Document that the built-in open() when called with a file descriptor ignores "mode" argument #95498

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1. piotr-dobrogost

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   docsDocumentation in the Doc dirDocumentation in the Doc dir

   on Jul 31, 2022
2. ronaldoussoren

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   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.