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

從 Lib/pathlib/init.py 中連結的 Path.iterdir() 實作（約第 835-843 行）開始，接著檢視相關的 issue 129871 和連結的 PR 136060。確認預期的迭代行為和相容性要求；當 issue 的串流行為得到處理，並由相關的 pathlib 測試涵蓋時，即視為完成。