--- a/tests/run-tests.py	Fri Jan 24 23:14:54 2025 +0100
+++ b/tests/run-tests.py	Sat Jan 25 23:49:24 2025 +0100
@@ -54,6 +54,7 @@
 import json
 import multiprocessing
 import os
+import pathlib
 import platform
 import queue
 import random
@@ -72,7 +73,6 @@
 import uuid
 import xml.dom.minidom as minidom
 
-
 # Don't compare sys.version_info directly, to prevent pyupgrade from dropping
 # the conditional.
 sys_version_info = sys.version_info
@@ -760,8 +760,25 @@
         )
         path_local_hg = os.path.join(reporootdir, venv_local, BINDIRNAME, b"hg")
         if not os.path.exists(path_local_hg):
-            # no local environment but we can still use ./hg to please test-run-tests.t
-            path_local_hg = os.path.join(reporootdir, b"hg")
+            if "HGTEST_REAL_HG" in os.environ:
+                # this file is run from a test (typically test-run-tests.t)
+                # no local environment but we can still use ./hg to please test-run-tests.t
+                path_local_hg = os.path.join(reporootdir, b"hg")
+            else:
+                message = (
+                    f"run-tests.py called with --local but {_bytes2sys(venv_local)} does not exist.\n"
+                    f'To create it, run \nmake local PYTHON="{sys.executable}"'
+                )
+                paths_venv = sorted(
+                    pathlib.Path(_bytes2sys(reporootdir)).glob(".venv_*")
+                )
+                if paths_venv:
+                    message += (
+                        "\nAlternatively, call run-tests.py with a Python "
+                        f"corresponding to {[p.name for p in paths_venv]}."
+                    )
+                print(message, file=sys.stderr)
+                sys.exit(1)
 
         pathandattrs = [(path_local_hg, 'with_hg')]
         if options.chg: