handle missing exc_info in ExceptionDictTransformer (#657)

* handle missing exc_info in ExceptionDictTransformer

- Added a new utility function is_missing_exc_info to check if exc_info is missing.
- Updated ExceptionDictTransformer to return an empty list when exc_info is missing.
- Modified _format_exception to use the new is_missing_exc_info function.
- Added a test case to verify that ExceptionDictTransformer returns an empty list when exc_info is missing.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Update tests/test_tracebacks.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Hynek Schlawack <[email protected]>

src/structlog/_frames.py

@@ -16,13 +16,20 @@
 from .typing import ExcInfo
 
 
+def is_missing_exc_info(exc_info: ExcInfo) -> bool:
+    """
+    Return True if exc_info is the missing value.
+    """
+    return exc_info == (None, None, None)  # type: ignore[comparison-overlap]
+
+
 def _format_exception(exc_info: ExcInfo) -> str:
     """
     Prettyprint an `exc_info` tuple.
 
     Shamelessly stolen from stdlib's logging module.
     """
-    if exc_info == (None, None, None):  # type: ignore[comparison-overlap]
+    if is_missing_exc_info(exc_info):
         return "MISSING"
 
     sio = StringIO()

src/structlog/tracebacks.py

@@ -22,6 +22,8 @@
 from types import ModuleType, TracebackType
 from typing import Any, Iterable, Sequence, Tuple, Union
 
+from ._frames import is_missing_exc_info
+
 
 try:
     import rich
@@ -412,6 +414,8 @@ def __init__(
         self.use_rich = use_rich
 
     def __call__(self, exc_info: ExcInfo) -> list[dict[str, Any]]:
+        if is_missing_exc_info(exc_info):
+            return []
         trace = extract(
             *exc_info,
             show_locals=self.show_locals,

tests/test_tracebacks.py

@@ -730,3 +730,14 @@ def test_json_traceback_value_error(
         monkeypatch.setattr(kwargs["suppress"][0], "__file__", None)
     with pytest.raises(ValueError, match=next(iter(kwargs.keys()))):
         tracebacks.ExceptionDictTransformer(**kwargs)
+
+
+def test_exception_dict_transformer_missing_exc_info():
+    """
+    ExceptionDictTransformer returns an empty list if exc_info is missing.
+    """
+    transformer = tracebacks.ExceptionDictTransformer()
+
+    result = transformer(exc_info=(None, None, None))
+
+    assert [] == result