python 2.7 and 3.3 subprocess module difference

Question

I have the following code

from __future__ import print_function
import subprocess

p = subprocess.Popen(['cat'], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
p.kill()
p.wait()
print(p.stdin.write(b'some'))

and I ran it with python 2.7 and python 3.3. With python 2.7 an exception is thrown when I try to write to stdin. With python 3.3 this code prints 4. What is the difference between python 2.7 subprocess module and python 3.3?

Answer 1

The difference is in the default value for bufsize. It is 0 (unbuffered) in Python 2.7 therefore write causes EPIPE error. Pipes are fully buffered in Python 3.2+ i.e., until you flush the buffer, no error is detected. From subprocess' docs:

Changed in version 3.3.1: bufsize now defaults to -1 to enable buffering by default to match the behavior that most code expects. In versions prior to Python 3.2.4 and 3.3.1 it incorrectly defaulted to 0 which was unbuffered and allowed short reads. This was unintentional and did not match the behavior of Python 2 as most code expected.