Python 3.9 UWP does not create key in PythonCore

[bintoss]

BPO	44027
Nosy	@pfmoore, @tjguk, @zware, @zooba, @BinToss

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-05-04.19:55:55.204>
created_at = <Date 2021-05-04.04:30:58.686>
labels = ['type-bug', 'invalid', '3.9', 'OS-windows']
title = 'Python 3.9 UWP does not create key in PythonCore'
updated_at = <Date 2021-05-04.19:55:55.204>
user = 'https://github.com/bintoss'

bugs.python.org fields:

activity = <Date 2021-05-04.19:55:55.204>
actor = 'steve.dower'
assignee = 'none'
closed = True
closed_date = <Date 2021-05-04.19:55:55.204>
closer = 'steve.dower'
components = ['Windows']
creation = <Date 2021-05-04.04:30:58.686>
creator = 'BinToss'
dependencies = []
files = []
hgrepos = []
issue_num = 44027
keywords = []
message_count = 2.0
messages = ['392851', '392940']
nosy_count = 5.0
nosy_names = ['paul.moore', 'tim.golden', 'zach.ware', 'steve.dower', 'BinToss']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44027'
versions = ['Python 3.9']

[bintoss]

On Windows, dependent applications such as VapourSynth look for Python's path via the entries in HKLM\\SOFTWARE\\Python\\PythonCore\\.
However, the Python 3.8 and 3.9 UWP releases don't create and write to their respective subkeys. Only the Win32 releases write to their subkeys.

See vapoursynth/vapoursynth#684

[zooba]

Correct. It does, however, write to the lookaside space, which Windows will map back if you access it.

So reading HKLM\Software\Python\PythonCore\3.9 will return all the values that would be there. It's only enumeration that doesn't work. I've reported that to the team responsible, and they think it's not necessary, so I have no qualms about saying that you should *also* look in "SOFTWARE\\Microsoft\\AppModel\\Lookaside\\user\\Software\\Python\\PythonCore". (And if another redistributor also does a UWP package of Python, they'll have their own registry section for it to go into.)

Not writing into shared registry locations is one of the *best* features of this form of packaging, so I'm not inclined to try too hard to break it. Overall the robustness and stability is worthwhile, so it'll just fall to tool developers to catch up a bit, or alternatively win over the team that could just fix it properly.