check what files are open in Python

On Linux, you can look at the contents of /proc/self/fd:

$ ls -l /proc/self/fd/
total 0
lrwx------ 1 foo users 64 Jan  7 15:15 0 -> /dev/pts/3
lrwx------ 1 foo users 64 Jan  7 15:15 1 -> /dev/pts/3
lrwx------ 1 foo users 64 Jan  7 15:15 2 -> /dev/pts/3
lr-x------ 1 foo users 64 Jan  7 15:15 3 -> /proc/9527/fd

To list all open files in a cross-platform manner, I would recommend psutil.

#!/usr/bin/env python
import psutil

for proc in psutil.process_iter():
    print(proc.open_files())

The original question implicitly restricts the operation to the currently running process, which can be accessed through psutil's Process class.

proc = psutil.Process()
print(proc.open_files())

Lastly, you'll want to run the code using an account with the appropriate permissions to access this information or you may see AccessDenied errors.

I ended up wrapping the built-in file object at the entry point of my program. I found out that I wasn't closing my loggers.

import io
import sys
import builtins
import traceback
from functools import wraps


def opener(old_open):
    @wraps(old_open)
    def tracking_open(*args, **kw):
        file = old_open(*args, **kw)

        old_close = file.close
        @wraps(old_close)
        def close():
            old_close()
            open_files.remove(file)
        file.close = close
        file.stack = traceback.extract_stack()

        open_files.add(file)
        return file
    return tracking_open


def print_open_files():
    print(f'### {len(open_files)} OPEN FILES: [{", ".join(f.name for f in open_files)}]', file=sys.stderr)
    for file in open_files:
        print(f'Open file {file.name}:\n{"".join(traceback.format_list(file.stack))}', file=sys.stderr)


open_files = set()
io.open = opener(io.open)
builtins.open = opener(builtins.open)

Although the solutions above that wrap opens are useful for one's own code, I was debugging my client to a third party library including some c extension code, so I needed a more direct way. The following routine works under darwin, and (I hope) other unix-like environments:

def get_open_fds():
    '''
    return the number of open file descriptors for current process

    .. warning: will only work on UNIX-like os-es.
    '''
    import subprocess
    import os

    pid = os.getpid()
    procs = subprocess.check_output( 
        [ "lsof", '-w', '-Ff', "-p", str( pid ) ] )

    nprocs = len( 
        filter( 
            lambda s: s and s[ 0 ] == 'f' and s[1: ].isdigit(),
            procs.split( '\n' ) )
        )
    return nprocs

If anyone can extend to be portable to windows, I'd be grateful.