fix: name the python install command per system

Without Python the bootstrap told every system to run sudo apt install python3. It now reads Termux, macOS and /etc/os-release to name pkg, xcode-select, apt, dnf, pacman, zypper or apk, and in Termux offers to run pkg install python, which needs no root.

install.sh

@@ -118,17 +118,65 @@ if [ -z "$INSTALLER" ] || [ ! -f "$INSTALLER" ]; then
   INSTALLER="$TEMP_INSTALLER"
 fi
 
-PYTHON=""
-for command_name in python3 python; do
-  if command -v "$command_name" >/dev/null 2>&1 \
-    && "$command_name" -c 'import sys; raise SystemExit(sys.version_info < (3, 8))' 2>/dev/null; then
-    PYTHON=$command_name
-    break
+find_python() {
+  PYTHON=""
+  for command_name in python3 python; do
+    if command -v "$command_name" >/dev/null 2>&1 \
+      && "$command_name" -c 'import sys; raise SystemExit(sys.version_info < (3, 8))' 2>/dev/null; then
+      PYTHON=$command_name
+      return 0
+    fi
+  done
+  return 1
+}
+
+# The command that installs Python here, named for this system: Termux has
+# no sudo and installs with pkg, macOS has no apt.
+python_install_command() {
+  case "${TERMUX_VERSION:-}:${PREFIX:-}" in
+    ?*:*|*com.termux*) echo "pkg install python"; return ;;
+  esac
+  if [ "$(uname -s 2>/dev/null)" = "Darwin" ]; then
+    echo "xcode-select --install"
+    return
   fi
-done
+  os_ids=""
+  if [ -r /etc/os-release ]; then
+    os_ids=$(sed -n 's/^ID\(_LIKE\)\{0,1\}=//p' /etc/os-release | tr -d '"' | tr '\n' ' ')
+  fi
+  for os_id in $os_ids; do
+    case "$os_id" in
+      debian|ubuntu) echo "sudo apt install python3"; return ;;
+      fedora|rhel|centos) echo "sudo dnf install python3"; return ;;
+      arch) echo "sudo pacman -S python"; return ;;
+      opensuse*|suse) echo "sudo zypper install python3"; return ;;
+      alpine) echo "apk add python3"; return ;;
+    esac
+  done
+  echo ""
+}
+
+if ! find_python; then
+  install_command=$(python_install_command)
+  # Termux installs without root: at a terminal, the bootstrap offers to run it.
+  if [ "$install_command" = "pkg install python" ] && (: </dev/tty) 2>/dev/null; then
+    printf 'Python 3.8 or newer is required. Install it now with "%s"? [Y/n] ' \
+      "$install_command" >/dev/tty
+    read -r answer </dev/tty || answer="n"
+    case "$answer" in
+      [Nn]*) ;;
+      *) pkg install -y python </dev/tty && find_python || true ;;
+    esac
+  fi
+fi
 if [ -z "$PYTHON" ]; then
   echo "Error: Python 3.8 or newer is required." >&2
-  echo "  Install it, then run this command again. On Debian or Ubuntu: sudo apt install python3" >&2
+  if [ -n "$install_command" ]; then
+    echo "  Install it with: $install_command" >&2
+    echo "  Then run this command again." >&2
+  else
+    echo "  Install it with this system's package manager, then run this command again." >&2
+  fi
   exit 1
 fi
 

tests/test_install.py

@@ -2378,6 +2378,43 @@ class RetroArchWhereItReallyLives(unittest.TestCase):
             self.assertIn(("retroarch", support / "system"), found)
 
 
+class NoPythonNamesThisSystemsCommand(unittest.TestCase):
+    """Without Python the bootstrap said `sudo apt install python3` on every
+    system: Termux has no sudo and installs with `pkg install python`, and
+    macOS has no apt."""
+
+    def _run(self, extra_env: dict, uname: str = "Linux") -> subprocess.CompletedProcess:
+        root = Path(_tempdir(self))
+        fake_bin = root / "bin"
+        fake_bin.mkdir()
+        for tool in ("sed", "tr", "dirname", "cat"):
+            found = shutil.which(tool)
+            if found:
+                (fake_bin / tool).symlink_to(found)
+        (fake_bin / "uname").write_text(f"#!/bin/sh\necho {uname}\n", encoding="utf-8")
+        (fake_bin / "uname").chmod(0o755)
+        # No terminal: the Termux offer is a question asked on /dev/tty.
+        return subprocess.run(
+            ["/bin/sh", str(REPO_ROOT / "install.sh")], cwd=root,
+            env={"PATH": str(fake_bin), "HOME": str(root), **extra_env},
+            capture_output=True, text=True, timeout=30, check=False,
+            start_new_session=True,
+        )
+
+    def test_termux_is_told_pkg(self):
+        proc = self._run({"TERMUX_VERSION": "0.118",
+                          "PREFIX": "/data/data/com.termux/files/usr"})
+        self.assertEqual(proc.returncode, 1)
+        self.assertIn("Install it with: pkg install python", proc.stderr)
+        self.assertNotIn("sudo", proc.stderr)
+
+    def test_macos_is_told_xcode_select(self):
+        proc = self._run({}, uname="Darwin")
+        self.assertEqual(proc.returncode, 1)
+        self.assertIn("Install it with: xcode-select --install", proc.stderr)
+        self.assertNotIn("apt", proc.stderr)
+
+
 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/first-install.md

@@ -154,8 +154,12 @@ its BIOS folder, see [Getting started](getting-started.md).
 ### Error: Python 3.8 or newer is required.
 
 No Python 3.8 or newer was found: Python is missing, or older. The message
-names the command that installs it, `sudo apt install python3` on Debian and
-Ubuntu. On a handheld running Batocera or Recalbox it is already there, so
+names the command that installs it on the system it runs on: `sudo apt install
+python3` on Debian and Ubuntu, `sudo dnf install python3` on Fedora, `sudo
+pacman -S python` on Arch, `xcode-select --install` on macOS and `pkg install
+python` in Termux, where it offers to run that command itself (Termux installs
+without root). On Windows, `install.ps1` offers to run `winget install
+Python.Python.3.12`. On a handheld running Batocera or Recalbox it is already there, so
 this message usually means the command ran on the computer rather than on the
 console.