An Erlang Port in Python

pascal4123

Recipe 534162: An Erlang Port in Python
   
Erlang has two built-in interoperability mechanisms. One is
distributed Erlang nodes and the other one is ports.
Ports provide the basic Erlang mechanism for communication with the
external world. They provide a byte-oriented interface to an external
program. When a port has been created, Erlang can communicate with it
by sending and receiving lists of bytes. This recipe cooks an Erlang
port in python. Making it easy for Erlang to instantiate and use python
objects. Like most simple port implementations it uses an external
python program and lets Erlang communicate via standard input and write
to standard output. Theoretically, the external program could be
written in any programming language. This recipe is pretty abstract and
you will have to implement your own encode and decoding scheme.
  
   
      
        
        Python
      
      
        1
  2
  3
  4
  5
  6
  7
  8
  9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114import sys, os, struct, traceback
from cStringIO import StringIO
   
class ErlangPort(object):
    PACK = '!h'
    def __init__(self):
        self._in = sys.stdin
        self._out = sys.stdout
        
    def recv(self):
        buf = self._in.read(2)
        if len(buf) ==2:
            (sz,) = struct.unpack(self.PACK, buf)
            return self._in.read(sz)
        
    def send(self, what):
        sz = len(what)
        buf = struct.pack(self.PACK, sz)
        self._out.write(buf)
        return self._out.write(what)
    def run(self):
        buf = self.recv()
        while buf:
            try:
                result = self.process(buf)
            except:
                result = traceback.format_exc()
            self.send(result)
            buf = self.recv()
class ErlangPortTest(ErlangPort):
    cmds = (0,lambda x: x+2, lambda x: x*2)
    def process(self, message):
        fn,arg = struct.unpack('!BB', message)
        res = self.cmds[fn](arg)
        return struct.pack('!B', res)
class ErlangPyTest(ErlangPortTest):
    class SandBox:
        def process(self, message):
            exec message
    sandbox = SandBox()
    def process(self, code):
        try:
            realout = sys.stdout
            sys.stdout = StringIO()
            self.sandbox.process(code)
            result = sys.stdout.getvalue()
        finally:
            if sys.stdout: sys.stdout.close()
            if realout: sys.stdout = realout
        return result
            
if __name__ =='__main__':
    import sys
    try:
        command = sys.argv[1]
        if command == 'PortTest':
            ErlangPortTest().run()
        elif command =='pytest':
            ErlangPyTest().run()
    except IndexError:
        print """
Usage:
First of all see the c Port section in the Erlang guide.
http://www.erlang.org/doc/tutorial/c_port.html#4
1. Start Erlang and compile the Erlang user guide example code:
http://www.erlang.org/doc/tutorial/complex1.erl
unix> erl
Erlang (BEAM) emulator version 4.9.1.2
Eshell V4.9.1.2 (abort with ^G)
1> c(complex1).
{ok,complex1}
   
3. Run the example.
2> complex1:start("python -u port.py PortTest").
3> complex1:foo(3).
4
4> complex1:bar(5).
10
5> complex1:stop().
stop
   
For more fun try.
6> c("c:\\tg\\python.erl").
:/tg/python.erl:42: Warning: variable 'Reason' is unused
{ok,python}
7> python:start("c:\\python25\\python.exe -u c:\\tg\\port.py pytest").
8> python:exec("import os")
9> python:exec("self.x = os.environ['PATH']").
* 2: syntax error before: python **
10> python:exec("import os").                  
[]
11> python:exec("self.x = os.environ['PATH']").
[]
12> python:exec("print self.x").
"H:\\PROGRA~1\\ERL55~1.5\\ERTS-5~1.5\\bin;H:\\PROGRA~1\\ERL55~1.5\\bin
m Files\\ActivePositionManager\\;C:\\WINNT\\system32;C:\\WINNT;C:\\WI
32\\Wbem;"
"""
      
   
  
  
   
      
      Discussion
      
      
      
First of all see the c Port section in the Erlang guide.
http://www.erlang.org/doc/tutorial/c_port.html#4
.
Erlang
is a great language for distributed programming especially if you use
the OPT frame work, but python is nicer to program in. This class is my
first of many Erlang/Python utility classes that will let me program in
Python distributed nodes and yet manage them with Erlang code.
ErlangPort is the main base class in this recipe. For most cases all
you will have to do is override process to return a string or a struct
representation in network safe mode. Also remember to run python with
the -u (unbuffered) flag set.
ErlangPortTest is just a pythonesque port of the c example given in the Erlang guide. You can find the  Erlang example at
http://www.erlang.org/doc/tutorial/complex1.erl
ErlangPyTest lets Erlang execute python code block by block. In this
class I use a letter envelop idiom (Sandbox) to save Erlang form
clobbering the port object. Be careful how you use it!
For This use python.erl
-module(python).
-export([start/1, stop/0, init/1]).
-export([exec/1]).
start(ExtPrg) ->
    spawn(?MODULE, init, [ExtPrg]).
stop() ->
    python ! stop.
exec(X) ->
    call_port(X).
call_port(Msg) ->
    python ! {call, self(), Msg},
    receive
    {python, Result} ->
        Result
    end.
init(ExtPrg) ->
    register(python, self()),
    process_flag(trap_exit, true),
    Port = open_port({spawn, ExtPrg}, [{packet, 2}]),
    loop(Port).
loop(Port) ->
    receive
    {call, Caller, Msg} ->
        Port ! {self(), {command, Msg}},
        receive
        {Port, {data, Data}} ->
            Caller ! {python, Data}
        end,
        loop(Port);
    stop ->
        Port ! {self(), close},
        receive
        {Port, closed} ->
            exit(normal)
        end;
    {'EXIT', Port, Reason} ->
        exit(port_terminated)
    end.
      
   
  
  
Sign in
to comment
               
               
               

本文来自ChinaUnix博客，如果查看原文请点：http://blog.chinaunix.net/u/16651/showart_2004039.html