Incorporate mypy into pre-commit

.pre-commit-config.yaml

@@ -17,6 +17,17 @@ repos:
     rev: v0.3.3
     hooks:
       - id: ruff
+  - repo: https://github.com/pre-commit/mirrors-mypy
+    rev: v1.9.0
+    hooks:
+      - id: mypy
+        # Passing filenames to mypy can do odd things.
+        # pyproject.toml determines the set of files that will actually be checked.
+        pass_filenames: false
+        # The pre-commit hook passes some options, but we set options in pyproject.toml.
+        args: []
+        additional_dependencies:
+          - pytest==8.1.1
   - repo: https://github.com/jazzband/pip-tools
     rev: 7.4.1
     hooks:

pyproject.toml

@@ -52,5 +52,14 @@ debug = false
 profile = "black"
 known_first_party = ["katcp_codec"]
 
+[tool.mypy]
+python_version = "3.8"
+files = "python"
+
+[[tool.mypy.overrides]]
+# The Rust module doesn't have type hints
+module = "katcp_codec._lib"
+ignore_missing_imports = true
+
 [tool.pytest.ini_options]
 testpaths = "python/tests"

python/katcp_codec/__init__.py

@@ -18,7 +18,7 @@
 from dataclasses import dataclass
 from typing import List, Optional, Union
 
-from . import _lib
+from . import _lib  # type: ignore
 
 
 # Note: the values must correspond to those in message.rs
@@ -51,7 +51,7 @@ def __bytes__(self) -> bytes:
 def _message_from_rust(
     message: Union[_lib.Message, ValueError]
 ) -> Union[Message, ValueError]:
-    if isinstance(message, Exception):
+    if isinstance(message, ValueError):
         return message
     else:
         return Message(