually. I verified that setting a "default program" for windows will put the deficient association (without argument passing) into the registry. 

Either way, I would expect that after installing Python 3.8, my scripts would really run with 3.8, and with their arguments. :-) 

Danke & Grüße von 
nobi

An "Applications\python.exe" progid gets created by browsing for "python.exe" when configuring the file association. For example, it gets created by the Windows shell (Explorer) via "open with" -> "choose another app" -> "look for another app on this PC". The shell has no option in this case to indicate that the filetype is a script that needs command-line arguments via "%*", so the "shell\open\command" template that it assigns is incorrect.

If the user also selects to "always use this app", the shell sets a "UserChoice" key in a key named for the file extension under "HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts". The user choice overrides the default association computed via HKCR settings. Applications are not supposed to meddle with the "UserChoice" key. Its contents are doubly protected by the key's security and a hash value.

The shell key for the file extension usually has two other subkeys: "OpenWithList" and "OpenWithProgids". These can also override the HKCR file association. The "OpenWithList" key lists executables that have been used to open the filetype via the "open with" menu, and it also contains an "MRUList" value that sorts the list by order of most recently used. The "OpenWithProgids" key caches progids that are used to open the program. Usually there's just one cached progid. But if there's more than one, in my experience the shell selects the progid that's associated with the first exectuable in the "OpenWithList", by order of the "MRUList" value. In this particular and peculiar case, the current file association follows whichever program was most recently selected in the "open with" list.

> I found this report which fixed the problem: 
> https://stackoverflow.com/questions/15281951

Continuing to use the shell-generated "Applications\python.exe" progid means that you won't have shebang support provided by the py.exe launcher (e.g. for associating scripts with particular virtual environments), the "Edit with IDLE" menu, or the shell-extension drop handler. 

I recommend selecting the installed "Python.File" progid instead. It's the "Python" app with the launcher icon that has a rocket on it. If you use the "open with" menu to select this progid, also select the option to always use the selected app. This sets the shell's "UserChoice" key to lock down the file association.

FYI, never modify the HKCR merged view as suggested by the linked SO answer. Modify the source hive in "HKCU\Software\Classes" or "HKLM\Software\Classes". If you modify HKCR directly, the key(s) that you end up modifying could be in one or the other hive depending on what's already present (e.g. you could modify a value just for the current user, though your intent is to modify it for all users). You should know whether you need to modify system settings, user settings, or both. If you don't know, you need to do more research before proceeding.

Eryk - thanks for your response. 

> I recommend selecting the installed "Python.File" progid instead. 
> It's the "Python" app with the launcher icon that has a rocket on it.

I don't have a Python with rocket launcher. When I use "Open With" on Windows Explorer, I only have two Python icons with the yin-yan snakes, one with white background (I presume that's still 2.7, as the  file icons were white before), and one with black background (which is available since I installed 3.8). If I select "Choose default program" (or similar - I have a German Windows), the list still only contains these two Python icons, and lot of unrelated stuff (zip, thunderbird, etc.). Initially, the black-background Python was not in this list at all, and I continued by "Further Options" and used the resulting browser to pick the 3.8 version. Since then, I have white and black Pythons. 

How do I get to the "rocket" launcher? 

Danke & Grüße von nobi

> How do I get to the "rocket" launcher? 

The py.exe launcher is installed by default with the Python 3 distribution from python.org. Did you install that, or did you install the app distribution from the Microsoft Store?

Eryk, I installed from python.org (didn't even know that it would be available from Microsoft Store). 

Nevertheless, I don't find py.exe. Should it be installed besides python.exe? If not, where else? 

I understand that py.exe is at least in part used to make handling multiple python versions easier. As I planned to only have one version (which might have been a bad idea in hindsight :-(  I might have unchecked any option that sounded like this... 

Danke & Grüße von nobi

> I understand that py.exe is at least in part used to make handling 
> multiple python versions easier. 

On the command line the launcher supports multiple installed versions via the "-X[.Y][-32|-64]" option. In scripts it supports multiple versions via virtual shebangs, e.g. "#!python3.8-32". But shebangs can also use fully-qualified Windows paths, which allows directly running a script in a particular virtual environment, e.g. "#!C:\VEnv\SparkEnv\Scripts\python.exe". Without the launcher, the .py file association is locked on a single Python installation.

> I don't find py.exe. Should it be installed besides python.exe? 
> If not, where else? 

If installed for all users, the py launcher is in the Windows directory. Otherwise it's in the user's local application data in the directory "%LocalAppData%\Programs\Python\Launcher".

Try repairing the installation to ensure the py launcher is installed, preferably for all users if you have admin access.