-- run in Windows 10

1 - 
f = open('test', "w+")
f.write('xxx\nyyy\nzzz')

f.seek(0)

f.readline()
print(f.tell())  # output 5 (xxx\r\n)

x = f.truncate()
print(x)  # output 13 (xxx\r\nyyy\r\nzzz), why it is 13, not 5?

2 - 
f = open('test', "w+")
f.write('xxx\nyyy\nzzz')

f.seek(0)

f.readline()
print(f.tell())  # output 5 (xxx\r\n)

f.seek(f.tell())

x = f.truncate()
print(x)  # output 5

# Run in Windows 10

# Code snippet 1
f = open('test', "w+")
f.write('xxx\nyyy\nzzz')

f.seek(0)

f.readline()
print(f.tell())  # 5

x = f.truncate()
print(x)  # 13 - why it is 13, not 5?


# Code snippet 2
f = open('test', "w+")
f.write('xxx\nyyy\nzzz')

f.seek(0)

f.readline()
print(f.tell())  # 5

f.seek(f.tell())

x = f.truncate()
print(x)  # 5

This is the same as Issue 26158. Truncating text files is not clearly documented for a start, and truncating after reading doesn’t seem to be considered much in the implementations.

Your question is answered at <https://bugs.python.org/issue26158#msg258619>. Your code calls the C implementation of “io.TextIOWrapper.truncate”. This implementation does not consider that there are 8 unread bytes in a buffer, so it truncates the file at the end of the buffer (5 read + 8 unread = 13 bytes).