Good idea for a more robust caching-focused library like dogpile, but I'd say this is out of scope of unstdlib.py, per:

> We value simplicity and elegance over robustness and optimization. This library should serve as a good foundation for your own application-specific code instead of a complete framework. In other words, it is preferred to have a 2-line function that covers 80% of use cases than a 20-line function that covers 100% of the use cases. Unexpected behaviour on bad input is fine as long as expected usage is well documented.