offset can not be negative in File.seek()?

[jollydragon]

It's intersting I can not put the "offset" to non-zero if I set the second parameter as "1" or "2" in file.seek(), and please refer to the below screenshot for more details.
I don't believe it's a bug of Python Idle environment, but how to explain and fix it?

   

[Gribouillis]

What is csv_file ?

[DeaD_EyE]

With the right whence-flag, you can use negative numbers.
Look here: https://docs.python.org/3/library/io.htm...OBase.seek

In [1]: from pathlib import Path                                                              

In [2]: raspbian = Path('Downloads/2019-07-10-raspbian-buster-lite.zip')                      

In [3]: with raspbian.open('rb') as fd: 
   ...:     print('Size:', raspbian.stat().st_size) 
   ...:     print('Pos:', fd.tell()) 
   ...:     print('Seek to -10 bytes from end') 
   ...:     fd.seek(-10, 2) 
   ...:     print('New position:', fd.tell()) 
   ...:                                                                                       
Size: 426250971
Pos: 0
Seek to -10 bytes from end
New position: 426250961

Instead of recognizing the number for 'whence', you can use os.SEEK_END:

In [10]: print(os.SEEK_SET, os.SEEK_CUR, os.SEEK_END)                                         
0 1 2

---

In addition, I've used pathlib.Path, which is better to handle.
You can still use the built-in function open().
The resulting file-object is the same as from Path.open(mode).

[jollydragon]

What is csv_file ?

csv_file = open("1.csv", "r")


I don't want to use any additional module, saying "os", because it's a simple work to seek backward with "file.seek". Just confused why it doesn't work as descripted.

[Gribouillis]

The negative seek works if the file is opened in binary mode 'rb'.

[jollydragon]

The negative seek works if the file is opened in binary mode 'rb'.

Thank you so much, it's really the root cause!
May I know why it only works in "rb" mode? Why not in "r" mode?

[jollydragon]

Found the explaination on internet, and thank you again.