Adding read/readline() after write appends to EOF, not to the position

Bug report

Bug description:

with open("testfile", "r+") as my_file:

    text = my_file.write("========")
    print(my_file.readline())

Instead of writing to the beginning of the file, the program writes to the EOF even if readline() is placed after write.

CPython versions tested on:

3.8, 3.9

Operating systems tested on:

Windows

I'm a beginner contributor and I'm looking to work on this issue, what should I do?

I've been working on this issue, and it seems that the bug doesn't occur when tell() and seek() are used between the write and readline calls. I'm struggling to properly use the built in seek and tell functions within the textio.c file however, if someone has experience in this module I would appreciate the help.

For anyone who wants to take over this task, here are some test cases I wrote.
`

def compare(file1, file2):
    with open(file1, 'r') as file1:
        content1 = file1.read()
    with open(file2, 'r') as file2:
        content2 = file2.read()
    if content1 == content2:
        return True
    else:
        return False

def restore(modified, original):
    for m, o in zip(modified, original):
        with open(o, 'r') as src_file:
            content = src_file.read()
        with open(m, 'w') as dest_file:
            dest_file.write(content)
        src_file.close()
        dest_file.close()


def test1a():
    filename = 'test1.txt'
    with open(filename, "r+") as my_file:
        my_file.write("========")
        r = my_file.readline()

        my_file.close()
    if compare('test1.txt', 'test1_corr.txt'):
        print('Test 1a - PASSED')
    else:
        print('Test 1a - FAILED')


def test1b():
    filename = 'test1.txt'
    with open(filename, "r+") as my_file:
        my_file.write("========")
        my_file.close()
    if compare('test1.txt', 'test1_corr.txt'):
        print('Test 1b - PASSED')
    else:
        print('Test 1b - FAILED')

def test2a():
    filename = 'test2.txt'
    f = open(filename, 'r+')
    f.write('....')
    f.read()
    f.close()
    if compare('test2.txt', 'test2_corr.txt'):
        print('Test 2a - PASSED')
    else:
        print('Test 2a - FAILED')

def test2b():
    filename = 'test2.txt'
    f = open(filename, 'w+')
    f.write('....')
    f.readline()
    f.close()
    if compare('test2.txt', 'test2_corr.txt'):
        print('Test 2b - PASSED')
    else:
        print('Test 2b - FAILED')

def test2c():
    filename = 'test2.txt'
    f = open(filename, 'r+')
    f.write('....')
    f.readline(3)
    f.close()
    if compare('test2.txt', 'test2_corr.txt'):
        print('Test 2c - PASSED')
    else:
        print('Test 2c - FAILED')

def test3a():
    filename = 'test3.txt'
    f = open(filename, 'r+')
    f.write('^^^^^^^^')
    f.readline()
    f.close()
    if compare('test3.txt', 'test3_corr.txt'):
        print('Test 3a - PASSED')
    else:
        print('Test 3a - FAILED')

def test3b():
    filename = 'test3.txt'
    f = open(filename, 'r+')
    print("Initial:", f.tell())
    f.write('^^^^^^^^')
    # print("After write:", f.tell())
    f.readline(3)
    print("End:", f.tell())
    f.close()
    if compare('test3.txt', 'test3_corr.txt'):
        print('Test 3b - PASSED')
    else:
        print('Test 3b - FAILED')

def test3c():
    filename = 'test3.txt'
    f = open(filename, 'r+')
    f.write('^^^^^^^^')
    f.flush()
    f.readline()
    f.close()
    if compare('test3.txt', 'test3_corr.txt'):
        print('Test 3c - PASSED')
    else:
        print('Test 3c - FAILED')

`
test1.txt
test1_corr.txt
test1_orig.txt
test2.txt
test2_corr.txt
test2_orig.txt
test3.txt
test3_corr.txt
test3_orig.txt

Might be related to the existing bug reports about writing after reading: #82891 and #70346

The reproducers here pass on python 3.13, 3.14, and 3.15. They seem to have been fixed by gh-115059 which was backported to 3.11.

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Adding read/readline() after write appends to EOF, not to the position #117095

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Operating systems tested on:

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Adding read/readline() after write appends to EOF, not to the position #117095

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Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

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TooPercentMilk commented on Apr 2, 2024

TooPercentMilk commented on Apr 23, 2024

TooPercentMilk commented on Apr 25, 2024

vadmium commented on Jul 26, 2024

cmaloney commented on Oct 9, 2026

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