fix: name the old Python the bootstrap found

install.sh

@@ -127,10 +127,18 @@ fi
 find_python() {
   PYTHON=""
   PYTHON_LACKING=""
+  PYTHON_OLD=""
   for command_name in python3 python; do
     command -v "$command_name" >/dev/null 2>&1 || continue
-    "$command_name" -c 'import sys; raise SystemExit(sys.version_info < (3, 8))' \
-      2>/dev/null || continue
+    if ! "$command_name" -c 'import sys; raise SystemExit(sys.version_info < (3, 8))' \
+        2>/dev/null; then
+      found_version=$("$command_name" -c \
+        'import sys; print("%d.%d.%d" % tuple(sys.version_info[:3]))' 2>/dev/null)
+      if [ -n "$found_version" ]; then
+        PYTHON_OLD=${PYTHON_OLD:-"$command_name here is $found_version"}
+      fi
+      continue
+    fi
     if "$command_name" -c 'import ssl, unicodedata, zlib' 2>/dev/null; then
       PYTHON=$command_name
       return 0
@@ -184,6 +192,15 @@ if [ -z "$PYTHON" ] && [ -n "$PYTHON_LACKING" ]; then
   echo "  (ssl, unicodedata or zlib). Run it with a full Python 3.8 or newer." >&2
   exit 1
 fi
+if [ -z "$PYTHON" ] && [ -n "$PYTHON_OLD" ]; then
+  # Debian Buster ships 3.7: apt answers that python3 is already the newest
+  # version, and the same advice looped.
+  echo "Error: Python 3.8 or newer is required, and $PYTHON_OLD." >&2
+  echo "  Install a newer one: from this system's packages if they offer one, by" >&2
+  echo "  updating the system, or from https://www.python.org/downloads/." >&2
+  echo "  Then run this command again." >&2
+  exit 1
+fi
 if [ -z "$PYTHON" ]; then
   echo "Error: Python 3.8 or newer is required." >&2
   if [ -n "$install_command" ]; then

tests/test_install.py

@@ -2628,6 +2628,18 @@ class ABootstrapPythonCanDoHttps(unittest.TestCase):
         self.assertEqual(proc.returncode, 0, proc.stderr)
         self.assertIn("installer ran", proc.stdout)
 
+    def test_an_old_python_is_named_with_its_version(self):
+        """Debian Buster's python3 3.7 got "Install it with: sudo apt install
+        python3", which apt answers with "already the newest version"."""
+        root, fake_bin = self._bin()
+        (fake_bin / "python3").write_text(
+            '#!/bin/sh\ncase "$2" in *print*) echo 3.7.3 ;; esac\nexit 1\n')
+        proc = self._run(root, fake_bin)
+        self.assertEqual(proc.returncode, 1)
+        self.assertIn("Python 3.8 or newer is required, and python3 here is 3.7.3.",
+                      proc.stderr)
+        self.assertNotIn("Install it with: sudo apt", proc.stderr)
+
     def test_the_missing_module_is_named(self):
         root, fake_bin = self._bin()
         proc = self._run(root, fake_bin)