Python "with" statement can occur TOCTOU?

Question

Assume that, file.txt is closed or deleted during the delay between open and write. (or it can ?)

Then, this situation can occur TOCTOU ?

with statement sure that atomic until with block or not?

with open("file.txt") as f :
    # ...delayed...
    f.write("something")

`with` blocks aren’t atomic (neither in general nor with files). On Windows, IIRC, you can’t delete a file that’s being written to; elsewhere, unlinking a file doesn’t invalidate handles to it. Either way, there’s no problem with continuing to write. **tl;dr: no** — Ry-, Mar 31 '17 at 05:44
No. As long as the file is open, it won’t actually be deleted by conventional means. — Ry-, Mar 31 '17 at 06:11

score 2 · Accepted Answer · edited May 23 '17 at 12:25

Can this occur?

Use case 1: Python itself deletes the file*

yes it can happen. I just tested like this:

In [1]: with open("file.txt", "w") as f :
   ...:     f.write("Something Old")
   ...:

In [2]: !cat ./file.txt
Something Old
In [3]: import os
   ...: with open("file.txt","w") as f:
   ...:     os.remove("./file.txt")
   ...:     print f.write("Something new")
   ...:
None

In [4]: !cat ./file.txt
cat: ./file.txt: No such file or directory

Use Case 2: Other than python deletes the file.

Then also, found the behavior to be same.

In [1]: !cat ./file.txt
Something Old
In [2]: import os
   ...: import time
   ...:
   ...: with open("file.txt","w") as f:
   ...:     time.sleep(15)
   ...:     print f.write("Something new")
   ...:
None

In [3]: !cat ./file.txt
cat: ./file.txt: No such file or directory

How to avoid it?

You can use exclusive lock from fcntl.lockf()

Edit: There is one more caveat here. Locking the file may not be straight forward and may be OS dependent like What is the best way to open a file for exclusive access in Python?

It's pretty interested example. Thank you ! – Ho0ony Mar 31 '17 at 06:14 — Ho0ony, Mar 31 '17 at 06:14

Python "with" statement can occur TOCTOU?

1 Answers1