popen communicate in Python

Question

1st.py creates a REPL (read-eval-print-loop):

print "Something to print"
while True:
    r = raw_input()
    if r == 'n':
        print "exiting"
        break
    else:
        print "continuing"

I then launched 1st.py with the following code:

p = subprocess.Popen(["python","1st.py"], stdin=PIPE, stdout=PIPE)

And then tried this:

print p.communicate()[0]

It failed, providing this traceback:

Traceback (most recent call last):
  File "1st.py", line 3, in <module>
    r = raw_input()
EOFError: EOF when reading a line

Can you explain what is happening here please? When I use p.stdout.read(), it hangs forever.

Answer 1

Your second bit of code starts the first bit of code as a subprocess with piped input and output. It then closes its input and tries to read its output.

The first bit of code tries to read from standard input, but the process that started it closed its standard input, so it immediately reaches an end-of-file, which Python turns into an exception.