feat: offer winget when windows lacks python

Without Python, install.ps1 at a console now offers to run winget install Python.Python.3.12, rereads PATH from the registry and carries on. The Store's python.exe placeholder fails the version check like a missing Python.

install.ps1

@@ -64,24 +64,51 @@
             $installer = $temporary
         }
 
-        $launcher = @()
-        $python = Get-Command py -ErrorAction SilentlyContinue
-        if ($python) {
-            $launcher = @("-3")
-        } else {
-            $python = Get-Command python3 -ErrorAction SilentlyContinue
-            if (-not $python) {
-                $python = Get-Command python -ErrorAction SilentlyContinue
+        # The py launcher first, then python3 and python. The Microsoft Store
+        # places a python.exe that only points at the Store: it fails the
+        # version check like a missing one.
+        $findPython = {
+            $found = Get-Command py -ErrorAction SilentlyContinue
+            if ($found) { return ,@($found, @("-3")) }
+            foreach ($name in @("python3", "python")) {
+                $found = Get-Command $name -ErrorAction SilentlyContinue
+                if ($found) { return ,@($found, @()) }
             }
+            return $null
+        }
+        $usable = {
+            param($candidate)
+            if (-not $candidate) { return $false }
+            $arguments = $candidate[1]
+            & $candidate[0].Source @arguments -c "import sys; raise SystemExit(sys.version_info < (3, 8))" | Out-Null
+            return ($LASTEXITCODE -eq 0)
         }
         $needPython = "Python 3.8 or newer is required. Install it with " +
             "'winget install Python.Python.3.12' or from https://www.python.org/downloads/ " +
             "(tick 'Add python.exe to PATH'), then open a new PowerShell window and run this command again."
-        if (-not $python) {
-            throw $needPython
+        $candidate = & $findPython
+        if (-not (& $usable $candidate)) {
+            # winget ships with Windows 10 and 11: at a console the bootstrap
+            # offers to run the install the message names, then carries on.
+            $winget = Get-Command winget -ErrorAction SilentlyContinue
+            if ($winget -and -not [Console]::IsInputRedirected) {
+                $answer = Read-Host "Python 3.8 or newer is required. Install Python 3.12 now with winget? [Y/n]"
+                if ($answer -notmatch '^\s*[nNqQ]') {
+                    & $winget.Source install --id Python.Python.3.12 --exact --source winget `
+                        --accept-package-agreements --accept-source-agreements
+                    # The installer adds Python to the PATH of new sessions:
+                    # this one reads it again from the registry.
+                    if ($env:OS -eq "Windows_NT") {
+                        $env:Path = [Environment]::GetEnvironmentVariable("Path", "Machine") + ";" +
+                            [Environment]::GetEnvironmentVariable("Path", "User")
+                    }
+                    $candidate = & $findPython
+                }
+            }
+            if (-not (& $usable $candidate)) { throw $needPython }
         }
-        & $python.Source @launcher -c "import sys; raise SystemExit(sys.version_info < (3, 8))"
-        if ($LASTEXITCODE -ne 0) { throw $needPython }
+        $python = $candidate[0]
+        $launcher = $candidate[1]
         # The installer rebuilds this command in its hints, and the user has
         # no install.py of their own to run.
         $previousBootstrap = $env:RETROBIOS_BOOTSTRAP

tests/test_install.py

@@ -2415,6 +2415,52 @@ class NoPythonNamesThisSystemsCommand(unittest.TestCase):
         self.assertNotIn("apt", proc.stderr)
 
 
+@unittest.skipUnless(shutil.which("pwsh") and shutil.which("script"), "needs PowerShell and a terminal")
+class WindowsWithoutPythonIsOfferedWinget(unittest.TestCase):
+    """A Windows machine without Python was told to install it, open a new
+    window and run the command again, while winget, present on Windows 10
+    and 11, could do it from the bootstrap."""
+
+    def _folder(self) -> tuple[Path, Path]:
+        folder = Path(_tempdir(self))
+        shutil.copy(REPO_ROOT / "install.ps1", folder / "install.ps1")
+        (folder / "install.py").write_text("print('installer ran')\n")
+        fake_bin = folder / "bin"
+        fake_bin.mkdir()
+        python = shutil.which("python3") or sys.executable
+        (fake_bin / "winget").write_text(
+            f"#!/bin/sh\necho \"winget $*\"\n{shutil.which('ln')} -s {shlex.quote(python)} "
+            f"{shlex.quote(str(fake_bin / 'python3'))}\n"
+        )
+        (fake_bin / "winget").chmod(0o755)
+        return folder, fake_bin
+
+    def _run(self, folder: Path, fake_bin: Path, answer: str) -> subprocess.CompletedProcess:
+        pwsh = shutil.which("pwsh")
+        command = (f"{shlex.quote(pwsh)} -NoProfile -File "
+                   f"{shlex.quote(str(folder / 'install.ps1'))}")
+        return subprocess.run(
+            [shutil.which("script"), "-qec", command, "/dev/null"], input=answer,
+            env={**os.environ, "PATH": f"{fake_bin}{os.pathsep}/usr/bin/nonexistent"},
+            capture_output=True, text=True, timeout=120, check=False,
+        )
+
+    def test_yes_installs_and_carries_on(self):
+        folder, fake_bin = self._folder()
+        proc = self._run(folder, fake_bin, "y\n")
+        self.assertIn("Install Python 3.12 now with winget?", proc.stdout)
+        self.assertIn("winget install --id Python.Python.3.12", proc.stdout)
+        self.assertIn("installer ran", proc.stdout)
+        self.assertEqual(proc.returncode, 0, proc.stdout)
+
+    def test_no_names_the_command(self):
+        folder, fake_bin = self._folder()
+        proc = self._run(folder, fake_bin, "n\n")
+        self.assertNotIn("installer ran", proc.stdout)
+        self.assertIn("winget install Python.Python.3.12", proc.stdout)
+        self.assertFalse((fake_bin / "python3").exists())
+
+
 class ThePowerShellWrapperPassesTheStatusOn(unittest.TestCase):
     """Every non-zero exit became a red "installer failed" exception and
     code 1: a --check that found work read as a crash, 2 and 130 were lost."""

wiki/installer.md

@@ -62,8 +62,13 @@ the same way: waiting five minutes settles it. Any change to `install.py`
 therefore means recomputing that pin in both wrappers.
 
 They locate a Python 3.8 or newer interpreter (`python3` then `python` on
-POSIX, `py -3` then `python3` then `python` on Windows) and refuse to continue
-without one. `install.sh` needs `curl` or `wget` and `sha256sum` or `shasum`.
+POSIX, `py -3` then `python3` then `python` on Windows). Without one,
+`install.sh` names the command that installs it on the system it runs on
+(`pkg install python` in Termux, which it offers to run since Termux installs
+without root, `xcode-select --install` on macOS, apt, dnf, pacman, zypper or apk
+from `/etc/os-release`), and `install.ps1` at a console offers to run `winget
+install Python.Python.3.12`, then reads the new PATH and carries on.
+`install.sh` needs `curl` or `wget` (GNU or BusyBox) and `sha256sum` or `shasum`.
 Windows PowerShell 5.1 still negotiates SSL 3.0 and TLS 1.0 by default and
 GitHub refuses both, so `install.ps1` raises the floor to TLS 1.2 before
 downloading, and `install.sh` pins the same floor with `curl --tlsv1.2`.