maint: add mypy to project

Adds all type annotations to satisfy mypy. There should be 0 functional
changes.

Author: nicklafleur

.pre-commit-config.yaml

@@ -20,3 +20,17 @@ repos:
     -   id: ruff
         args: [--fix, --exit-non-zero-on-fix]
     -   id: ruff-format
+-   repo: https://github.com/pre-commit/mirrors-mypy
+    rev: v1.19.1
+    hooks:
+    -   id: mypy
+        args: [--config-file=mypy.ini]
+        exclude: (tests/|docs/)
+        additional_dependencies:
+        -    click>=8.0.0
+        -    coverage>=7.3.0
+        -    libcst>=1.8.5
+        -    pytest>=6.2.5
+        -    setproctitle>=1.1.0
+        -    textual>=1.0.0
+        -    types-toml>=0.10.2

mypy.ini

@@ -0,0 +1,12 @@
+[mypy]
+mypy_path = src:types
+strict = True
+check_untyped_defs = True
+explicit_package_bases = True
+warn_redundant_casts = True
+
+[mypy-hammett]
+ignore_missing_imports = True
+
+[mypy-e2e_projects.*]
+ignore_errors = True

src/mutmut/__init__.py

@@ -1,19 +1,25 @@
+from __future__ import annotations
+
 import importlib.metadata
 from collections import defaultdict
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from mutmut.__main__ import Config
 
 __version__ = importlib.metadata.version("mutmut")
 
 
-duration_by_test = defaultdict(float)
-stats_time = None
-config = None
+duration_by_test: defaultdict[str, float] = defaultdict(float)
+stats_time: float | None = None
+config: Config | None = None
 
-_stats = set()
-tests_by_mangled_function_name = defaultdict(set)
-_covered_lines = None
+_stats: set[str] = set()
+tests_by_mangled_function_name: defaultdict[str, set[str]] = defaultdict(set)
+_covered_lines: dict[str, set[int]] | None = None
 
 
-def _reset_globals():
+def _reset_globals() -> None:
     global duration_by_test, stats_time, config, _stats, tests_by_mangled_function_name
     global _covered_lines