how to get output from a pipe connection before cl

In R, we can open a pipe connection using pipe() and write to it. I observed the following situation that I do not quite understand. Let's use a python pipe for example:

z = pipe('python', open='w+')

cat('x=1\n', file=z)
cat('print(x)\n', file=z)
cat('print(x+2)\n', file=z)
cat('print(x+2\n', file=z)
cat(')\n', file=z)

close(z)

What I was expecting was the output from print() would be immediately shown in the R console, but the fact is the output comes only after I close the pipe connection:

> z = pipe('python', open='w+')
> 
> cat('x=1\n', file=z)
> cat('print(x)\n', file=z)
> cat('print(x+2)\n', file=z)
> cat('print(x+2\n', file=z)
> cat(')\n', file=z)
> 
> close(z)
1
3
3

So my question is, how can I get the output before I close the connection? Note that it does not seem to be possible to capture the output using capture.output(), either:

> z = pipe('python', open='w+')
> 
> cat('x=1\n', file=z)
> cat('print(x)\n', file=z)
> cat('print(x+2)\n', file=z)
> cat('print(x+2\n', file=z)
> cat(')\n', file=z)
> 
> x = capture.output(close(z))
1
3
3
> x
character(0)

The background of this question is the knitr engines. For the interpreted languages like Python, I wish I can open a persistent "terminal" so that I can keep on writing code into it and get output from it. I'm not sure if pipe() is the correct way to go, though.

Python notices that the input is not interactive and waits until the connection is closed to parse and execute the code. You can use the -i option to force it to stay in interactive mode. (but the output is a bit mangled).

z = pipe('python -i', open='w')
cat('x=1\n', file=z)
cat('print(x)\n', file=z)
cat('print(x+2)\n', file=z)
cat('print(x+2\n', file=z)
cat(')\n', file=z)
Sys.sleep(2)
# Python 2.7.4 (default, Apr 19 2013, 18:28:01) 
# [GCC 4.7.3] on linux2
# Type "help", "copyright", "credits" or "license" for more information.
# >>> >>> 1
# >>> 3
# >>> ... 3
# >>> 
close(z)

Your actual problem is more complicated: you need to both read and write to the same connection. I do not know how to do that in a portable way, but you can use a pipe and a named pipe (a "fifo") on platforms that support them.

stopifnot( capabilities("fifo") )
system('mkfifo /tmp/Rpython.fifo')
output <- fifo('/tmp/Rpython.fifo', 'r')
input  <- pipe('python -i > /tmp/Rpython.fifo', 'w')
python_code <- "
x=1
print(x)
print(x+2)
print(x+2
)
"
cat( python_code, file = input )
flush( input )
Sys.sleep(2) # Wait for the results
result <- readLines(output)
result
# [1] "1" "3" "3"