chore(debugger): better no local error message (#11405)

We improve the error message reported when an expression fails to
resolve a reference to a local variable.

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [ ] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

ddtrace/debugging/_expressions.py

@@ -86,6 +86,13 @@ def instanceof(value: Any, type_qname: str) -> bool:
     return False
 
 
+def get_local(_locals: Mapping[str, Any], name: str) -> Any:
+    try:
+        return _locals[name]
+    except KeyError:
+        raise NameError(f"No such local variable: '{name}'")
+
+
 class DDCompiler:
     @classmethod
     def __getmember__(cls, o, a):
@@ -235,11 +242,9 @@ def _compile_direct_operation(self, ast: DDASTType) -> Optional[List[Instr]]:
             if arg == "@it":
                 return [Instr("LOAD_FAST", "_dd_it")]
 
-            return [
-                Instr("LOAD_FAST", "_locals"),
-                Instr("LOAD_CONST", self.__ref__(arg)),
-                Instr("BINARY_SUBSCR"),
-            ]
+            return self._call_function(
+                get_local, [Instr("LOAD_FAST", "_locals")], [Instr("LOAD_CONST", self.__ref__(arg))]
+            )
 
         return None
 

tests/debugging/test_debugger.py

@@ -772,7 +772,7 @@ def test_debugger_condition_eval_error_get_reported_once():
         evaluationErrors = snapshot["debugger"]["snapshot"]["evaluationErrors"]
         assert 1 == len(evaluationErrors)
         assert "foo == 42" == evaluationErrors[0]["expr"]
-        assert "'foo'" == evaluationErrors[0]["message"]
+        assert "No such local variable: 'foo'" == evaluationErrors[0]["message"]
 
 
 def test_debugger_function_probe_eval_on_entry():
@@ -881,6 +881,7 @@ def test_debugger_log_line_probe_generate_messages():
                 probe_id="foo",
                 source_file="tests/submod/stuff.py",
                 line=36,
+                rate=float("inf"),
                 **compile_template(
                     "hello world ",
                     {"dsl": "foo", "json": {"ref": "foo"}},
@@ -899,8 +900,7 @@ def test_debugger_log_line_probe_generate_messages():
         assert "hello world ERROR 456!" == msg2["message"], msg2
 
         assert "foo" == msg1["debugger"]["snapshot"]["evaluationErrors"][0]["expr"], msg1
-        # not amazing error message for a missing variable
-        assert "'foo'" == msg1["debugger"]["snapshot"]["evaluationErrors"][0]["message"], msg1
+        assert "No such local variable: 'foo'" == msg1["debugger"]["snapshot"]["evaluationErrors"][0]["message"], msg1
 
         assert not msg1["debugger"]["snapshot"]["captures"]
 

tests/debugging/test_debugger_span_decoration.py

@@ -65,7 +65,7 @@ def test_debugger_span_decoration_probe_on_inner_function_active_span(self):
             assert span.get_tag("_dd.di.test.tag.probe_id") == "span-decoration"
             assert (
                 span.get_tag("_dd.di.test.bad.evaluation_error")
-                == "'Failed to evaluate expression \"test\": \\'notathing\\''"
+                == "'Failed to evaluate expression \"test\": No such local variable: \\'notathing\\''"
             )
 
             assert not d.test_queue
@@ -107,11 +107,15 @@ def test_debugger_span_decoration_probe_on_inner_function_active_span_unconditio
             assert span.get_tag("_dd.di.test.tag.probe_id") == "span-decoration"
             assert (
                 span.get_tag("_dd.di.test.bad.evaluation_error")
-                == "'Failed to evaluate expression \"test\": \\'notathing\\''"
+                == "'Failed to evaluate expression \"test\": No such local variable: \\'notathing\\''"
             )
 
             (signal,) = d.test_queue
-            assert signal.errors == [EvaluationError(expr="test", message="Failed to evaluate condition: 'notathing'")]
+            assert signal.errors == [
+                EvaluationError(
+                    expr="test", message="Failed to evaluate condition: No such local variable: 'notathing'"
+                )
+            ]
 
             (payload,) = d.uploader.wait_for_payloads()
             assert payload["message"] == "Condition evaluation errors for probe span-decoration"

tests/debugging/test_expressions.py

@@ -50,7 +50,7 @@ def __getitem__(self, name):
         ({"len": {"getmember": [{"ref": "self"}, "collectionField"]}}, {"self": CustomObject("expr")}, 10),
         ({"len": {"getmember": [{"ref": "self"}, "_privateField"]}}, {"self": CustomObject("expr")}, len("private")),
         ({"len": {"getmember": [{"ref": "self"}, "bogusField"]}}, {"self": CustomObject("expr")}, AttributeError),
-        ({"len": {"ref": "payload"}}, {}, KeyError),
+        ({"len": {"ref": "payload"}}, {}, NameError),
         # Test plain references
         ({"ref": "hits"}, {"hits": 42}, 42),
         ({"getmember": [{"ref": "self"}, "name"]}, {"self": CustomObject("test-me")}, "test-me"),
@@ -104,8 +104,8 @@ def __getitem__(self, name):
         (True, {}, True),
         ({"or": [{"ref": "bar"}, {"ref": "foo"}]}, {"bar": 42}, 42),
         ({"and": [{"ref": "bar"}, {"ref": "foo"}]}, {"bar": 0}, 0),
-        ({"or": [{"ref": "bar"}, {"ref": "foo"}]}, {"bar": 0}, KeyError),
-        ({"and": [{"ref": "bar"}, {"ref": "foo"}]}, {"bar": 42}, KeyError),
+        ({"or": [{"ref": "bar"}, {"ref": "foo"}]}, {"bar": 0}, NameError),
+        ({"and": [{"ref": "bar"}, {"ref": "foo"}]}, {"bar": 42}, NameError),
         ({"isDefined": "foobar"}, {"bar": 42}, False),
         ({"isDefined": "bar"}, {"bar": 42}, True),
         ({"instanceof": [{"ref": "bar"}, "int"]}, {"bar": 42}, True),