From 595555d03637117cfaa13e212917d3ae8ba6ffcf Mon Sep 17 00:00:00 2001
From: Max Jones <14077947+maxrjones@users.noreply.github.com>
Date: Wed, 1 Jul 2026 15:21:24 -0400
Subject: [PATCH 01/10] types: make donfig strict-clean and run mypy via a
 dependency group

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 .pre-commit-config.yaml     | 27 +++++-----
 donfig/_lock.py             | 23 ++++-----
 donfig/config_obj.py        | 87 ++++++++++++++++++--------------
 donfig/tests/test_config.py | 98 +++++++++++++++++++------------------
 donfig/tests/test_lock.py   |  8 +--
 donfig/utils.py             |  3 +-
 pyproject.toml              | 25 ++++++++--
 7 files changed, 151 insertions(+), 120 deletions(-)

diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 0fc5b88..018a9a9 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -20,24 +20,23 @@ repos:
       hooks:
         - id: flake8
           language_version: python3
-  - repo: https://github.com/pre-commit/mirrors-mypy
-    rev: v2.1.0
+  # mypy runs via `uv run --group typecheck`, so the type stubs live in the
+  # `typecheck` dependency group (pyproject.toml) rather than being duplicated
+  # here as additional_dependencies.
+  - repo: local
     hooks:
       - id: mypy
-        # Override default --ignore-missing-imports
-        # Use pyproject.toml if possible instead of adding command line parameters here
-        args: [ --warn-unused-configs ]
-        additional_dependencies:
-          # Type stubs
-          - types-docutils
-          - types-PyYAML
-          - types-setuptools
-          # Typed libraries
-          - numpy
-          - pytest
+        name: mypy
+        language: system
+        entry: uv run --group typecheck mypy
+        pass_filenames: false
+        always_run: true
+        types_or: [python, pyi]
 
 ci:
   # To trigger manually, comment on a pull request with "pre-commit.ci autofix"
   autofix_prs: false
   autoupdate_schedule: "monthly"
-  skip: []
+  # mypy is a `language: system` hook needing uv + the repo's dependency
+  # groups, which are unavailable on pre-commit.ci runners; run it in CI instead.
+  skip: [mypy]
diff --git a/donfig/_lock.py b/donfig/_lock.py
index 5b5bfaa..8d862e4 100644
--- a/donfig/_lock.py
+++ b/donfig/_lock.py
@@ -11,6 +11,7 @@
 
 import uuid
 from threading import Lock
+from typing import Any
 from weakref import WeakValueDictionary
 
 
@@ -42,9 +43,9 @@ class SerializableLock:
     The creation of locks is itself not threadsafe.
     """
 
-    _locks: WeakValueDictionary = WeakValueDictionary()
+    _locks: WeakValueDictionary[str, Lock] = WeakValueDictionary()
 
-    def __init__(self, token=None):
+    def __init__(self, token: str | None = None) -> None:
         self.token = token or str(uuid.uuid4())
         if self.token in SerializableLock._locks:
             self.lock = SerializableLock._locks[self.token]
@@ -52,28 +53,28 @@ def __init__(self, token=None):
             self.lock = Lock()
             SerializableLock._locks[self.token] = self.lock
 
-    def acquire(self, *args, **kwargs):
+    def acquire(self, *args: Any, **kwargs: Any) -> bool:
         return self.lock.acquire(*args, **kwargs)
 
-    def release(self, *args, **kwargs):
+    def release(self, *args: Any, **kwargs: Any) -> None:
         return self.lock.release(*args, **kwargs)
 
-    def __enter__(self):
+    def __enter__(self) -> None:
         self.lock.__enter__()
 
-    def __exit__(self, *args):
+    def __exit__(self, *args: Any) -> None:
         self.lock.__exit__(*args)
 
-    def locked(self):
+    def locked(self) -> bool:
         return self.lock.locked()
 
-    def __getstate__(self):
+    def __getstate__(self) -> str:
         return self.token
 
-    def __setstate__(self, token):
-        self.__init__(token)
+    def __setstate__(self, token: str) -> None:
+        self.__init__(token)  # type: ignore[misc]
 
-    def __str__(self):
+    def __str__(self) -> str:
         return f"<{self.__class__.__name__}: {self.token}>"
 
     __repr__ = __str__
diff --git a/donfig/config_obj.py b/donfig/config_obj.py
index 29a0035..6f3dadd 100644
--- a/donfig/config_obj.py
+++ b/donfig/config_obj.py
@@ -15,6 +15,7 @@
 from collections.abc import Mapping, MutableMapping, Sequence
 from contextlib import nullcontext
 from copy import deepcopy
+from types import TracebackType
 from typing import Any, Literal
 
 import yaml
@@ -51,8 +52,8 @@ def update(
     old: MutableMapping[str, Any],
     new: Mapping[str, Any],
     priority: Literal["old", "new", "new-defaults"] = "new",
-    defaults: Mapping | None = None,
-) -> Mapping[str, Any]:
+    defaults: Mapping[str, Any] | None = None,
+) -> MutableMapping[str, Any]:
     """Update a nested dictionary with values from another
 
     This is like dict.update except that it smoothly merges nested values
@@ -114,7 +115,7 @@ def update(
     return old
 
 
-def merge(*dicts: Mapping) -> dict:
+def merge(*dicts: Mapping[str, Any]) -> dict[str, Any]:
     """Update a sequence of nested dictionaries
 
     This prefers the values in the latter dictionaries to those in the former
@@ -131,13 +132,13 @@ def merge(*dicts: Mapping) -> dict:
     donfig.config_obj.update
 
     """
-    result: dict = {}
+    result: dict[str, Any] = {}
     for d in dicts:
         update(result, d)
     return result
 
 
-def collect_yaml(paths: Sequence[str]) -> list[dict]:
+def collect_yaml(paths: Sequence[str]) -> list[dict[str, Any]]:
     """Collect configuration from yaml files
 
     This searches through a list of paths, expands to find all yaml or json
@@ -174,7 +175,7 @@ def collect_yaml(paths: Sequence[str]) -> list[dict]:
     return configs
 
 
-def _load_config_file(path: str) -> dict | None:
+def _load_config_file(path: str) -> dict[str, Any] | None:
     try:
         with open(path) as f:
             config = yaml.safe_load(f.read())
@@ -193,7 +194,7 @@ def _load_config_file(path: str) -> dict | None:
 
 def collect_env(
     prefix: str, env: Mapping[str, str] | None = None, deprecations: MutableMapping[str, str | None] | None = None
-) -> dict:
+) -> dict[str, Any]:
     """Collect config from environment variables
 
     This grabs environment variables of the form "DASK_FOO__BAR_BAZ=123" and
@@ -223,7 +224,7 @@ def collect_env(
             except (SyntaxError, ValueError):
                 d[varname] = value
 
-    result: dict = {}
+    result: dict[str, Any] = {}
     # fake thread lock to use set functionality
     lock = nullcontext()
     ConfigSet(result, lock, deprecations or {}, d)
@@ -252,16 +253,16 @@ class ConfigSet:
 
     def __init__(
         self,
-        config: MutableMapping,
-        lock: SerializableLock | contextlib.AbstractContextManager,
-        deprecations: MutableMapping[str, str | None],
-        arg: Mapping | None = None,
-        **kwargs,
-    ):
+        config: MutableMapping[str, Any],
+        lock: SerializableLock | contextlib.AbstractContextManager[Any],
+        deprecations: Mapping[str, str | None],
+        arg: Mapping[str, Any] | None = None,
+        **kwargs: Any,
+    ) -> None:
         with lock:
             self.config = config
             self.deprecations = deprecations
-            self._record: list[tuple[str, tuple, Any]] = []
+            self._record: list[tuple[str, tuple[str, ...], Any]] = []
 
             if arg is not None:
                 for key, value in arg.items():
@@ -273,7 +274,7 @@ def __init__(
                     key = self._check_deprecations(key)
                     self._assign(key.split("."), value, config)
 
-    def _check_deprecations(self, key: str):
+    def _check_deprecations(self, key: str) -> str:
         """Check if the provided value has been renamed or removed.
 
         Parameters
@@ -298,10 +299,15 @@ def _check_deprecations(self, key: str):
         else:
             return key
 
-    def __enter__(self):
+    def __enter__(self) -> MutableMapping[str, Any]:
         return self.config
 
-    def __exit__(self, type, value, traceback):
+    def __exit__(
+        self,
+        exc_type: type[BaseException] | None,
+        exc_value: BaseException | None,
+        exc_tb: TracebackType | None,
+    ) -> None:
         for op, path, value in reversed(self._record):
             d = self.config
             if op == "replace":
@@ -321,7 +327,7 @@ def _assign(
         self,
         keys: Sequence[str],
         value: Any,
-        d: MutableMapping,
+        d: MutableMapping[str, Any],
         path: tuple[str, ...] = (),
         record: bool = True,
     ) -> None:
@@ -361,7 +367,7 @@ def _assign(
             self._assign(keys[1:], value, d[key], path, record=record)
 
 
-def expand_environment_variables(config):
+def expand_environment_variables(config: Any) -> Any:
     """Expand environment variables in a nested config dictionary
 
     This function will recursively search through any nested dictionaries
@@ -396,7 +402,7 @@ class Config:
     def __init__(
         self,
         name: str,
-        defaults: list[Mapping[str, Any]] | None = None,
+        defaults: Sequence[Mapping[str, Any]] | None = None,
         paths: list[str] | None = None,
         env: Mapping[str, str] | None = None,
         env_var: str | None = None,
@@ -436,27 +442,32 @@ def __init__(
         self.env = env
         self.main_path = main_path
         self.paths = paths
-        self.defaults = defaults or []
+        # Preserve the historical contract that a caller-supplied ``defaults``
+        # list is aliased (``update_defaults`` appends to it in place); only copy
+        # when a non-list Sequence is passed.
+        self.defaults: list[Mapping[str, Any]] = (
+            defaults if isinstance(defaults, list) else list(defaults) if defaults else []
+        )
         self.deprecations = deprecations
 
         self.config: MutableMapping[str, Any] = {}
         self.config_lock = SerializableLock()
         self.refresh()
 
-    def __contains__(self, item):
+    def __contains__(self, item: Any) -> bool:
         try:
             self[item]
             return True
         except (TypeError, IndexError, KeyError):
             return False
 
-    def __getitem__(self, item):
+    def __getitem__(self, item: str) -> Any:
         return self.get(item)
 
-    def pprint(self, **kwargs):
+    def pprint(self, **kwargs: Any) -> None:
         return pprint.pprint(self.config, **kwargs)
 
-    def collect(self, paths: list[str] | None = None, env: Mapping[str, str] | None = None) -> dict:
+    def collect(self, paths: list[str] | None = None, env: Mapping[str, str] | None = None) -> dict[str, Any]:
         """Collect configuration from paths and environment variables
 
         Parameters
@@ -482,16 +493,16 @@ def collect(self, paths: list[str] | None = None, env: Mapping[str, str] | None
             paths = self.paths
         if env is None:
             env = self.env
-        configs = []
+        configs: list[Mapping[str, Any]] = []
 
-        if yaml:
-            configs.extend(collect_yaml(paths=paths))
+        # yaml is a hard dependency, so its loader is always available.
+        configs.extend(collect_yaml(paths=paths))
 
         configs.append(collect_env(self.env_prefix, env=env))
 
         return merge(*configs)
 
-    def refresh(self, **kwargs) -> None:
+    def refresh(self, **kwargs: Any) -> None:
         """Update configuration by re-reading yaml files and env variables.
 
         This goes through the following stages:
@@ -555,7 +566,7 @@ def get(self, key: str, default: Any = no_default) -> Any:
                     raise
         return result
 
-    def update_defaults(self, new: Mapping) -> None:
+    def update_defaults(self, new: Mapping[str, Any]) -> None:
         """Add a new set of defaults to the configuration
 
         It does two things:
@@ -570,7 +581,7 @@ def update_defaults(self, new: Mapping) -> None:
         self.defaults.append(new)
         update(self.config, new, priority="new-defaults", defaults=current_defaults)
 
-    def to_dict(self):
+    def to_dict(self) -> MutableMapping[str, Any]:
         """Return dictionary copy of configuration.
 
         .. warning::
@@ -582,19 +593,19 @@ def to_dict(self):
         """
         return deepcopy(self.config)
 
-    def clear(self):
+    def clear(self) -> None:
         """Clear all existing configuration."""
         self.config.clear()
 
-    def merge(self, *dicts):
+    def merge(self, *dicts: Mapping[str, Any]) -> None:
         """Merge this configuration with multiple dictionaries.
 
         See :func:`~donfig.config_obj.merge` for more information.
 
         """
-        self.config = merge(self.config, dicts)
+        self.config = merge(self.config, *dicts)
 
-    def update(self, new, priority="new"):
+    def update(self, new: Mapping[str, Any], priority: Literal["old", "new", "new-defaults"] = "new") -> None:
         """Update the internal configuration dictionary with `new`.
 
         See :func:`~donfig.config_obj.update` for more information.
@@ -610,7 +621,7 @@ def expand_environment_variables(self) -> None:
         """
         self.config = expand_environment_variables(self.config)
 
-    def rename(self, aliases: Mapping) -> None:
+    def rename(self, aliases: Mapping[str, str]) -> None:
         """Rename old keys to new keys
 
         This helps migrate older configuration versions over time
@@ -629,7 +640,7 @@ def rename(self, aliases: Mapping) -> None:
 
         self.set(new)
 
-    def set(self, arg=None, **kwargs):
+    def set(self, arg: Mapping[str, Any] | None = None, **kwargs: Any) -> ConfigSet:
         """Set configuration values within a context manager.
 
         Parameters
diff --git a/donfig/tests/test_config.py b/donfig/tests/test_config.py
index 50f7993..055c5c9 100644
--- a/donfig/tests/test_config.py
+++ b/donfig/tests/test_config.py
@@ -1,13 +1,17 @@
 #!/usr/bin/env python
 # Copyright (c) 2018- Donfig Developers
 # Copyright (c) 2014-2018, Anaconda, Inc. and contributors
+from __future__ import annotations
+
 import os
 import site
 import stat
 import subprocess
 import sys
 from collections import OrderedDict
+from collections.abc import Iterator
 from contextlib import contextmanager
+from typing import Any
 
 import cloudpickle
 import pytest
@@ -30,7 +34,7 @@
 ENV_PREFIX = CONFIG_NAME.upper() + "_"
 
 
-def test_canonical_name():
+def test_canonical_name() -> None:
     c = {"foo-bar": 1, "fizz_buzz": 2}
     assert canonical_name("foo-bar", c) == "foo-bar"
     assert canonical_name("foo_bar", c) == "foo-bar"
@@ -40,7 +44,7 @@ def test_canonical_name():
     assert canonical_name("new_key", c) == "new_key"
 
 
-def test_update():
+def test_update() -> None:
     a = {"x": 1, "y": {"a": 1}}
     b = {"x": 2, "z": 3, "y": OrderedDict({"b": 2})}
     update(b, a)
@@ -52,7 +56,7 @@ def test_update():
     assert b == {"x": 2, "y": {"a": 3, "b": 2}, "z": 3}
 
 
-def test_update_new_defaults():
+def test_update_new_defaults() -> None:
     d = {"x": 1, "y": 1, "z": {"a": 1, "b": 1}}
     o = {"x": 1, "y": 2, "z": {"a": 1, "b": 2}, "c": 2, "c2": {"d": 2}}
     n = {"x": 3, "y": 3, "z": OrderedDict({"a": 3, "b": 3}), "c": 3, "c2": {"d": 3}}
@@ -67,7 +71,7 @@ def test_update_new_defaults():
     assert update(o, n, priority="new-defaults", defaults=None) == update(o, n, priority="old")
 
 
-def test_update_defaults():
+def test_update_defaults() -> None:
     defaults = [
         {"a": 1, "b": {"c": 1}},
         {"a": 2, "b": {"d": 2}},
@@ -86,7 +90,7 @@ def test_update_defaults():
     assert config.to_dict() == {"a": 0, "b": {"c": 0, "d": 3}, "extra": 0, "new-extra": 0}
 
 
-def test_merge():
+def test_merge() -> None:
     a = {"x": 1, "y": {"a": 1}}
     b = {"x": 2, "z": 3, "y": {"b": 2}}
 
@@ -96,7 +100,7 @@ def test_merge():
     assert c == expected
 
 
-def test_collect_yaml_paths():
+def test_collect_yaml_paths() -> None:
     a = {"x": 1, "y": {"a": 1}}
     b = {"x": 2, "z": 3, "y": {"b": 2}}
 
@@ -117,7 +121,7 @@ def test_collect_yaml_paths():
             assert config == expected
 
 
-def test_collect_yaml_dir():
+def test_collect_yaml_dir() -> None:
     a = {"x": 1, "y": {"a": 1}}
     b = {"x": 2, "z": 3, "y": {"b": 2}}
 
@@ -139,7 +143,7 @@ def test_collect_yaml_dir():
 
 
 @contextmanager
-def no_read_permissions(path):
+def no_read_permissions(path: str) -> Iterator[None]:
     perm_orig = stat.S_IMODE(os.stat(path).st_mode)
     perm_new = perm_orig ^ stat.S_IREAD
     try:
@@ -151,7 +155,7 @@ def no_read_permissions(path):
 
 @pytest.mark.skipif(sys.platform == "win32", reason="Can't make writeonly file on windows")
 @pytest.mark.parametrize("kind", ["directory", "file"])
-def test_collect_yaml_permission_errors(tmpdir, kind):
+def test_collect_yaml_permission_errors(tmpdir: Any, kind: str) -> None:
     a = {"x": 1, "y": 2}
     b = {"y": 3, "z": 4}
 
@@ -166,7 +170,7 @@ def test_collect_yaml_permission_errors(tmpdir, kind):
 
     if kind == "directory":
         cant_read = dir_path
-        expected = {}
+        expected: dict[str, int] = {}
     else:
         cant_read = a_path
         expected = b
@@ -176,7 +180,7 @@ def test_collect_yaml_permission_errors(tmpdir, kind):
         assert config == expected
 
 
-def test_collect_yaml_malformed_file(tmpdir):
+def test_collect_yaml_malformed_file(tmpdir: Any) -> None:
     dir_path = str(tmpdir)
     fil_path = os.path.join(dir_path, "a.yaml")
 
@@ -190,7 +194,7 @@ def test_collect_yaml_malformed_file(tmpdir):
     assert "original error message" in str(rec.value)
 
 
-def test_collect_yaml_no_top_level_dict(tmpdir):
+def test_collect_yaml_no_top_level_dict(tmpdir: Any) -> None:
     dir_path = str(tmpdir)
     fil_path = os.path.join(dir_path, "a.yaml")
 
@@ -204,7 +208,7 @@ def test_collect_yaml_no_top_level_dict(tmpdir):
     assert "must have a dict" in str(rec.value)
 
 
-def test_env():
+def test_env() -> None:
     env = {
         ENV_PREFIX + "A_B": "123",
         ENV_PREFIX + "C": "True",
@@ -229,10 +233,10 @@ def test_env():
     assert res == expected
 
 
-def test_collect():
+def test_collect() -> None:
     a = {"x": 1, "y": {"a": 1}}
     b = {"x": 2, "z": 3, "y": {"b": 2}}
-    env = {ENV_PREFIX + "W": 4}
+    env: dict[str, Any] = {ENV_PREFIX + "W": 4}
 
     expected = {
         "w": 4,
@@ -249,21 +253,21 @@ def test_collect():
             with open(fn2, "w") as f:
                 yaml.dump(b, f)
 
-            config = config.collect([fn1, fn2], env=env)
-            assert config == expected
+            result = config.collect([fn1, fn2], env=env)
+            assert result == expected
 
 
-def test_collect_env_none():
+def test_collect_env_none() -> None:
     os.environ[ENV_PREFIX + "FOO"] = "bar"
     config = Config(CONFIG_NAME)
     try:
-        config = config.collect([])
-        assert config == {"foo": "bar"}
+        result = config.collect([])
+        assert result == {"foo": "bar"}
     finally:
         del os.environ[ENV_PREFIX + "FOO"]
 
 
-def test_get():
+def test_get() -> None:
     test_config = Config(CONFIG_NAME)
     test_config.config = {"x": 1, "y": {"a": 2}}
 
@@ -278,7 +282,7 @@ def test_get():
         test_config["y.b"]
 
 
-def test_contains():
+def test_contains() -> None:
     test_config = Config(CONFIG_NAME)
     test_config.config = {"x": 1, "y": {"a": 2}}
 
@@ -287,7 +291,7 @@ def test_contains():
     assert "y.b" not in test_config
 
 
-def test_ensure_file(tmpdir):
+def test_ensure_file(tmpdir: Any) -> None:
     a = {"x": 1, "y": {"a": 1}}
     b = {"x": 123}
 
@@ -329,7 +333,7 @@ def test_ensure_file(tmpdir):
     assert not result
 
 
-def test_set():
+def test_set() -> None:
     config = Config(CONFIG_NAME)
     with config.set(abc=123):
         assert config.config["abc"] == 123
@@ -352,7 +356,7 @@ def test_set():
     assert config.config["abc"]["x"] == 123
 
 
-def test_set_kwargs():
+def test_set_kwargs() -> None:
     config = Config(CONFIG_NAME)
     with config.set(foo__bar=1, foo__baz=2):
         assert config.config["foo"] == {"bar": 1, "baz": 2}
@@ -369,7 +373,7 @@ def test_set_kwargs():
     assert "foo" not in config.config
 
 
-def test_set_nested():
+def test_set_nested() -> None:
     config = Config(CONFIG_NAME)
     with config.set({"abc": {"x": 123}}):
         assert config.config["abc"] == {"x": 123}
@@ -379,7 +383,7 @@ def test_set_nested():
     assert "abc" not in config.config
 
 
-def test_set_hard_to_copyables():
+def test_set_hard_to_copyables() -> None:
     import threading
 
     config = Config(CONFIG_NAME)
@@ -389,7 +393,7 @@ def test_set_hard_to_copyables():
 
 
 @pytest.mark.parametrize("mkdir", [True, False])
-def test_ensure_file_directory(mkdir, tmpdir):
+def test_ensure_file_directory(mkdir: bool, tmpdir: Any) -> None:
     a = {"x": 1, "y": {"a": 1}}
 
     source = os.path.join(str(tmpdir), "source.yaml")
@@ -408,7 +412,7 @@ def test_ensure_file_directory(mkdir, tmpdir):
     assert os.path.exists(os.path.join(dest, "source.yaml"))
 
 
-def test_ensure_file_defaults_to_TEST_CONFIG_directory(tmpdir):
+def test_ensure_file_defaults_to_TEST_CONFIG_directory(tmpdir: Any) -> None:
     a = {"x": 1, "y": {"a": 1}}
     source = os.path.join(str(tmpdir), "source.yaml")
     with open(source, "w") as f:
@@ -428,7 +432,7 @@ def test_ensure_file_defaults_to_TEST_CONFIG_directory(tmpdir):
     assert os.path.split(fn)[1] == os.path.split(source)[1]
 
 
-def test_rename():
+def test_rename() -> None:
     config = Config(CONFIG_NAME)
     aliases = {"foo_bar": "foo.bar"}
     config.config = {"foo-bar": 123}
@@ -436,8 +440,8 @@ def test_rename():
     assert config.config == {"foo": {"bar": 123}}
 
 
-def test_refresh():
-    defaults = []
+def test_refresh() -> None:
+    defaults: list[dict[str, Any]] = []
     config = Config(CONFIG_NAME, defaults=defaults)
 
     config.update_defaults({"a": 1})
@@ -463,7 +467,7 @@ def test_refresh():
         ({"a": "A", "b": [1, "2", "$FOO"]}, {"a": "A", "b": [1, "2", "foo"]}),
     ],
 )
-def test_expand_environment_variables(inp, out):
+def test_expand_environment_variables(inp: Any, out: Any) -> None:
     try:
         os.environ["FOO"] = "foo"
         assert expand_environment_variables(inp) == out
@@ -471,7 +475,7 @@ def test_expand_environment_variables(inp, out):
         del os.environ["FOO"]
 
 
-def test_env_var_canonical_name(monkeypatch):
+def test_env_var_canonical_name(monkeypatch: pytest.MonkeyPatch) -> None:
     value = 3
     monkeypatch.setenv(ENV_PREFIX + "A_B", str(value))
     config = Config(CONFIG_NAME)
@@ -480,7 +484,7 @@ def test_env_var_canonical_name(monkeypatch):
     assert config.get("a-b") == value
 
 
-def test_get_set_canonical_name():
+def test_get_set_canonical_name() -> None:
     c = {"x-y": {"a_b": 123}}
     config = Config(CONFIG_NAME)
     config.update(c)
@@ -500,7 +504,7 @@ def test_get_set_canonical_name():
 
 
 @pytest.mark.parametrize("key", ["custom_key", "custom-key"])
-def test_get_set_roundtrip(key):
+def test_get_set_roundtrip(key: str) -> None:
     value = 123
     config = Config(CONFIG_NAME)
     with config.set({key: value}):
@@ -508,11 +512,11 @@ def test_get_set_roundtrip(key):
         assert config.get("custom-key") == value
 
 
-def test_merge_none_to_dict():
+def test_merge_none_to_dict() -> None:
     assert merge({"a": None, "c": 0}, {"a": {"b": 1}}) == {"a": {"b": 1}, "c": 0}
 
 
-def test_pprint(capsys):
+def test_pprint(capsys: pytest.CaptureFixture[str]) -> None:
     test_config = Config(CONFIG_NAME)
     test_config.config = {"x": 1, "y": {"a": 2}}
     test_config.pprint()
@@ -520,7 +524,7 @@ def test_pprint(capsys):
     assert cap_out == """{'x': 1, 'y': {'a': 2}}\n"""
 
 
-def test_to_dict():
+def test_to_dict() -> None:
     test_config = Config(CONFIG_NAME)
     test_config.config = {"x": 1, "y": {"a": 2}}
     d = test_config.to_dict()
@@ -532,7 +536,7 @@ def test_to_dict():
     assert d["y"] != test_config.config["y"]
 
 
-def test_path_includes_site_prefix():
+def test_path_includes_site_prefix() -> None:
     command = (
         "import site, os; "
         'prefix = os.path.join("include", "this", "path"); '
@@ -546,7 +550,7 @@ def test_path_includes_site_prefix():
     subprocess.check_call([sys.executable, "-c", command])
 
 
-def test__get_paths(monkeypatch):
+def test__get_paths(monkeypatch: pytest.MonkeyPatch) -> None:
     # These settings, if present, would interfere with these tests
     # We temporarily remove them to avoid interference from the
     # machine where tests are being run.
@@ -586,14 +590,14 @@ def test__get_paths(monkeypatch):
         assert len(paths) == len(set(paths))
 
 
-def test_serialization():
+def test_serialization() -> None:
     config = Config(CONFIG_NAME)
     config.set(one_key="one_value")
     new_config = cloudpickle.loads(cloudpickle.dumps(config))
     assert new_config.get("one_key") == "one_value"
 
 
-def test_deprecations_rename():
+def test_deprecations_rename() -> None:
     config = Config(CONFIG_NAME, deprecations={"fuse_ave_width": "optimization.fuse.ave-width"})
     with pytest.warns(Warning) as info, config.set(fuse_ave_width=123):
         assert config.get("optimization.fuse.ave-width") == 123
@@ -601,19 +605,19 @@ def test_deprecations_rename():
     assert "optimization.fuse.ave-width" in str(info[0].message)
 
 
-def test_deprecations_removed():
+def test_deprecations_removed() -> None:
     config = Config(CONFIG_NAME, deprecations={"fuse_ave_width": None})
     with pytest.raises(ValueError):
         config.set(fuse_ave_width=123)
 
 
-def test_config_serialization_functions():
+def test_config_serialization_functions() -> None:
     serialized = serialize({"array": {"svg": {"size": 150}}})
     config_dict = deserialize(serialized)
     assert config_dict["array"]["svg"]["size"] == 150
 
 
-def test_config_object_serialization():
+def test_config_object_serialization() -> None:
     config = Config(CONFIG_NAME)
     config.set({"array.svg.size": 150})
     ser_config = config.serialize()
@@ -623,7 +627,7 @@ def test_config_object_serialization():
     assert deser_config.get("array.svg.size") == 150
 
 
-def test_config_inheritance(monkeypatch):
+def test_config_inheritance(monkeypatch: pytest.MonkeyPatch) -> None:
     ser_dict = serialize({"array": {"svg": {"size": 150}}})
     monkeypatch.setenv(f"{ENV_PREFIX}_INTERNAL_INHERIT_CONFIG", ser_dict)
     config = Config(CONFIG_NAME)
diff --git a/donfig/tests/test_lock.py b/donfig/tests/test_lock.py
index 5ea08d8..0c4d509 100644
--- a/donfig/tests/test_lock.py
+++ b/donfig/tests/test_lock.py
@@ -7,7 +7,7 @@
 from .._lock import SerializableLock
 
 
-def test_SerializableLock():
+def test_SerializableLock() -> None:
     a = SerializableLock()
     b = SerializableLock()
     with a:
@@ -42,7 +42,7 @@ def test_SerializableLock():
                     pass
 
 
-def test_SerializableLock_name_collision():
+def test_SerializableLock_name_collision() -> None:
     a = SerializableLock("a")
     b = SerializableLock("b")
     c = SerializableLock("a")
@@ -53,7 +53,7 @@ def test_SerializableLock_name_collision():
     assert d.lock not in (a.lock, b.lock, c.lock)
 
 
-def test_SerializableLock_locked():
+def test_SerializableLock_locked() -> None:
     a = SerializableLock("a")
     assert not a.locked()
     with a:
@@ -61,7 +61,7 @@ def test_SerializableLock_locked():
     assert not a.locked()
 
 
-def test_SerializableLock_acquire_blocking():
+def test_SerializableLock_acquire_blocking() -> None:
     a = SerializableLock("a")
     assert a.acquire(blocking=True)
     assert not a.acquire(blocking=False)
diff --git a/donfig/utils.py b/donfig/utils.py
index 87197ae..2b998da 100644
--- a/donfig/utils.py
+++ b/donfig/utils.py
@@ -4,11 +4,12 @@
 import os
 import shutil
 import tempfile
+from collections.abc import Iterator
 from contextlib import contextmanager, suppress
 
 
 @contextmanager
-def tmpfile(extension="", dir=None):
+def tmpfile(extension: str = "", dir: str | None = None) -> Iterator[str]:
     extension = "." + extension.lstrip(".")
     handle, filename = tempfile.mkstemp(extension, dir=dir)
     os.close(handle)
diff --git a/pyproject.toml b/pyproject.toml
index 4764039..5e7b4b6 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -36,6 +36,19 @@ docs = [
     "cloudpickle",
 ]
 
+# Type-checking toolchain (PEP 735 dependency group): mypy plus the type stubs.
+# This is the single source of truth for the stubs — the pre-commit mypy hook
+# runs `uv run --group typecheck mypy` rather than duplicating the list.
+[dependency-groups]
+typecheck = [
+    "mypy",
+    "types-docutils",
+    "types-PyYAML",
+    "types-setuptools",
+    "pytest",  # so the test suite type-checks against real pytest types
+    "cloudpickle",
+]
+
 [tool.setuptools]
 include-package-data = true
 
@@ -48,14 +61,16 @@ python_version = "3.10"
 platform = "linux"
 # platform = win32
 # platform = darwin
+strict = true
 allow_untyped_decorators = false
 ignore_missing_imports = true
-no_implicit_optional = true
-show_error_codes = true
-warn_redundant_casts = true
-warn_unused_ignores = true
 warn_unreachable = true
-warn_unused_configs = true
+enable_error_code = ["ignore-without-code", "redundant-expr", "truthy-bool"]
+
+# `donfig/version.py` is generated by versioneer; do not type-check it.
+[[tool.mypy.overrides]]
+module = "donfig.version"
+ignore_errors = true
 
 [tool.black]
 line-length = 120

From afe64658f6dc28293364005cce9ca22d64bb4e08 Mon Sep 17 00:00:00 2001
From: Max Jones <14077947+maxrjones@users.noreply.github.com>
Date: Thu, 23 Jul 2026 17:31:11 -0400
Subject: [PATCH 02/10] chore: type-check in CI

---
 .github/workflows/ci.yaml | 15 +++++++++++++++
 .pre-commit-config.yaml   |  2 ++
 pyproject.toml            | 18 ++++--------------
 3 files changed, 21 insertions(+), 14 deletions(-)

diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 35e0cd1..1c6433a 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -9,6 +9,21 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  typecheck:
+    runs-on: "ubuntu-latest"
+    steps:
+      - name: Checkout source
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          # hatch-vcs needs the full history to resolve the version when uv
+          # builds the project environment
+          fetch-depth: 0
+          persist-credentials: false
+
+      - name: Run mypy
+        # uv is preinstalled on GitHub-hosted runners
+        run: uv run --group typecheck mypy donfig
+
   test:
     runs-on: ${{ matrix.os }}
     strategy:
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 117bf69..55bdb35 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -23,12 +23,14 @@ repos:
         # Use pyproject.toml if possible instead of adding command line parameters here
         args: [ --warn-unused-configs ]
         additional_dependencies:
+          # Keep in sync with the `typecheck` dependency group in pyproject.toml
           # Type stubs
           - types-docutils
           - types-PyYAML
           - types-setuptools
           # Typed libraries
           - pytest
+          - cloudpickle
   - repo: https://github.com/scientific-python/cookie
     rev: 2026.04.04
     hooks:
diff --git a/pyproject.toml b/pyproject.toml
index 46be6f1..4d62d96 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -42,10 +42,11 @@ docs = [
 [dependency-groups]
 test = ["pytest", "cloudpickle"]
 docs = ["sphinx>=4.0.0", "numpydoc", "pytest", "cloudpickle"]
-dev = [{ include-group = "test" }]
+dev = [{ include-group = "test" }, { include-group = "typecheck" }]
 # Type-checking toolchain (PEP 735 dependency group): mypy plus the type stubs.
-# This is the single source of truth for the stubs — the pre-commit mypy hook
-# runs `uv run --group typecheck mypy` rather than duplicating the list.
+# The mirrors-mypy pre-commit hook cannot read dependency groups, so its
+# additional_dependencies in .pre-commit-config.yaml must be kept in sync with
+# this list by hand.
 typecheck = [
     "mypy",
     "types-docutils",
@@ -83,12 +84,6 @@ ignore = [
     # (`donfig/tests/`) rather than a src-layout; both are intentional.
     "PY004",
     "PY005",
-    # MyPy strictness is tightened in a separate typing PR, not here.
-    "MY101",
-    "MY102",
-    "MY104",
-    "MY105",
-    "MY106",
     # Adding the doc/markdown formatters (blacken-docs etc.) is out of scope for
     # this packaging change.
     "PC111",
@@ -108,11 +103,6 @@ ignore_missing_imports = true
 warn_unreachable = true
 enable_error_code = ["ignore-without-code", "redundant-expr", "truthy-bool"]
 
-# `donfig/version.py` is generated by versioneer; do not type-check it.
-[[tool.mypy.overrides]]
-module = "donfig.version"
-ignore_errors = true
-
 [tool.codespell]
 # Vendored, minified js-yaml bundle used by the docs config converter — not prose.
 skip = "doc/_static/js-yaml.min.js"

From db46b353ed3bca3f64f67711e357b1978394fb7d Mon Sep 17 00:00:00 2001
From: Max Jones <14077947+maxrjones@users.noreply.github.com>
Date: Thu, 23 Jul 2026 17:31:37 -0400
Subject: [PATCH 03/10] chore: test config.merge

---
 donfig/config_obj.py        | 6 +++---
 donfig/tests/test_config.py | 8 ++++++++
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/donfig/config_obj.py b/donfig/config_obj.py
index d8b2206..876a857 100644
--- a/donfig/config_obj.py
+++ b/donfig/config_obj.py
@@ -444,9 +444,9 @@ def __init__(
         self.env = env
         self.main_path = main_path
         self.paths = paths
-        # Preserve the historical contract that a caller-supplied ``defaults``
-        # list is aliased (``update_defaults`` appends to it in place); only copy
-        # when a non-list Sequence is passed.
+        # A caller-supplied list (even an empty one) is aliased, not copied, so
+        # that ``update_defaults`` appends to it in place; any other Sequence is
+        # copied into a new list.
         self.defaults: list[Mapping[str, Any]] = (
             defaults if isinstance(defaults, list) else list(defaults) if defaults else []
         )
diff --git a/donfig/tests/test_config.py b/donfig/tests/test_config.py
index 055c5c9..c4b8b6f 100644
--- a/donfig/tests/test_config.py
+++ b/donfig/tests/test_config.py
@@ -100,6 +100,14 @@ def test_merge() -> None:
     assert c == expected
 
 
+def test_config_merge() -> None:
+    config = Config(CONFIG_NAME)
+    config.clear()
+    config.update({"x": 1, "y": {"a": 1}})
+    config.merge({"y": {"b": 2}}, {"z": 3})
+    assert config.to_dict() == {"x": 1, "y": {"a": 1, "b": 2}, "z": 3}
+
+
 def test_collect_yaml_paths() -> None:
     a = {"x": 1, "y": {"a": 1}}
     b = {"x": 2, "z": 3, "y": {"b": 2}}

From 418dcbdb2cd44fdbee8fd1d733a3dbfb255be35b Mon Sep 17 00:00:00 2001
From: Max Jones <14077947+maxrjones@users.noreply.github.com>
Date: Thu, 23 Jul 2026 17:31:50 -0400
Subject: [PATCH 04/10] chore: add py.typed

---
 donfig/py.typed | 0
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 donfig/py.typed

diff --git a/donfig/py.typed b/donfig/py.typed
new file mode 100644
index 0000000..e69de29

From 301ba5a68133b7c090dbd1f444d92085f0c5500f Mon Sep 17 00:00:00 2001
From: Max Jones <14077947+maxrjones@users.noreply.github.com>
Date: Fri, 24 Jul 2026 13:56:29 -0400
Subject: [PATCH 05/10] chore: scope ignore_missing_imports to cloudpickle only

---
 pyproject.toml | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/pyproject.toml b/pyproject.toml
index 4d62d96..0cfd83c 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -99,10 +99,14 @@ platform = "linux"
 # platform = darwin
 strict = true
 allow_untyped_decorators = false
-ignore_missing_imports = true
 warn_unreachable = true
 enable_error_code = ["ignore-without-code", "redundant-expr", "truthy-bool"]
 
+[[tool.mypy.overrides]]
+# cloudpickle ships neither inline types nor a stubs package.
+module = ["cloudpickle"]
+ignore_missing_imports = true
+
 [tool.codespell]
 # Vendored, minified js-yaml bundle used by the docs config converter — not prose.
 skip = "doc/_static/js-yaml.min.js"

From 56d68ba2514173f927c19ba1cee764a9715fee5c Mon Sep 17 00:00:00 2001
From: Max Jones <14077947+maxrjones@users.noreply.github.com>
Date: Fri, 24 Jul 2026 14:26:24 -0400
Subject: [PATCH 06/10] Configure mypy job

---
 .github/workflows/ci.yaml | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 1c6433a..e8f80cf 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -20,8 +20,10 @@ jobs:
           fetch-depth: 0
           persist-credentials: false
 
+      - name: Install uv
+        uses: astral-sh/setup-uv@c771a70e6277c0a99b617c7a806ffedaca235ff9 # v9.0.0
+
       - name: Run mypy
-        # uv is preinstalled on GitHub-hosted runners
         run: uv run --group typecheck mypy donfig
 
   test:

From 973f111e92b09a408049bcfb81a4f4f073d53f9d Mon Sep 17 00:00:00 2001
From: Max Jones <14077947+maxrjones@users.noreply.github.com>
Date: Fri, 24 Jul 2026 16:23:49 -0400
Subject: [PATCH 07/10] refactor: type defaults as MutableSequence to make the
 aliasing contract explicit

---
 donfig/config_obj.py        | 13 +++++--------
 donfig/tests/test_config.py |  6 +++---
 2 files changed, 8 insertions(+), 11 deletions(-)

diff --git a/donfig/config_obj.py b/donfig/config_obj.py
index 876a857..aa7fc08 100644
--- a/donfig/config_obj.py
+++ b/donfig/config_obj.py
@@ -12,7 +12,7 @@
 import site
 import sys
 import warnings
-from collections.abc import Mapping, MutableMapping, Sequence
+from collections.abc import Mapping, MutableMapping, MutableSequence, Sequence
 from contextlib import nullcontext
 from copy import deepcopy
 from types import TracebackType
@@ -404,7 +404,7 @@ class Config:
     def __init__(
         self,
         name: str,
-        defaults: Sequence[Mapping[str, Any]] | None = None,
+        defaults: MutableSequence[Mapping[str, Any]] | None = None,
         paths: list[str] | None = None,
         env: Mapping[str, str] | None = None,
         env_var: str | None = None,
@@ -444,12 +444,9 @@ def __init__(
         self.env = env
         self.main_path = main_path
         self.paths = paths
-        # A caller-supplied list (even an empty one) is aliased, not copied, so
-        # that ``update_defaults`` appends to it in place; any other Sequence is
-        # copied into a new list.
-        self.defaults: list[Mapping[str, Any]] = (
-            defaults if isinstance(defaults, list) else list(defaults) if defaults else []
-        )
+        # Aliased, not copied: ``update_defaults`` appends into the
+        # caller-supplied sequence in place.
+        self.defaults: MutableSequence[Mapping[str, Any]] = defaults if defaults is not None else []
         self.deprecations = deprecations
 
         self.config: MutableMapping[str, Any] = {}
diff --git a/donfig/tests/test_config.py b/donfig/tests/test_config.py
index c4b8b6f..54c3d42 100644
--- a/donfig/tests/test_config.py
+++ b/donfig/tests/test_config.py
@@ -9,7 +9,7 @@
 import subprocess
 import sys
 from collections import OrderedDict
-from collections.abc import Iterator
+from collections.abc import Iterator, Mapping
 from contextlib import contextmanager
 from typing import Any
 
@@ -72,7 +72,7 @@ def test_update_new_defaults() -> None:
 
 
 def test_update_defaults() -> None:
-    defaults = [
+    defaults: list[Mapping[str, Any]] = [
         {"a": 1, "b": {"c": 1}},
         {"a": 2, "b": {"d": 2}},
     ]
@@ -449,7 +449,7 @@ def test_rename() -> None:
 
 
 def test_refresh() -> None:
-    defaults: list[dict[str, Any]] = []
+    defaults: list[Mapping[str, Any]] = []
     config = Config(CONFIG_NAME, defaults=defaults)
 
     config.update_defaults({"a": 1})

From 3d09a7c41742777724fe391453e380a5085c0d53 Mon Sep 17 00:00:00 2001
From: Max Jones <14077947+maxrjones@users.noreply.github.com>
Date: Mon, 27 Jul 2026 17:26:01 -0400
Subject: [PATCH 08/10] types: simplify to dict

---
 donfig/config_obj.py | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/donfig/config_obj.py b/donfig/config_obj.py
index eb8dbbe..1d78ba4 100644
--- a/donfig/config_obj.py
+++ b/donfig/config_obj.py
@@ -49,11 +49,11 @@ def canonical_name(k: str, config: Mapping[str, Any]) -> str:
 
 
 def update(
-    old: MutableMapping[str, Any],
+    old: dict[str, Any],
     new: Mapping[str, Any],
     priority: Literal["old", "new", "new-defaults"] = "new",
     defaults: Mapping[str, Any] | None = None,
-) -> MutableMapping[str, Any]:
+) -> dict[str, Any]:
     """Update a nested dictionary with values from another
 
     This is like dict.update except that it smoothly merges nested values
@@ -193,7 +193,7 @@ def _load_config_file(path: str) -> dict[str, Any] | None:
 
 
 def collect_env(
-    prefix: str, env: Mapping[str, str] | None = None, deprecations: MutableMapping[str, str | None] | None = None
+    prefix: str, env: Mapping[str, str] | None = None, deprecations: Mapping[str, str | None] | None = None
 ) -> dict[str, Any]:
     """Collect config from environment variables
 
@@ -262,7 +262,7 @@ def __init__(
         with lock:
             self.config = config
             self.deprecations = deprecations
-            self._record: list[tuple[str, tuple[str, ...], Any]] = []
+            self._record: list[tuple[Literal["replace", "insert"], tuple[str, ...], Any]] = []
 
             if arg is not None:
                 for key, value in arg.items():
@@ -447,7 +447,7 @@ def __init__(
         self.defaults: list[Mapping[str, Any]] = list(defaults) if defaults is not None else []
         self.deprecations = deprecations
 
-        self.config: MutableMapping[str, Any] = {}
+        self.config: dict[str, Any] = {}
         self.config_lock = SerializableLock()
         self.refresh()
 
@@ -578,7 +578,7 @@ def update_defaults(self, new: Mapping[str, Any]) -> None:
         self.defaults.append(new)
         update(self.config, new, priority="new-defaults", defaults=current_defaults)
 
-    def to_dict(self) -> MutableMapping[str, Any]:
+    def to_dict(self) -> dict[str, Any]:
         """Return dictionary copy of configuration.
 
         .. warning::

From 511cdae2b77755f3a220d72a6e78c308e220898b Mon Sep 17 00:00:00 2001
From: David Hoese <david.hoese@ssec.wisc.edu>
Date: Mon, 3 Aug 2026 09:42:15 -0500
Subject: [PATCH 09/10] Add missing type annotations in test_config.py

---
 donfig/tests/test_config.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/donfig/tests/test_config.py b/donfig/tests/test_config.py
index ed19cc5..2c7d3d5 100644
--- a/donfig/tests/test_config.py
+++ b/donfig/tests/test_config.py
@@ -610,7 +610,7 @@ def test__get_paths(monkeypatch: pytest.MonkeyPatch) -> None:
         assert len(paths) == len(set(paths))
 
 
-def test_paths_not_mutated(monkeypatch):
+def test_paths_not_mutated(monkeypatch) -> None:
     monkeypatch.setenv("MYPKG_CONFIG", "foo-bar")
     paths = ["/etc/mypkg"]
     config = Config("mypkg", paths=paths)
@@ -619,7 +619,7 @@ def test_paths_not_mutated(monkeypatch):
     assert paths == ["/etc/mypkg"]
 
 
-def test_paths_accepts_any_sequence(monkeypatch):
+def test_paths_accepts_any_sequence(monkeypatch) -> None:
     monkeypatch.setenv("MYPKG_CONFIG", "foo-bar")
     config = Config("mypkg", paths=("/etc/mypkg",))
     assert config.paths == ["/etc/mypkg", "foo-bar"]

From d2ccb8274b3e57ad7adc0679e032ef2ac10b4e26 Mon Sep 17 00:00:00 2001
From: Max Jones <14077947+maxrjones@users.noreply.github.com>
Date: Mon, 3 Aug 2026 12:11:36 -0400
Subject: [PATCH 10/10] chore: update tests

---
 donfig/tests/test_config.py | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/donfig/tests/test_config.py b/donfig/tests/test_config.py
index 2c7d3d5..d39277b 100644
--- a/donfig/tests/test_config.py
+++ b/donfig/tests/test_config.py
@@ -610,7 +610,7 @@ def test__get_paths(monkeypatch: pytest.MonkeyPatch) -> None:
         assert len(paths) == len(set(paths))
 
 
-def test_paths_not_mutated(monkeypatch) -> None:
+def test_paths_not_mutated(monkeypatch: pytest.MonkeyPatch) -> None:
     monkeypatch.setenv("MYPKG_CONFIG", "foo-bar")
     paths = ["/etc/mypkg"]
     config = Config("mypkg", paths=paths)
@@ -619,12 +619,6 @@ def test_paths_not_mutated(monkeypatch) -> None:
     assert paths == ["/etc/mypkg"]
 
 
-def test_paths_accepts_any_sequence(monkeypatch) -> None:
-    monkeypatch.setenv("MYPKG_CONFIG", "foo-bar")
-    config = Config("mypkg", paths=("/etc/mypkg",))
-    assert config.paths == ["/etc/mypkg", "foo-bar"]
-
-
 def test_serialization() -> None:
     config = Config(CONFIG_NAME)
     config.set(one_key="one_value")