Erlang: Port To Python Instance Not Responding

I am trying to communicate to an external python process through an Erlang port. First, a port is opened, then a message is sent to the external process via stdin. I am expecting a

Solution 1:

As you've discovered, ports don't do what you'd like for this problem, which is why alternatives like ErlPort exist. An old workaround for this problem is to use netcat to pipe commands into python so that a proper EOF occurs. Here's an example session:

1>PortOpts= [exit_status, stderr_to_stdout, {line,1000000}].
[exit_status,stderr_to_stdout,{line,1000000},use_stdio]
2>Port= open_port({spawn, "nc -l 51234 | python"}, PortOpts).
#Port<0.564>
3> {ok, S} = gen_tcp:connect("localhost", 51234, []).
{ok,#Port<0.565>}
4> gen_tcp:send(S, "print 'hello'\nprint 'hello again'\n").
ok
5> gen_tcp:send(S, "print 'hello, one more time'\n").
ok
6> gen_tcp:close(S).
ok
7> flush().
Shell got {#Port<0.564>,{data,{eol,"hello"}}}
Shell got {#Port<0.564>,{data,{eol,"hello again"}}}
Shell got {#Port<0.564>,{data,{eol,"hello, one more time"}}}
Shell got {#Port<0.564>,{exit_status,0}}
ok

This approach opens a port running netcat as a listener on port 51234 — you can choose whatever port you wish to, of course, as long as its not already in use — with its output piped into python. We then connect to netcat over the local TCP loopback and send python command strings into it, which it then forwards through its pipe to python. Closing the socket causes netcat to exit, which results in an EOF on python's stdin, which in turn causes it to execute the commands we sent it. Flushing the Erlang shell message queue shows we got the results we expected from python via the Erlang port.