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piotr-dobrogost

opened on Jul 31, 2022

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At https://docs.python.org/3/library/functions.html#open, in the documentation of the mode argument there is no information that this argument is ignored when the function is called with a file descriptor instead of a path.

See Why does os.fdopen ignore mode? SO question. The question talks about os.fdopen() but according to https://docs.python.org/3/library/os.html#os.fdopen

This is an alias of the open() built-in function and accepts the same arguments.

so I think it should be enough to place the information in the documentation of the mode argument of the built-in open() function. This way it would document the built-in open() function but also the os.fdopen() function by virtue of the latter being an alias of the former.

Linked PRs

• gh-95498: Document open() behavior with file descriptors #159324
• gh-95498: Document open() behavior with file descriptors #161193

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1. piotr-dobrogost

   added

   docsDocumentation in the Doc dirDocumentation in the Doc dir

   on Jul 31, 2022
2. ronaldoussoren

   added

   topic-IO

   on Jul 31, 2022
3. tiran commented on Jul 31, 2022

   

   tiran

   on Jul 31, 2022

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   builtin open() does not ignore the mode parameter. The argument is used to determinate the type of object. The open() function does not verify that the properties of the fd matches the arguments:

   >>> fd = os.open("testfile.txt", os.O_WRONLY | os.O_CREAT)
   

   >>> f1 = open(fd, mode="r")
   >>> f1
   <_io.TextIOWrapper name=3 mode='r' encoding='UTF-8'>
   >>> f1.read()
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   OSError: [Errno 9] Bad file descriptor
   >>> f1.write("foo")
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   io.UnsupportedOperation: not writable
   

   >>> f2 = open(fd, mode="rb")
   >>> f2
   <_io.BufferedReader name=3>
   
4. piotr-dobrogost commented on Jul 31, 2022

   

   piotr-dobrogost

   on Jul 31, 2022

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   The open() function does not verify that the properties of the fd matches the arguments

   Well, this is surprising. If the properties of the fd can't be verified (or it's undesired for them to be verified for some reason) then it should be clearly stated in the documentation that it's a caller's responsibility to match them with the value of the mode argument. Also in this case there should be information on what exactly matching the properties of the fd means.
5. kevinprimeraquecreo-stack

   added this to Project_VKon Oct 18, 2026
6. github-project-automation

   moved this to Todo in Project_VKon Oct 18, 2026
7. kevinprimeraquecreo-stack

   added this to Project_VKon Oct 18, 2026
8. raihanshiras

   added a commit that references this issue on Oct 29, 2026

   Merge pull request #1 from raihanshiras/pythongh-95498-open-fd-docs

   4acc5a3

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