Since RPython is a strict subset of Python, the PyPy Python interpreter can be run on top of another Python implementation untranslated. This is, of course, extremely slow but it makes it possible to quickly test changes in the interpreter. It also enables normal Python debugging tools to be used to debug the interpreter. Most of PyPy's interpreter tests can be run both on the untranslated interpreter and the translated interpreter. This allows quick testing during development as well as assurance that the translated interpreter behaves the same as the untranslated one