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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