Fix custom chat templates with a {system_message} placeholder (dead code in _change_system_message) (#6735)

---------

Co-authored-by: oobabooga <112222186+oobabooga@users.noreply.github.com>

tests/python/test_change_system_message.py

@@ -0,0 +1,76 @@
+import ast
+import re
+import types
+from pathlib import Path
+
+import pytest
+
+
+def _load_change_system_message():
+    # Extract just _change_system_message from chat_templates.py so the test runs
+    # without importing unsloth (which needs unsloth_zoo / a GPU). Same pattern as
+    # tests/saving/test_is_gpt_oss_detection.py.
+    source = Path(__file__).parents[2] / "unsloth" / "chat_templates.py"
+    tree = ast.parse(source.read_text(encoding = "utf-8"))
+    funcs = [
+        node
+        for node in tree.body
+        if isinstance(node, ast.FunctionDef) and node.name == "_change_system_message"
+    ]
+    namespace = {
+        "re": re,
+        "logger": types.SimpleNamespace(warning_once = lambda *a, **k: None),
+        "DEFAULT_SYSTEM_MESSAGE": {"unsloth": "You are a helpful assistant to the user"},
+    }
+    module = ast.Module(body = funcs, type_ignores = [])
+    ast.fix_missing_locations(module)
+    exec(compile(module, str(source), "exec"), namespace)
+    return namespace["_change_system_message"]
+
+
+CUSTOM = "mycustom"  # not in DEFAULT_SYSTEM_MESSAGE -> no predefined default
+
+
+def test_custom_template_fills_placeholder():
+    # A custom template with a {system_message} placeholder must be filled, not
+    # left with the literal placeholder.
+    fn = _load_change_system_message()
+    template, used = fn("System: {system_message}\nUser:", CUSTOM, "You are a pirate")
+    assert template == "System: You are a pirate\nUser:"
+    assert "{system_message}" not in template
+    assert used == "You are a pirate"
+
+
+def test_custom_template_preserves_backslashes():
+    # Why str.replace and not re.sub: a system message with backslashes (Windows
+    # paths, LaTeX, group-like text) must be inserted verbatim. re.sub treats the
+    # replacement specially -- r"C:\Users" raises bad-escape, r"\1" is a group ref.
+    fn = _load_change_system_message()
+    for msg in (r"C:\Users\me", r"\frac{a}{b}", r"see \1 here"):
+        template, used = fn("System: {system_message}", CUSTOM, msg)
+        assert template == f"System: {msg}"
+        assert used == msg
+
+
+def test_custom_template_requires_system_message():
+    # A custom template with a placeholder but no system message must raise,
+    # rather than silently leaving the placeholder in.
+    fn = _load_change_system_message()
+    with pytest.raises(ValueError):
+        fn("System: {system_message}", CUSTOM, None)
+
+
+def test_custom_template_without_placeholder_unchanged():
+    fn = _load_change_system_message()
+    template, used = fn("System: fixed", CUSTOM, "ignored")
+    assert template == "System: fixed"
+
+
+def test_predefined_template_uses_default_then_override():
+    # Predefined templates with a default are unaffected by the change.
+    fn = _load_change_system_message()
+    t1, u1 = fn("System: {system_message}", "unsloth", None)
+    assert t1 == "System: You are a helpful assistant to the user"
+    t2, u2 = fn("System: {system_message}", "unsloth", "Custom override")
+    assert t2 == "System: Custom override"
+    assert u2 == "Custom override"