Appendix-File-Descriptor

In the context of selectors, fileobj is used only for monitoring I/O readiness events.

Selectors are built around OS-level file descriptors, and file descriptors represent resources that support non-blocking I/O, such as:

• sockets
• pipes
• files
• character devices
• eventfd / timerfd (on Linux)
• custom objects exposing a .fileno() that maps to one of the above

Selectors do not care about anything else — only whether a file descriptor is ready for reading or writing.

What selectors do

A selector monitors file descriptors for events:

• EVENT_READ → "ready to read without blocking"
• EVENT_WRITE → "ready to write without blocking"

✔ What selectors do NOT do

Selectors do not:

• read or write data
• watch arbitrary objects
• handle application logic
• manage timeouts or threading by themselves

They exist purely to efficiently check I/O readiness in event loops.