Saturday, March 21, 2009

Maximum number of open files in Python

So here I was merrily writing a python module to extract neural data from the Cerebus system's giant .nevs file and split it into smaller files - one per neural unit. I had been extracting one channel at a time and all was well. So now I copy the code over to the lab machine and tell it to extract all the data and split it into 96*4 files.

Traceback (most recent call last):
File "", line 11, in
File "/Users/kghose/Experiments/Python/", line 270, in fragment
IOError: [Errno 24] Too many open files: '../Neural/DJ2009/20090320//Frag//electrode64unit02.bin'

Whaaa? Whats all this? Well, it turns out you can't have too many files open at the same time. And the resource module can tell you exactly how many files you can have open at a given time:

(256L, 9223372036854775807L)

Which means the current soft limit is 256 files and the hard limit is some number so large only astronomers and people who write stimulus bills can deal with it. And so for my application, I can change the limit by doing:

resource.setrlimit(resource.RLIMIT_NOFILE, (500,-1))

(-1 means set the hard limit to the maximum possible)


  1. Thanks for this! I didn't know about resource, and this totally solved a problem I had today!

  2. Some OS's don't support the ability to change this. My Gentoo box for example. It's probably a good idea to do this

    resource.setrlimit(resource.RLIMIT_NOFILE, (512, -1))
    except ValueError:
    pass # Don't complain about the error

  3. Hi "my blog",
    That is odd. Gentoo is a Linux distribution and the kernel should be consistent. Which version of Python are you using and what kind of permissions do you have? Have you run your code as root to check if it is a permissions issue?

  4. I did this, however, the permissions on my machine disallowed me from changing the file limit. A quick search, and I found that this was the solution:

  5. On Windows:

    import platform

    if platform.system() == 'Windows':
    import win32file
    print('Max number of file handles: {0}'.format(win32file._getmaxstdio()))

    2048 is the maximum.