pathlib: Path.iterdir() is surprisingly not streaming

Bug report

Suprisingly (contrary to its name and being a generator), Path.iterdir() does not stream directory entries:

It reads all directory entries into memory before yielding the first entry.

This can cause excessive memory usage when "iterating" over very large directories.

Users would expect by default that .iterdir() operates in a streaming way, like UNIX find or readdir(), streaming e.g. the results of the underlying system calls such as getdents64() on Linux.

But it does entries = list(scandir_it) instead:

https://github.com/python/cpython/blob/c419af9e277bea7dd78f4defefc752fe93b0b8ec/Lib/pathlib/__init__.py#L835-L843

This should be documented, and can hopefully be fixed without too much breakage.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Related issues

• https://github.com/python/cpython/issues/129871

Linked PRs

• gh-136060

Start with Lib/pathlib/init.py at the linked Path.iterdir() implementation around lines 835-843, then review related issue 129871 and linked PR 136060. Confirm the intended iteration behavior and compatibility expectations; done means the issue's streaming behavior is addressed and covered by the relevant pathlib tests.