I ended up not using this code, but it may be useful to others:

  • mock_setup_provider.py:
import sys  
from unittest.mock import Mock

class MockSetupProvider(Mock):  
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.captured_setup_requires = set()
    def setup(self, *args, **kwargs):
        self.captured_setup_requires.update(kwargs.get('setup_requires'))
  • setup_extractor.py:
import sys  
from .mock_setup_provider import MockSetupProvider

def extract_setup_requires(setup_dir_path):  
    real_setuptools = sys.modules.get('setuptools')
    real_distutils_core = sys.modules.get('distutils.core')
    sys.path.insert(0, setup_dir_path)
    try:
        mock_setup_provider = MockSetupProvider()
        sys.modules['setuptools'] = sys.modules['distutils.core'] = mock_setup_provider
        import setup
        return mock_setup_provider.captured_setup_requires
    finally:
        sys.path.pop(0)
        sys.modules.get('distutils.core') = real_distutils_core
        sys.modules.get('setuptools') = real_setuptools
  • test_setup_extractor.py:
from setup_extractor import extract_setup_requires

def test_ok():  
    assert extract_setup_requires('tests-fixtures') == set(['damn42'])
  • test-fixtures/setup.py:
from setuptools import setup, find_packages

setup(  
    name='toto',
    version='1.2.3',
    install_requires=['dummy'],
    setup_requires=['damn42'],
)