In Python, how to open a file for writing, but do not create it if the file does not exist?

Question

The file is a device under /dev. I don't want to mess things up so if the file does not exist, I should not create it. How to handle this in Python?

I hope this problem be solved by open method. That is, it should throw an IOError like mode "r" when the file does not exist. Don't redirect the question to "check if a file exists".

score 3 · Accepted Answer · answered Jun 24 '15 at 20:14

3

There's two ways to do this. Either

from os.path import exists
if exists(my_file_path):
    my_file = open(my_file_path, 'w+')

if you need to trigger things based on the file existing, that's the best way.

Otherwise, just do open(file_path, 'r+')

answered Jun 24 '15 at 20:14

Morgan Thrapp

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Great, I'm glad I could help! – Morgan Thrapp Jun 24 '15 at 20:27
Thanks. BTW, can you remove the "duplicate" suggestion? I don't think this question is duplicate. – bfrgzju Jun 24 '15 at 20:29
@bfrgzju I cannot. After you clarified, it's really a dupe of https://stackoverflow.com/questions/1466000/python-open-built-in-function-difference-between-modes-a-a-w-w-and-r but as I'm not a mod, I cannot clear a flag. – Morgan Thrapp Jun 24 '15 at 20:30
I see. Thanks anyway. – bfrgzju Jun 25 '15 at 13:37

score 1 · Answer 2 · edited May 23 '17 at 12:34

1

import os

if os.path.exists(dev):
    fd = os.open(dev, os.O_WRONLY) # or open(dev, 'wb+')
    ...

Also check How to perform low level I/O on Linux device file in Python?

edited May 23 '17 at 12:34

Community

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answered Jun 24 '15 at 20:13

fferri

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Using mode "w" will truncate the file to 0 length and then start writing. – Morgan Thrapp Jun 24 '15 at 20:17
that would be a device not a file – fferri Jun 24 '15 at 20:21

score -1 · Answer 3 · answered Jun 24 '15 at 20:32

Here is a simple python script that shows the files in a directory, where their file is and whether the real file exists or not

import os

def get_file_existency(filename, directory):
 path = os.path.join(directory, filename)
 realpath = os.path.realpath(path)
 exists = os.path.exists(path)
 if exists:
   file = open(realpath, 'w')
 else:
   file = open(realpath, 'r')

In Python, how to open a file for writing, but do not create it if the file does not exist?

3 Answers3