Is there a way to efficiently yield every file in a directory containing millions of files?

Question

I'm aware of os.listdir, but as far as I can gather, that gets all the filenames in a directory into memory, and then returns the list. What I want, is a way to yield a filename, work on it, and then yield the next one, without reading them all into memory.

Is there any way to do this? I worry about the case where filenames change, new files are added, and files are deleted using such a method. Some iterators prevent you from modifying the collection during iteration, essentially by taking a snapshot of the state of the collection at the beginning, and comparing that state on each move operation. If there is an iterator capable of yielding filenames from a path, does it raise an error if there are filesystem changes (add, remove, rename files within the iterated directory) which modify the collection?

There could potentially be a few cases that could cause the iterator to fail, and it all depends on how the iterator maintains state. Using S.Lotts example:

filea.txt
fileb.txt
filec.txt

Iterator yields filea.txt. During processing, filea.txt is renamed to filey.txt and fileb.txt is renamed to filez.txt. When the iterator attempts to get the next file, if it were to use the filename filea.txt to find it's current position in order to find the next file and filea.txt is not there, what would happen? It may not be able to recover it's position in the collection. Similarly, if the iterator were to fetch fileb.txt when yielding filea.txt, it could look up the position of fileb.txt, fail, and produce an error.

If the iterator instead was able to somehow maintain an index dir.get_file(0), then maintaining positional state would not be affected, but some files could be missed, as their indexes could be moved to an index 'behind' the iterator.

This is all theoretical of course, since there appears to be no built-in (python) way of iterating over the files in a directory. There are some great answers below, however, that solve the problem by using queues and notifications.

Edit:

The OS of concern is Redhat. My use case is this:

Process A is continuously writing files to a storage location. Process B (the one I'm writing), will be iterating over these files, doing some processing based on the filename, and moving the files to another location.

Edit:

Definition of valid:

Adjective 1. Well grounded or justifiable, pertinent.

(Sorry S.Lott, I couldn't resist).

I've edited the paragraph in question above.

Answer 1

tl;dr <update>: As of Python 3.5 (currently in beta) just use os.scandir </update>

As I've written earlier, since "iglob" is just a facade for a real iterator, you will have to call low level system functions in order to get one at a time like you want. Fortunately, calling low level functions is doable from Python. The low level functions are different for Windows and Posix/Linux systems.

• If you are on Windows, you should check if win32api has any call to read "the next entry from a dir" or how to proceed otherwise.
• If you are on Posix/Linux, you can proceed to call libc functions straight through ctypes and get a file-dir entry (including naming information) a time.

The documentation on the C functions is here: http://www.gnu.org/s/libc/manual/html_node/Opening-a-Directory.html#Opening-a-Directory

http://www.gnu.org/s/libc/manual/html_node/Reading_002fClosing-Directory.html#Reading_002fClosing-Directory

I have provided a snippet of Python code that demonstrates how to call the low-level C functions on my system but this code snippet may not work on your system[footnote-1]. I recommend opening your /usr/include/dirent.h header file and verifying the Python snippet is correct (your Python Structure must match the C struct) before using the snippet.

Here is the snippet using ctypes and libc I've put together that allow you to get each filename, and perform actions on it. Note that ctypes automatically gives you a Python string when you do str(...) on the char array defined on the structure. (I am using the print statement, which implicitly calls Python's str)

#!/usr/bin/env python2
from ctypes import *

libc = cdll.LoadLibrary( "libc.so.6")
dir_ = c_voidp( libc.opendir("/home/jsbueno"))

class Dirent(Structure):
    _fields_ = [("d_ino",  c_voidp),
                ("off_t", c_int64),
                ("d_reclen", c_ushort),
                ("d_type", c_ubyte),
                ("d_name", c_char * 2048)
            ]

while True:
    p  = libc.readdir64(dir_)
    if not p:
        break
    entry = Dirent.from_address( p)
    print entry.d_name

---

update: Python 3.5 is now in beta - and in Python 3.5 the new os.scandir function call is available as the materialization of PEP 471 ("a better and faster directory iterator") which does exactly what is asked for here, besides a lot other optimizations that can deliver up to 9 fold speed increase over os.listdir on large-directories listing under Windows (2-3 fold increase in Posix systems).

[footnote-1] The dirent64 C struct is determined at C compile time for each system.

Answer 2

The glob module Python from 2.5 onwards has an iglob method which returns an iterator. An iterator is exactly for the purposes of not storing huge values in memory.

glob.iglob(pathname)
Return an iterator which yields the same values as glob() without
actually storing them all simultaneously.

For example:

import glob
for eachfile in glob.iglob('*'):
    # act upon eachfile

Answer 3

Since you are using Linux, you might want to look at pyinotify. It would allow you to write a Python script which monitors a directory for filesystem changes -- such as the creation, modification or deletion of files.

Every time such a filesystem event occurs, you can arrange for the Python script to call a function. This would be roughly like yielding each filename once, while being able to react to modifications and deletions.

It sounds like you already have a million files sitting in a directory. In this case, if you were to move all those files to a new, pyinotify-monitored directory, then the filesystem events generated by the creation of new files would yield the filenames as desired.