human
Solve the following python programming problem given the following code:
@pytest.fixture(autouse=True)
def mock_fetch_cache(monkeypatch):
assistant
#Here is the Python program you requested.

    """Substitutes spack.fetch_cache with a mock object that does nothing
    and raises on fetch.
    """
    class MockCache(object, ):
        def store(self, copyCmd, relativeDst):
            pass
        def fetcher(self, targetPath, digest, **kwargs):
            return MockCacheFetcher()
    class MockCacheFetcher(object, ):
        def set_stage(self, stage):
            pass
        def fetch(self):
            raise FetchError('Mock    cache    always    fails    for    tests')
        def __str__(self):
            return '[mock    fetch    cache]'
    monkeypatch.setattr(spack, 'fetch_cache', MockCache())