Make sure locals_max_{length,string} work as expected in case they are None (hynek#675)

* Make sure locals_max_{length,string} work as expected in case they are None

* Update tests/test_tracebacks.py

---------

Co-authored-by: Hynek Schlawack <hs@ox.cx>

Author: Sh4pe

src/structlog/tracebacks.py

@@ -382,10 +382,10 @@ def __init__(
         max_frames: int = MAX_FRAMES,
         use_rich: bool = True,
     ) -> None:
-        if locals_max_length < 0:
+        if locals_max_length is not None and locals_max_length < 0:
             msg = f'"locals_max_length" must be >= 0: {locals_max_length}'
             raise ValueError(msg)
-        if locals_max_string < 0:
+        if locals_max_string is not None and locals_max_string < 0:
             msg = f'"locals_max_string" must be >= 0: {locals_max_string}'
             raise ValueError(msg)
         if max_frames < 2:

tests/test_tracebacks.py

@@ -531,6 +531,46 @@ def bar(n):
     ]
 
 
+@pytest.mark.parametrize(
+    ("kwargs", "local_variable"),
+    [
+        (
+            {"locals_max_string": None},
+            "x" * (tracebacks.LOCALS_MAX_STRING + 10),
+        ),
+        (
+            {"locals_max_length": None},
+            list(range(tracebacks.LOCALS_MAX_LENGTH + 10)),
+        ),
+    ],
+)
+def test_exception_dict_transformer_locals_max_accept_none_argument(
+    kwargs, local_variable
+):
+    """
+    ExceptionDictTransformer works when either locals_max_string or
+    locals_max_length is set to None.
+    """
+
+    try:
+        my_local = local_variable
+        1 / 0
+    except Exception as e:
+        format_json = tracebacks.ExceptionDictTransformer(
+            show_locals=True, **kwargs
+        )
+        result = format_json((type(e), e, e.__traceback__))
+
+    _ = my_local
+
+    assert len(result) == 1
+    assert len(result[0]["frames"]) == 1
+
+    frame = result[0]["frames"][0]
+
+    assert frame["locals"]["my_local"].strip("'") == str(local_variable)
+
+
 def test_json_traceback():
     """
     Tracebacks are formatted to JSON with all information.