Why can't I run freshly-installed Python from the command line on Windows?

Generally, programs installed "for all users" on Windows go into separate folders, side by side, within C:\Program Files (or a similarly named directory). Each program is bundled together with its associated libraries, documentation etc. in its own folder. This is different from the default Linux organization, where the main programs go directly side-by-side in /usr/bin (or similar) and the supporting files might go into /usr/lib, /usr/share (for documentation), etc. Because of this, installers for Windows programs need to add that sub-folder to PATH to make the program discoverable - entries on PATH are only checked directly, not searched.

Since the introduction of the Python Launcher for Windows in 2011, the default choice in the Windows installer is not to add the new Python to PATH. This is partly to avoid polluting the PATH (or even corrupting it: there's a length limit, and lots of other programs want to add themselves too), and partly to avoid confusion when multiple versions are installed.

After all, the entire point of the launcher is that it uses its own logic to find a Python installation (and it can even emulate how Linux treats shebang lines). So, since every installer ensures that the launcher is available, there's no need to add to PATH - unless you want python to work as a command instead of py.

In May 2019, a Windows 10 update added some Microsoft Store "helpers" for Python. The idea was to make it easier for Windows users who wanted to learn Python, to obtain it and start using it right away - without having to do research to find the Python web site, for example. So if a new student tried to run python, Windows would provide a helpful message and a direct way to install Python - without needing to pre-install it as part of Windows.

Unfortunately, there are some serious drawbacks to installing Python this way. It bypasses the "lengthy" setup wizard... which contains some options that some users find very useful. The update apparently also caused other issues for some users who already had Python installed when the update was released. Finally, this distribution of Python is a bit non-standard - in particular, because it's a Store app, it has limitations on file system access. Hilariously, at first, py didn't know how to launch the Store version of Python, either.

In order to make sure that these helpers are found, Windows puts their containing folder on the PATH. To try to make sure that the helpers are only found when Python is not installed, Windows puts said folder at the end of the PATH.

Which would work brilliantly, if it weren't for the fact that official Python installers had, for the last 8 years or so, been defaulting to not adding the newly installed Python to the PATH.

Oops.

(You'd think someone at Microsoft would have been aware of the potential issue. Maybe instead of a special shortcut link, they could have made an actual script that checks whether the launcher is installed. Considering that it would always be named %WINDOWS%\py.exe, that ought to be pretty easy. I'm rather disappointed at the failure to coordinate, honestly.)