> Also, I'm reluctant to have two copies of the code for _read_block(); it makes the code harder to read, and increases the chance of introducing a bug when changing the code.

Recursive inline _check_can_read() will be enough. Now this check calls 4 Python functions (_check_can_read(), readable(), _check_non_closed(), closed). Recursive inlining only readable() in _check_can_read() is achieved significant but less (about 30%) effect.