I do not really agree with you on this one, and the fact that the original postgresql library (libpq) is loading the credentials from the \~/.pgpass file is a really good example for implementing the same behaviour into pg8000.

By not introducing this you just enforce the users or pg8000 to implement it in their own apps, making them more complex and duplicating code.

Considering that pg8000 is a replacement for the native postgresql library, we already have a model to follow.