classification
Title: unsynchronized write pointer in io.TextIOWrapper in 'r+' mode
Type: behavior Stage:
Components: IO, Library (Lib) Versions: Python 3.7
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: Manuel Ignacio Pérez Alcolea, josh.r
Priority: normal Keywords:

Created on 2019-11-06 04:04 by Manuel Ignacio Pérez Alcolea, last changed 2019-11-07 00:36 by josh.r.

Messages (1)
msg356089 - (view) Author: Manuel Ignacio Pérez Alcolea (Manuel Ignacio Pérez Alcolea) Date: 2019-11-06 04:04
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
History
Date User Action Args
2019-11-07 00:36:28josh.rsetnosy: + josh.r
2019-11-07 00:29:02josh.rsetcomponents: + Library (Lib)
2019-11-06 04:04:15Manuel Ignacio Pérez Alcoleacreate