Add a format module

Have a central location to view and evaluate all format variables used in the evaluation of the `pyproject.toml`

Signed-off-by: Cristian Le <[email protected]>

Author: LecrisUT

docs/api/scikit_build_core.rst

@@ -41,6 +41,14 @@ scikit\_build\_core.errors module
    :show-inheritance:
    :undoc-members:
 
+scikit\_build\_core.format module
+---------------------------------
+
+.. automodule:: scikit_build_core.format
+   :members:
+   :show-inheritance:
+   :undoc-members:
+
 scikit\_build\_core.program\_search module
 ------------------------------------------
 

src/scikit_build_core/format.py

@@ -0,0 +1,39 @@
+"""Format variables available in the ``pyproject.toml`` evaluation"""
+
+from __future__ import annotations
+
+import dataclasses
+from collections.abc import Mapping
+from typing import TYPE_CHECKING, Any
+
+if TYPE_CHECKING:
+    from collections.abc import Iterator
+
+    from scikit_build_core.settings.skbuild_model import ScikitBuildSettings
+
+__all__ = [
+    "Formatter",
+]
+
+
+@dataclasses.dataclass
+class Formatter(Mapping[str, Any]):
+    """Formatting helper"""
+
+    _dict: dict[str, Any] = dataclasses.field(init=False)
+    """Dict storage of the acceptable variable keys and their expanded values."""
+    settings: ScikitBuildSettings
+    """Parsed settings."""
+
+    def __getitem__(self, key: str) -> Any:
+        return self._dict[key]
+
+    def __len__(self) -> int:
+        return len(self._dict)
+
+    def __iter__(self) -> Iterator[Any]:
+        return iter(self._dict)
+
+    def __post_init__(self) -> None:
+        # TODO: Initialize the container with all known values
+        self._dict = {}