I created a new file for researching truncate() method. and I writed  this text:
alma
armud
heyva
nar
qarpiz
yemis

I wanted to delete all the text apart from first line by truncate method.

I writed these codes on python idle:
__________________________________________
#First, I read text for showing it's content
>>> dosya = open('deneme.txt','r')
>>> dosya.read()
'alma\narmud\nheyva\nnar\nqarpiz\nyemis'
>>> dosya.close()


#Then writing codes for operation which I wanted
>>> dosya = open('deneme.txt','r+')
>>> dosya.readline()
'alma\n'
>>> dosya.tell()
6
>>> dosya.truncate()
38
>>> dosya.seek(0)
0
>>> dosya.read()
'alma\narmud\nheyva\nnar\nqarpiz\nyemis'
>>> dosya.close()
#I thought I must closed the file, then read again:


#Then I saw nothing has changed
>>> dosya = open('deneme.txt','r')
>>> dosya.read()
'alma\narmud\nheyva\nnar\nqarpiz\nyemis'
>>> dosya.close()

#But when I writed codes in this way, it is worked
>>> dosya = open('deneme.txt','r+')
>>> dosya.readline()
'alma\n'
>>> dosya.tell()
6
>>> dosya.seek(6)
6
>>> dosya.truncate()
6
>>> dosya.close()
>>> dosya = open('deneme.txt','r')
>>> dosya.read()
'alma\n'

# I was on 6th byte in both. But it is worked which I used seek() #method. I think both of them were returnden same things

This looks like a similar report to issue26158 where truncate refers to the position of the underlying buffer and could be documented better.