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

从 Lib/pathlib/init.py 中链接的 Path.iterdir() 实现（约第 835-843 行）开始，然后查看相关 issue 129871 和链接的 PR 136060。确认预期的迭代行为和兼容性要求；当 issue 的流式行为得到处理并由相关的 pathlib 测试覆盖时，即视为完成。