pythongh-121016: Add test for PYTHON_BASIC_REPL envioronment variab…

…le (python#121017)

(cherry picked from commit 9e45fd9)

Lib/test/support/__init__.py

@@ -2607,3 +2607,9 @@ def wrapper(*args, **kwargs):
                 if value is not None:
                     os.environ[key] = value
     return wrapper
+
+
+def initialized_with_pyrepl():
+    """Detect whether PyREPL was used during Python initialization."""
+    # If the main module has a __file__ attribute it's a Python module, which means PyREPL.
+    return hasattr(sys.modules["__main__"], "__file__")

Lib/test/test_pyrepl/test_pyrepl.py

@@ -873,6 +873,31 @@ def test_dumb_terminal_exits_cleanly(self):
         self.assertNotIn("Exception", output)
         self.assertNotIn("Traceback", output)
 
+    @force_not_colorized
+    def test_python_basic_repl(self):
+        env = os.environ.copy()
+        commands = ("from test.support import initialized_with_pyrepl\n"
+                    "initialized_with_pyrepl()\n"
+                    "exit()\n")
+
+        env.pop("PYTHON_BASIC_REPL", None)
+        output, exit_code = self.run_repl(commands, env=env)
+        if "can\'t use pyrepl" in output:
+            self.skipTest("pyrepl not available")
+        self.assertEqual(exit_code, 0)
+        self.assertIn("True", output)
+        self.assertNotIn("False", output)
+        self.assertNotIn("Exception", output)
+        self.assertNotIn("Traceback", output)
+
+        env["PYTHON_BASIC_REPL"] = "1"
+        output, exit_code = self.run_repl(commands, env=env)
+        self.assertEqual(exit_code, 0)
+        self.assertIn("False", output)
+        self.assertNotIn("True", output)
+        self.assertNotIn("Exception", output)
+        self.assertNotIn("Traceback", output)
+
     def run_repl(self, repl_input: str | list[str], env: dict | None = None) -> tuple[str, int]:
         master_fd, slave_fd = pty.openpty()
         process = subprocess.Popen(