仅当文件已存在时才附加到文件

Question

At the start of my script, the file script.log is expected to exist and messages are being appended to it. But if the file does not exist (the user has decided to delete the file), the file should not be created again, the messages will be printed to stdout instead. Fill the bucket only if you see it.

How to do that?
Loads of people must have had a similar problem but I couldn't find any solution.

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I aimed at something like the following code but the append mode does not trigger any exception when the file is not found:

if os.path.exists('script.log'):
    with open('script.log', 'a') as f:
        f.write('foo')
else:
    print('foo')

I know that the following code should work but ideally I would like to avoid it because it contains a race condition :

 if os.path.exists('script.log'): with open('script.log', 'a') as f: f.write('foo') else: print('foo')

Answer 1

Use os.open and os.fdopen separately rather than open .

The "a" mode used by open tries to open the file using the flags os.O_APPEND and os.O_CREAT , creating the file if it doesn't already exist. We'll use os.fdopen to use just the os.O_APPEND flag, causing a FileNotFoundError to be raised if it doesn't already exist.

Assuming it succeeds, we'll use os.fdopen to wrap the file descriptor returned by fdopen in a file-like object. (This requires, unfortunately, a seemingly redundant "a" flag.)

import os
import sys

try:
    fd = os.open('script.log', os.O_APPEND)
    with os.fdopen(fd, "a") as f:
        f.write("foo")
except FileNotFoundError:
    print("foo")