fix: preserve existing sys.settrace when entering/exiting DebugContext

- Store previous trace function in _previous_trace before setting new trace
- Restore previous trace on exit instead of unconditionally clearing to None
- Skip sys.settrace calls entirely when context is disabled
- Add tests for disabled context behavior and previous trace restoration

Author: KJonline

src/helper/debugger.py

@@ -12,16 +12,21 @@ def __init__(self, name, enabled):
         self.name = name
         self.enabled = enabled
         self.logging = logging.getLogger(__name__)
+        self._previous_trace = None
 
     def __enter__(self):
         """Set trace calls on entering debugger."""
         self.logging.debug("Entering debug context for %s", self.name)
+        if not self.enabled:
+            return self
+        self._previous_trace = sys.gettrace()
         sys.settrace(self.trace_calls)
         return self
 
     def __exit__(self, exc_type, exc_val, exc_tb):
         """Remove trace on exiting debugger."""
-        sys.settrace(None)
+        if self.enabled:
+            sys.settrace(self._previous_trace)
         return False
 
     def trace_calls(self, frame, event, _arg):

tests/unit/test_debugger.py

@@ -41,6 +41,13 @@ def test_returns_self(self):
             returned = ctx.__enter__()
         assert returned is ctx
 
+    def test_disabled_context_does_not_set_sys_trace(self):
+        ctx = DebugContext("my_func", False)
+        with patch.object(sys, "settrace") as mock_settrace:
+            returned = ctx.__enter__()
+        mock_settrace.assert_not_called()
+        assert returned is ctx
+
 
 class TestDebugContextExit:
     """Tests for DebugContext.__exit__."""
@@ -51,6 +58,20 @@ def test_clears_sys_trace(self):
             ctx.__exit__(None, None, None)
             mock_settrace.assert_called_once_with(None)
 
+    def test_restores_previous_trace(self):
+        previous_trace = object()
+        ctx = DebugContext("my_func", True)
+        ctx._previous_trace = previous_trace
+        with patch.object(sys, "settrace") as mock_settrace:
+            ctx.__exit__(None, None, None)
+        mock_settrace.assert_called_once_with(previous_trace)
+
+    def test_disabled_context_exit_does_not_clear_trace(self):
+        ctx = DebugContext("my_func", False)
+        with patch.object(sys, "settrace") as mock_settrace:
+            ctx.__exit__(None, None, None)
+        mock_settrace.assert_not_called()
+
     def test_returns_false(self):
         ctx = DebugContext("my_func", True)
         with patch.object(sys, "settrace"):