Raphael: Can you point to the implementation code that handles file objects without a file descriptor (or give a demonstration of it)? I suspect there is no such support and you are mistaken.

Perhaps we can instead clarify in the “subprocess” documentation that “fileno” is required. Issue 19992 already proposes this.

Issue 1260171 and Issue 10482 are also related, about streaming data between Python file objects in the parent and pipes connected to the child.