Indeed. And if you're mocking eg. API calls in a client library, try to have tests for the real things as well. They don't have to be part of your normal test suite, they can run only if env vars are set with the API keys needed.
VCR.py (https://github.com/kevin1024/vcrpy) is a great utility for mocking APIs.
It will run each request once, save the responses to YAML files, and then replay the responses every time you re-run the tests. It's also very useful for caching API responses (e.g. you have a trial account with limited number of requests).
Unfortunately, if used for testing, it will not cover the case when the original API changes its interface.
