add mypy to linting checks (#19)

Reading through this project's code for work on
rapidsai/build-planning#31, I noticed it isn't
using type hints or running a type-checking

This proposes the following:

* adding `mypy` to checks run via pre-commit
* adding type hints for a few cases

I think this'll make the code a little easier to understand, and improve
the change of catching some types of bugs (like treating a value which
can be `None` as if it was unconditionally not `None) during
development.

### How I tested this

```shell
pre-commit run --all-files
```

.gitignore

@@ -1,5 +1,7 @@
 __pycache__
 .cache
+.mypy_cache/
+.ruff_cache/
 .vscode
 *.swp
 .DS_Store

.pre-commit-config.yaml

@@ -26,7 +26,15 @@ repos:
     hooks:
       - id: verify-copyright
         args: [--fix, --main-branch=main]
-
+  - repo: https://github.com/pre-commit/mirrors-mypy
+    rev: 'v1.10.0'
+    hooks:
+      - id: mypy
+        args: [
+          "--config-file=pyproject.toml",
+          "rapids_build_backend/"
+        ]
+        pass_filenames: false
 
 default_language_version:
       python: python3

pyproject.toml

@@ -33,6 +33,9 @@ license-files = ["LICENSE"]
 [tool.setuptools.packages.find]
 include = ["rapids_build_backend*"]
 
+[tool.mypy]
+ignore_missing_imports = true
+
 [tool.ruff]
 lint.select = ["E", "F", "W", "I", "N", "UP"]
 lint.fixable = ["ALL"]

rapids_build_backend/config.py

@@ -1,17 +1,30 @@
 # Copyright (c) 2024, NVIDIA CORPORATION.
 
 import os
+from typing import TYPE_CHECKING
 
 from .utils import _get_pyproject
 
+if TYPE_CHECKING:
+    from typing import Callable
+
+    # config options can be one of these types...
+    config_val_type = str | bool | None
+
+    # ... or a callable that returns one of those or some other mutable types
+    mutable_config_val_type = list[str]
+    config_val_callable = Callable[[], config_val_type | mutable_config_val_type]
+
+    config_options_type = dict[str, tuple[config_val_type | config_val_callable, bool]]
+
 
 class Config:
     """Manage the build configuration for the project."""
 
     # Mapping from config option to default value (None indicates that option is
     # required) and whether it may be overridden by an environment variable or a config
     # setting.
-    config_options = {
+    config_options: "config_options_type" = {
         "build-backend": (None, False),
         "commit-file": ("", False),
         "dependencies-file": ("dependencies.yaml", True),

rapids_build_backend/impls.py

@@ -78,7 +78,7 @@ def _get_cuda_version(require_cuda=False):
 
 
 @lru_cache
-def _get_cuda_suffix(require_cuda=False):
+def _get_cuda_suffix(require_cuda=False) -> str:
     """Get the CUDA suffix based on nvcc.
 
     Parameters
@@ -99,7 +99,7 @@ def _get_cuda_suffix(require_cuda=False):
 
 
 @lru_cache
-def _get_git_commit():
+def _get_git_commit() -> str | None:
     """Get the current git commit.
 
     Returns None if git is not in the PATH or if it fails to find the commit.

rapids_build_backend/utils.py

@@ -5,7 +5,7 @@
 import tomli
 
 
-def _get_pyproject(dirname="."):
+def _get_pyproject(dirname: str = ".") -> dict:
     """Parse and return the pyproject.toml file."""
     with open(os.path.join(dirname, "pyproject.toml"), "rb") as f:
         return tomli.load(f)