CODEWITHSUNDEEP
python
Ho0ony
Ho0ony

Reputation: 158

Python "with" statement can occur TOCTOU?

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")

Upvotes: 3

Views: 562

Answers (1)

Gurupad Hegde
Gurupad Hegde

Reputation: 2155

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?

Upvotes: 2

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