Python won't write to file unless it already exists: permissions related issue

Question

I've got the following Python code:

class Server(object):

  def __init__(self, options):
    self.data_file = '/etc/my_module/resources.json'

  # Triggered when new data is received
  def write(self, data):
    try:
      f = open(self.data_file, 'w')
      json.dump(data, f)
    except Exception, e:
      print e
    finally:
      f.close()

If the file /etc/my_module/resources.json does not exist then I get an error message when I call the write method:

[Errno 2] No such file or directory: '/etc/my_module/resources.json'

If I create the file manually and call the write method again, then everything works and the json is written to the file.

I'm running my application with sudo: sudo python my_module.py, and I've added my user (vagrant) to /etc/sudoers.

So python won't create the file if it doesn't exist, on the grounds that it doesn't have permission to write to /etc, but it will happily write to the file if it does exist. Why is this?

score 4 · Accepted Answer · answered Sep 09 '14 at 12:52

4

Change

f = open(self.data_file, 'w')

To

f = open(self.data_file, 'w+')

answered Sep 09 '14 at 12:52

score 2 · Answer 2 · answered Sep 09 '14 at 12:52

2

Creating a new file involves writing a new file entry to the directory which requires write permissions on the directory. Modifying an existing file bypasses this.

answered Sep 09 '14 at 12:52

Ignacio Vazquez-Abrams

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1

Assuming he's running the file using sudo wouldn't it have those necessary write permissions? – Sep 09 '14 at 12:55
That would imply that `O_CREAT` is not set in the `open(2)` call, which sounds like an OS bug. – Ignacio Vazquez-Abrams Sep 09 '14 at 13:01
Thanks, it turns out @iChar got the right answer above, I needed to use `open(self.data_file, 'w+')`. – stephenmurdoch Sep 09 '14 at 13:03
@marflar: But that doesn't fully explain *why*. (Not that mine does either.) – Ignacio Vazquez-Abrams Sep 09 '14 at 13:04

Python won't write to file unless it already exists: permissions related issue

2 Answers2