ENH: implement variables substitution in configuration

meson.build

@@ -11,6 +11,7 @@ py.install_sources(
     'mesonpy/_compat.py',
     'mesonpy/_editable.py',
     'mesonpy/_rpath.py',
+    'mesonpy/_substitutions.py',
     'mesonpy/_tags.py',
     'mesonpy/_util.py',
     'mesonpy/_wheelfile.py',

mesonpy/__init__.py

@@ -47,6 +47,7 @@
 
 import mesonpy._compat
 import mesonpy._rpath
+import mesonpy._substitutions
 import mesonpy._tags
 import mesonpy._util
 import mesonpy._wheelfile
@@ -660,6 +661,12 @@ def __init__(  # noqa: C901
         # load meson args from pyproject.toml
         pyproject_config = _validate_pyproject_config(pyproject)
         for key, value in pyproject_config.get('args', {}).items():
+            # apply variable interpolation
+            try:
+                value = [mesonpy._substitutions.interpolate(x) for x in value]
+            except ValueError as exc:
+                raise ConfigError(
+                    f'Cannot interpret value for "tool.meson-python.args.{key}" configuration entry: {exc.args[0]}') from None
             self._meson_args[key].extend(value)
 
         # meson arguments from the command line take precedence over

mesonpy/_substitutions.py

@@ -0,0 +1,96 @@
+# SPDX-FileCopyrightText: 2023 The meson-python developers
+#
+# SPDX-License-Identifier: MIT
+
+from __future__ import annotations
+
+import ast
+import operator
+import typing
+
+
+if typing.TYPE_CHECKING:  # pragma: no cover
+    from typing import Any, Callable, Mapping, Optional, Type
+
+
+_methods = {}
+
+
+def _register(nodetype: Type[ast.AST]) -> Callable[..., Callable[..., Any]]:
+    def closure(method: Callable[[Interpreter, ast.AST], Any]) -> Callable[[Interpreter, ast.AST], Any]:
+        _methods[nodetype] = method
+        return method
+    return closure
+
+
+class Interpreter:
+
+    _operators = {
+        ast.Add: operator.add,
+        ast.Sub: operator.sub,
+        ast.Mult: operator.mul,
+        ast.Div: operator.truediv,
+    }
+
+    def __init__(self, variables: Mapping[str, Any]):
+        self._variables = variables
+
+    def eval(self, string: str) -> Any:
+        try:
+            expr = ast.parse(string, mode='eval')
+            return self._eval(expr)
+        except KeyError as exc:
+            raise ValueError(f'unknown variable "{exc.args[0]}"') from exc
+        except NotImplementedError as exc:
+            raise ValueError(f'invalid expression {string!r}') from exc
+
+    __getitem__ = eval
+
+    def _eval(self, node: ast.AST) -> Any:
+        # Cannot use functools.singlemethoddispatch as long as Python 3.7 is supported.
+        method = _methods.get(type(node), None)
+        if method is None:
+            raise NotImplementedError
+        return method(self, node)
+
+    @_register(ast.Expression)
+    def _expression(self, node: ast.Expression) -> Any:
+        return self._eval(node.body)
+
+    @_register(ast.BinOp)
+    def _binop(self, node: ast.BinOp) -> Any:
+        func = self._operators.get(type(node.op))
+        if func is None:
+            raise NotImplementedError
+        return func(self._eval(node.left), self._eval(node.right))
+
+    @_register(ast.Constant)
+    def _constant(self, node: ast.Constant) -> Any:
+        return node.value
+
+    # Python 3.7, replaced by ast.Constant is later versions.
+    @_register(ast.Num)
+    def _num(self, node: ast.Num) -> Any:
+        return node.n
+
+    # Python 3.7, replaced by ast.Constant is later versions.
+    @_register(ast.Str)
+    def _str(self, node: ast.Str) -> Any:
+        return node.s
+
+    @_register(ast.Name)
+    def _variable(self, node: ast.Name) -> Any:
+        value = self._variables[node.id]
+        if callable(value):
+            value = value()
+        return value
+
+
+def _ncores() -> int:
+    return 42
+
+
+def interpolate(string: str, variables: Optional[Mapping[str, Any]] = None) -> str:
+    if variables is None:
+        variables = {'ncores': _ncores}
+    return string % Interpreter(variables)

tests/packages/substitutions-invalid/meson.build

@@ -0,0 +1,5 @@
+# SPDX-FileCopyrightText: 2023 The meson-python developers
+#
+# SPDX-License-Identifier: MIT
+
+project('substitutions', version: '0.0.1')

tests/packages/substitutions-invalid/pyproject.toml

@@ -0,0 +1,10 @@
+# SPDX-FileCopyrightText: 2023 The meson-python developers
+#
+# SPDX-License-Identifier: MIT
+
+[build-system]
+build-backend = 'mesonpy'
+requires = ['meson-python']
+
+[tool.meson-python.args]
+compile = ['-j', '%(xxx)d']

tests/packages/substitutions/meson.build

@@ -0,0 +1,5 @@
+# SPDX-FileCopyrightText: 2023 The meson-python developers
+#
+# SPDX-License-Identifier: MIT
+
+project('substitutions', version: '0.0.1')

tests/packages/substitutions/pyproject.toml

@@ -0,0 +1,10 @@
+# SPDX-FileCopyrightText: 2023 The meson-python developers
+#
+# SPDX-License-Identifier: MIT
+
+[build-system]
+build-backend = 'mesonpy'
+requires = ['meson-python']
+
+[tool.meson-python.args]
+compile = ['-j', '%(ncores / 2 + 2)d']

tests/test_substitutions.py

@@ -0,0 +1,34 @@
+# SPDX-FileCopyrightText: 2023 The meson-python developers
+#
+# SPDX-License-Identifier: MIT
+
+import pytest
+
+import mesonpy
+
+
+def test_interpolate():
+    assert mesonpy._substitutions.interpolate('%(x * 2 + 3 - 4 / 1)d', {'x': 1}) == '1'
+
+
+def test_interpolate_key_error():
+    with pytest.raises(RuntimeError, match='unknown variable "y"'):
+        mesonpy._substitutions.interpolate('%(y)d', {'x': 1})
+
+
+def test_interpolate_not_implemented():
+    with pytest.raises(RuntimeError, match='invalid expression'):
+        mesonpy._substitutions.interpolate('%(x ** 2)d', {'x': 1})
+
+
+def test_substitutions(package_substitutions, monkeypatch):
+    monkeypatch.setattr(mesonpy._substitutions, '_ncores', lambda: 2)
+    with mesonpy._project() as project:
+        assert project._meson_args['compile'] == ['-j', '3']
+
+
+def test_substitutions_invalid(package_substitutions_invalid, monkeypatch):
+    monkeypatch.setattr(mesonpy._substitutions, '_ncores', lambda: 2)
+    with pytest.raises(mesonpy.ConfigError, match=''):
+        with mesonpy._project():
+            pass