I haven't tried the approach in Python, but example generation is compositional: Generate arbitrary credit cards when you're testing the credit card code. Then

I haven't tried the approach in Python, but example generation is compositional: Generate arbitrary credit cards when you're testing the credit card code. Then later when you're testing a customer code, generate a customer who has 0..n credit cards. "But isn't it hard to set up your input data exactly right to make the test pass?" That's the point! You see what your system does when you give it data you didn't expect to give it.