I'm writing this issue on behalf of the Rust project.

The build system for the Rust compiler is a Python 3 script `x.py`, which orchestrates the build process for a user even if they don't already have Rust installed. (For instance, `x.py build`, `x.py test`, and various command-line arguments for more complex cases.)

We currently run into various issues making this script easy for people to use on all common platforms people build Rust on: Windows, macOS, and Linux.

If we use a shebang of `#!/usr/bin/env python3`, then x.py works for macOS and Linux users, and also works on Windows systems that install Python via the Windows store, but fails to run on Windows systems that install via the official Python installer, requiring users to explicitly invoke Python 3 on the script, and adding friction, support issues, and complexity to our documentation to help users debug that situation.

If we use a shebang of `#!/usr/bin/env python`, then x.py works for Windows users, fails on some modern macOS systems, works on other modern macOS systems (depending on installation method I think, e.g. homebrew vs Apple), fails on some modern Linux systems, and on macOS and Linux systems where it *does* work, it might be python2 or python3. So in practice, people often have to explicitly run `python3 x.py`, which again results in friction, support issues, and complexity in our documentation.

We've even considered things like `#!/bin/sh` and then writing a shell script hidden inside a Python triple-quoted string, but that doesn't work well on Windows where we can't count on the presence of a shell.

We'd love to write a single shebang that works for all of Windows, macOS, and Linux systems, and doesn't resort in recurring friction or support issues for us across the wide range of systems that our users use.

As far as we can tell, `#!/usr/bin/env python3` would work on all platforms, if the Python installer for Windows shipped a `python3.exe` and handled that shebang by using `python3.exe` as the interpreter.

Is that something that the official Python installer could consider adding, to make it easy for us to supply cross-platform Python 3 scripts that work out of the box for all our users?

Thank you,
Josh Triplett, on behalf of many Rust team members

Correction to the above evaluation of `#!/usr/bin/env python3`, based on some retesting on Windows systems:

The failure case we encounter reasonably often involves the official Python installer for Windows, but applies specifically in the case of third-party shells such as MSYS2, which fail with that shebang. `#!/usr/bin/env python3` does work with the official Python installer when running from cmd or PowerShell, it just doesn't work from third-party shells.

We have enough users that cases like this come up reasonably often, and it'd be nice to Just Work in those cases too.

Thank you.

(MSYS2 maintainer here) If you run a Python script in MSYS2 bash, the shebang gets interpreted by bash, which looks up in PATH (which by default doesn't include the full Windows PATH to avoid conflicts) and if installed leads to the MSYS2 included CPython to be invoked, either the Cygwin based one, or the MinGW fork, depending on the active MSYS2 environment.

In short, there is no System CPython involved in any of this.