Add function for reading and caching PEP 518 spec

colcon_core/python_project/spec.py

@@ -0,0 +1,49 @@
+# Copyright 2022 Open Source Robotics Foundation, Inc.
+# Licensed under the Apache License, Version 2.0
+
+try:
+    # Python 3.11+
+    from tomllib import load as toml_load
+except ImportError:
+    from tomli import load as toml_load
+
+
+SPEC_NAME = 'pyproject.toml'
+
+_DEFAULT_BUILD_SYSTEM = {
+    'build-backend': 'setuptools.build_meta:__legacy__',
+    'requires': ['setuptools >= 40.8.0', 'wheel'],
+}
+
+
+def load_spec(project_path):
+    """
+    Load build system specifications for a Python project.
+
+    :param project_path: Path to the root directory of the project
+    """
+    spec_file = project_path / SPEC_NAME
+    try:
+        with spec_file.open('rb') as f:
+            spec = toml_load(f)
+    except FileNotFoundError:
+        spec = {}
+
+    spec.setdefault('build-system', _DEFAULT_BUILD_SYSTEM)
+
+    return spec
+
+
+def load_and_cache_spec(desc):
+    """
+    Get the cached spec for a package descriptor.
+
+    If the spec has not been loaded yet, load and cache it.
+
+    :param desc: The package descriptor
+    """
+    spec = desc.metadata.get('python_project_spec')
+    if spec is None:
+        spec = load_spec(desc.path)
+        desc.metadata['python_project_spec'] = spec
+    return spec

setup.cfg

@@ -39,6 +39,7 @@ install_requires =
   pytest-repeat
   pytest-rerunfailures
   setuptools>=30.3.0
+  tomli>=1.0.0;python_version < "3.11"
 packages = find:
 zip_safe = false
 

test/spell_check.words

@@ -4,6 +4,7 @@ apache
 argparse
 asyncio
 autouse
+backend
 backported
 basepath
 bazqux
@@ -85,6 +86,7 @@ prepending
 proactor
 purelib
 pydocstyle
+pyproject
 pytest
 pytests
 pythondontwritebytecode
@@ -136,6 +138,9 @@ testsuite
 thomas
 tmpdir
 todo
+toml
+tomli
+tomllib
 traceback
 tryfirst
 tuples

test/test_pyproject_spec.py

@@ -0,0 +1,53 @@
+# Copyright 2024 Open Source Robotics Foundation, Inc.
+# Licensed under the Apache License, Version 2.0
+
+from colcon_core.package_descriptor import PackageDescriptor
+from colcon_core.python_project.spec import load_and_cache_spec
+
+
+def test_pyproject_missing(tmp_path):
+    desc = PackageDescriptor(tmp_path)
+
+    spec = load_and_cache_spec(desc)
+    assert spec.get('build-system') == {
+        'build-backend': 'setuptools.build_meta:__legacy__',
+        'requires': ['setuptools >= 40.8.0', 'wheel'],
+    }
+
+
+def test_pyproject_empty(tmp_path):
+    desc = PackageDescriptor(tmp_path)
+
+    (tmp_path / 'pyproject.toml').write_text('')
+
+    spec = load_and_cache_spec(desc)
+    assert spec.get('build-system') == {
+        'build-backend': 'setuptools.build_meta:__legacy__',
+        'requires': ['setuptools >= 40.8.0', 'wheel'],
+    }
+
+
+def test_specified(tmp_path):
+    desc = PackageDescriptor(tmp_path)
+
+    (tmp_path / 'pyproject.toml').write_text('\n'.join((
+        '[build-system]',
+        'build-backend = "my_build_backend.meta"',
+        'requires = ["my-build-backend"]',
+    )))
+
+    spec = load_and_cache_spec(desc)
+    assert spec.get('build-system') == {
+        'build-backend': 'my_build_backend.meta',
+        'requires': ['my-build-backend'],
+    }
+
+    # truncate the pyproject.toml and call again
+    # this verifies that the spec is cached
+    (tmp_path / 'pyproject.toml').write_text('')
+
+    spec = load_and_cache_spec(desc)
+    assert spec.get('build-system') == {
+        'build-backend': 'my_build_backend.meta',
+        'requires': ['my-build-backend'],
+    }