Related to #32090, right?

It is at least related in the sense that GFile does not behave as normal Python file like objects.

This specific bug seems to be caused by an assumption that only readable files can be seeked.
It seems that tell() had the same behaviour until this commit.

Well, yes, the API is incomplete.

My work on modular filesystems will first separate all filesystems into DSOs and then write a test for functionality requirements and then we can start completing the API and checking in every system to make sure it works. Maybe even fuzz the entire thing

test_seek_v1 works but test_seek_v2 not

def test_seek_v1():
  with open('file.txt', 'w+') as f:   # create a new file or truncates it
    f.write("test 1\n")
    f.write("test 2\n")
    f.write("test 3\n")             # now the file pointer is at the end
    f.seek(0)                       # move the file pointer to the beginning
    lines = f.read()                # read it, now we can read!
    print(lines)                    # print it

def test_seek_v2():
  with tf.io.gfile.GFile('file.txt', 'w+') as f:   # create a new file or truncates it
    f.write("test 1\n")
    f.write("test 2\n")
    f.write("test 3\n")             # now the file pointer is at the end
    f.seek(0)                       # move the file pointer to the beginning
    lines = f.read()                # read it, now we can read!
    print(lines)                    # print it

I think test_seek_v2 is actually correct. You are opening the file only for writing so of course you should not be able to read from it

The + argument to the built in open() function means that the file should be opened for updating (reading and writing), see the docs. So the w+ mode of the built in open() function means to open the file for reading and writing and truncating it first. I do not see any documentation of the mode parameter for tf.io.gfile.GFile in the TF docs, so I can only assume that the semantics are supposed to be the same as the mode parameter for the built in open().

This issue is about calling .seek() on a write only file (w), not about the behavior of the w+ mode. So test_seek_v1 and test_seek_v2 are not relevant for this issue, but that behavior might be a separate issue.

test_seek_v2 open mode is reading and writing

My bad, I misread the + meaning.

@eirism !
I can replicate this in 2.11 version. Attached gist in 2.11 for reference.
Thank you!

@eirism The provided code will trigger an error because you're attempting to call f.seek(0) on a file opened in write-only mode ('w'). Seeking is only supported for files opened in read or read/write modes.
Could you try to use it as follows;

with tf.io.gfile.GFile('test.txt', 'r+') as f:
  f.seek(0)
  # Your code that includes writing and seeking

Thank you!

@sushreebarsa I had the same comment a while ago and got #32122 (comment) as reply (which makes sense)

@sushreebarsa Normal Python file IO supports .seek() on files opened in write-only mode ('w'), so I expect GFile to also support it. In normal Python file IO .seek() moves the file object position and allows you to overwrite already written data. The following code using normal Python file IO will write "Hello" to the text file "test.txt" by using .seek() to overwrite the first character written to file:

with open("test.txt", "w") as f:
    f.write("hello")
    f.seek(0)  # Seek to start of file to overwrite characters from the start
    f.write("H")