Christian Heimes added the comment:
In case someone is wondering if the approach really reduces the amount of
syscalls: yes, it does. readdir() doesn't do a syscall for each entry. On Linux
it uses the internal syscall getdents() to fill a buffer of directory entry
structs. http://man7.org/linux/man-pages/man2/getdents.2.html
On my system os.listdir() does four syscalls:
$ strace python -c "import os; os.listdir('/home/heimes')"
openat(AT_FDCWD, "/home/heimes", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3
getdents(3, /* 381 entries */, 32768) = 12880
getdents(3, /* 0 entries */, 32768) = 0
close(3)
On Linux you can also use /proc/self/fd instead of /proc/YOURPID/fd.
Other operating systems have different APIs to get a list of open FDs. AFAK
/dev/fd is static on FreeBSD and Mac OS X:
FreeBSD:
http://www.manualpages.de/FreeBSD/FreeBSD-7.4-RELEASE/man3/kinfo_getfile.3.html
Darwin / Mac OS X:
proc_pidinfo()
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<http://bugs.python.org/issue13788>
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