tests: ensure tests never mutate DEFAULT_OPTS or cached TOML opts

Author: hukkin

src/mdformat/_cli.py

@@ -54,7 +54,11 @@ def run(cli_args: Sequence[str], cache_toml: bool = True) -> int:  # noqa: C901
             print_error(str(e))
             return 1
 
-        opts: Mapping = {**DEFAULT_OPTS, **toml_opts, **cli_core_opts}
+        opts = {**DEFAULT_OPTS, **toml_opts, **cli_core_opts}
+
+        # Merge plugin options from CLI.
+        # Make a copy of opts["plugin"] to not mutate DEFAULT_OPTS or cached TOML.
+        opts["plugin"] = dict(opts["plugin"])
         for plugin_id, plugin_opts in cli_plugin_opts.items():
             if plugin_id in opts["plugin"]:
                 opts["plugin"][plugin_id] |= plugin_opts

src/mdformat/_conf.py

@@ -2,20 +2,24 @@
 
 import functools
 from pathlib import Path
+from types import MappingProxyType
 from typing import Mapping
 
 from mdformat._compat import tomllib
+from mdformat._util import EMPTY_MAP
 
-DEFAULT_OPTS = {
-    "wrap": "keep",
-    "number": False,
-    "end_of_line": "lf",
-    "validate": True,
-    "exclude": [],
-    "plugin": {},
-    "extensions": None,
-    "codeformatters": None,
-}
+DEFAULT_OPTS = MappingProxyType(
+    {
+        "wrap": "keep",
+        "number": False,
+        "end_of_line": "lf",
+        "validate": True,
+        "exclude": (),
+        "plugin": EMPTY_MAP,
+        "extensions": None,
+        "codeformatters": None,
+    }
+)
 
 
 class InvalidConfError(Exception):