formatting strings for stdin.write() in python 3.x

Question

I'm having a problem where I get errors when I try to execute this code with python 3.2.2

working_file = subprocess.Popen(["/pyRoot/iAmAProgram"], stdout=subprocess.PIPE, stdin=subprocess.PIPE, stderr=subprocess.PIPE)

working_file.stdin.write('message')

I understand that python 3 changed the way it handles strings but I dont understand how to format the 'message'. Does anyone know how I'd change this code to be valid?

many thanks

jon

update: heres the error message i get

Traceback (most recent call last):
  File "/pyRoot/goRender.py", line 18, in <module>
    working_file.stdin.write('3')
TypeError: 'str' does not support the buffer interface

Answer 1

If you have a string variable that you want to write to a pipe (and not a bytes object), you have two choices:

1. Encode the string first before you write it to the pipe:

working_file.stdin.write('message'.encode('utf-8'))

2. Wrap the pipe into a buffered text interface that will do the encoding:

stdin_wrapper = io.TextIOWrapper(working_file.stdin, 'utf-8')
stdin_wrapper.write('message')

(Notice that the I/O is now buffered, so you may need to call stdin_wrapper.flush().)

Answer 2

Is your error message "TypeError: 'str' does not support the buffer interface"? That error message tells you pretty much exactly what is wrong. You don't write string objects to that sdtin. So what do you write? Well, anything supporting the buffer interface. Typically this is bytes objects.

Like:

working_file.stdin.write(b'message')