[New-bugs-announce] [issue38710] unsynchronized write pointer in io.TextIOWrapper in 'r+' mode

[Manuel Ignacio Pérez Alcolea]

New submission from Manuel Ignacio Pérez Alcolea <perezalcoleam at gmail.com>:

There seems to be a bug in the `io.TextIOWrapper` class while working in 'r+' mode, although I can't say the source of the problem is right there.

The write pointer doesn't match `file.tell()` after performing a read operation.

For example, this file, consisting of 3 lines:

  line one
  line two
  line three

Doesn't result in the expected modification running the following program:

with open('file', 'r+', buffering=1) as f:
    print(f.tell())                  # => 0
    print(f.readline().strip())      # we read 1 line
    print(f.tell())                  # => 9  
    print('Hello', file=f)           # we write "Hello\n"
    print(f.tell())                  # => 34

Instad of

  line one
  Hello
  wo
  line three

It results in

  line one
  line two
  line threeHello

But it works just fine if `f.seek(f.tell())` is added the program, right before the write operation.

There are several possible explanations on StackOverflow, involving the buffering for IO in text files:

https://stackoverflow.com/a/58722058/11601118

----------
components: IO
messages: 356089
nosy: Manuel Ignacio Pérez Alcolea
priority: normal
severity: normal
status: open
title: unsynchronized write pointer in io.TextIOWrapper in 'r+' mode
type: behavior
versions: Python 3.7

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38710>
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