When to use

To wrap a file-like object and count reads/writes.

Rules

Store the wrapped object as self._wrapped
Implement read(size=-1) by delegating to self._wrapped.read(size)
Increment counters by the length of the RETURNED bytes, NOT the requested size
For write: increment nwrites by the RETURN VALUE, or by len(data) if the wrapped write returns None
Expose read_bytes/nreads and write_bytes/nwrites as properties or attributes
enter returns self; exit calls self._wrapped.exit (or close()) and forwards the exception info
ALWAYS implement close() as a plain method for non-context-manager use
NEVER count requested bytes — only count what was actually returned/written

Thread safety: if the test uses threads, wrap counter updates in a threading.Lock.