Stop mutating styles in ConsoleRenderer (#669)

Fixes #643

CHANGELOG.md

@@ -28,13 +28,15 @@ You can find our backwards-compatibility policy [here](https://github.com/hynek/
   [#647](https://github.com/hynek/structlog/pull/647)
 
 
-
 ## Fixed
 
 - `structlog.traceback.ExceptionDictTransformer` now correctly handles missing exceptions.
 
   [#657](https://github.com/hynek/structlog/pull/657)
 
+- Instantiating `structlog.dev.ConsoleRenderer` does not mutate the passed *styles* dict anymore.
+
+  [#658](https://github.com/hynek/structlog/pull/658)
 
 
 ## [24.4.0](https://github.com/hynek/structlog/compare/24.3.0...24.4.0) - 2024-07-17

src/structlog/dev.py

@@ -635,7 +635,7 @@ def add_meaningless_arg(arg: str) -> None:
             self.get_default_level_styles(colors)
             if level_styles is None
             else level_styles
-        )
+        ).copy()
 
         for key in level_to_color:
             level_to_color[key] += styles.bright

tests/test_dev.py

@@ -614,6 +614,19 @@ def test_enforces_presence_of_exactly_one_default_formatter(self):
                 ]
             )
 
+    def test_does_not_modify_styles(self):
+        """
+        Instantiating ConsoleRenderer should not modify the styles passed in.
+
+        Ref #643
+        """
+        styles = {"info": "something"}
+        copy = styles.copy()
+
+        dev.ConsoleRenderer(level_styles=styles)
+
+        assert copy == styles
+
 
 class TestSetExcInfo:
     def test_wrong_name(self):