Better error message for invalid exclude types

CHANGES.md

@@ -32,6 +32,8 @@
 - `.pytest_cache`, `.ruff_cache` and `.vscode` are now excluded by default (#3691)
 - Fix black not honouring `pyproject.toml` settings when running `--stdin-filename` and
   the `pyproject.toml` found isn't in the current working directory (#3719)
+- Black will now error if `exclude` and `extend-exclude` have invalid data types in
+  `pyproject.toml`, instead of silently doing the wrong thing (#3764)
 
 ### Packaging
 

src/black/__init__.py

@@ -157,6 +157,16 @@ def read_pyproject_toml(
             "target-version", "Config key target-version must be a list"
         )
 
+    exclude = config.get("exclude")
+    if exclude is not None and not isinstance(exclude, str):
+        raise click.BadOptionUsage("exclude", "Config key exclude must be a string")
+
+    extend_exclude = config.get("extend_exclude")
+    if extend_exclude is not None and not isinstance(extend_exclude, str):
+        raise click.BadOptionUsage(
+            "extend-exclude", "Config key extend-exclude must be a string"
+        )
+
     default_map: Dict[str, Any] = {}
     if ctx.default_map:
         default_map.update(ctx.default_map)