ENH: implement variables substitution in configuration

Author: dnicolodi

mesonpy/_substitutions.py

@@ -0,0 +1,61 @@
+# SPDX-FileCopyrightText: 2023 The meson-python developers
+#
+# SPDX-License-Identifier: MIT
+
+from __future__ import annotations
+
+import ast
+import functools
+import operator
+
+from typing import Any, Dict
+
+
+class Interpreter:
+
+    _operators = {
+        ast.Add: operator.add,
+        ast.Sub: operator.sub,
+        ast.Mult: operator.mul,
+        ast.Div: operator.truediv,
+    }
+
+    def __init__(self, variables: Dict[str, Any]):
+        self._variables = variables
+
+    def eval(self, string: str) -> Any:
+        expr = ast.parse(string, mode='eval')
+        return self._eval(expr)
+
+    __getitem__ = eval
+
+    @functools.singledispatchmethod
+    def _eval(self, node: ast.Node) -> Any:
+        print(node, type(node))
+        raise ValueError
+
+    @_eval.register
+    def _expression(self, node: ast.Expression) -> Any:
+        return self._eval(node.body)
+
+    @_eval.register
+    def _binop(self, node: ast.BinOp) -> Any:
+        func = self._operators.get(type(node.op))
+        if func is None:
+            raise ValueError(node.op)
+        return func(self._eval(node.left), self._eval(node.right))
+
+    @_eval.register
+    def _constant(self, node: ast.Constant) -> Any:
+        return node.value
+
+    @_eval.register
+    def _variable(self, node: ast.Name) -> Any:
+        value = self._variables.get(node.id)
+        if value is None:
+            raise ValueError
+        return value
+
+
+def interpolate(string, **variables):
+    return string % Interpreter(variables)