File modes with '+' character does not give expected output

[anmolkejriwal]

BPO	38935
Nosy	@pfmoore, @tjguk, @stevendaprano, @zware, @zooba

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-11-28.07:29:19.327>
created_at = <Date 2019-11-28.07:17:15.092>
labels = ['3.8', 'invalid', 'type-bug', '3.7', 'OS-windows']
title = "File modes with '+' character does not give expected output"
updated_at = <Date 2019-11-28.07:29:19.314>
user = 'https://bugs.python.org/anmolkejriwal'

bugs.python.org fields:

activity = <Date 2019-11-28.07:29:19.314>
actor = 'steven.daprano'
assignee = 'none'
closed = True
closed_date = <Date 2019-11-28.07:29:19.327>
closer = 'steven.daprano'
components = ['Windows']
creation = <Date 2019-11-28.07:17:15.092>
creator = 'anmolkejriwal'
dependencies = []
files = []
hgrepos = []
issue_num = 38935
keywords = []
message_count = 3.0
messages = ['357609', '357610', '357611']
nosy_count = 6.0
nosy_names = ['paul.moore', 'tim.golden', 'steven.daprano', 'zach.ware', 'steve.dower', 'anmolkejriwal']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38935'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[anmolkejriwal]

The file modes a+, r+, w+ should read and write both. But a+ and w+ modes are not reading anything from the file. The RUN for the program is successful, but there is no output from the program.
Below is a simple program:

file=open("abcd.txt","w+")
l=["This is python.\nThis is an easy language.\nAnyone can learn this easily"]
file.writelines(l)
file.close()
file=open("abcd.txt","a+")   #Replacing with w+ also doesn't read.
file.write("123")      
t1=file.read()               #This read() does not work.
print(t1)                    #Does not print anything here.
file.close()

In r+ mode, it should write in the file without truncation and read it too. Instead, it is removing from the file, the equal number of characters I am trying to write in the file.
Below is the program:

file=open("abcd.txt","w+")
l=["This is python.\nThis is an easy language.\nAnyone can learn this easily"]
file.writelines(l)
file.close()
file=open("abcd.txt","r+")
file.write("123")
t1=file.read()
print(t1)

Output for this is:

s is python.
This is an easy language.
Anyone can learn this easily

[anmolkejriwal]

According to the doc,
https://docs.python.org/3/library/functions.html#open

The expected output for r+ mode in the program mentioned earlier should be:

123This is python.
This is an easy language.
Anyone can learn this easily

[stevendaprano]

This is expected behaviour. You read from the position of the file pointer, not the beginning of the file, so after writing to to the file, you have to use the seek() method to return to the beginning if you want to read what you just wrote.

https://docs.python.org/3/library/io.html

https://docs.python.org/3/tutorial/inputoutput.html#methods-of-file-objects