Why truncate when we open a file in 'w' mode in python

Question

I am going through Zed Shaw\'s Python Book. I am currently working on the opening and reading files chapters. I am wondering why we need to do a truncate, when we are alread

Answer 1

Scenario:
I was making a ransomware and needed to encrypt the file, My aim is not to encrypt the complete file but that much only to corrupt it because I want it to be fast in what it does and so saving time in encrypting it all, so I decided to edit some text only.
Now If I use write then my purpose is destroyed here because I would have to write the file a to z. Then what can I do?
well here truncate can be put in use.

Below is my code which just takes a token of last 16 digits in a file:

with open('saver.txt', 'rb+') as f:
    text_len = len(f.read())
    f.truncate(text_len-16)
    f.close()

I open the file
Truncate only 16 characters from file which will be replaced by me later.
Notice I am using it in read only mode, If I use in write mode than File is truncated completely and it will throw error when our truncate command comes in.
Answering this question after 8.4 years. :)