Why does truncating a BytesIO mess it up?

Question

Running this on Python 3.5.1 on OSX:

import io

b = io.BytesIO()

b.write(b'222')
print(b.getvalue())

b.truncate(0)
b.write(b'222')
print(b.getvalue())

Produces:

b'222'
b'\x00\x00\x00222'

So truncating the BytesIO somehow causes it to start inserting extra zero bytes in the beginning? Why?

Answer 1

truncate does not move the file pointer. So the next byte is written to the next position. You have also to seek to the beginning:

b.seek(0)
b.truncate()