TinyP2P使用15行Python代码实现了一个P2P的工具，下面是经过别人注释的代码：
# tinyp2p.py 1.0 (documentation at http://freedom-to-tinker.com/tinyp2p.html)
# (C) 2004, E.W. Felten
# license: http://creativecommons.org/licenses/by-nc-sa/2.0
# Annotated by Kochin Chang, Jan. 2005
# Usage:
# Server - python tinyp2p.py password server hostname portnum [otherurl]
# Client - python tinyp2p.py password client serverurl pattern
# ar[0] ar[1] ar[2] ar[3] ar[4] ar[5]
import sys, os, SimpleXMLRPCServer, xmlrpclib, re, hmac
# Import libraries used in the program.
# sys : system variables and functions.
# os : portable OS dependent functionalities.
# SimpleXMLRPCServer : basic XML-RPC server framework.
# xmlrpclib : XML-RPC client support.
# re : regular expression support.
# hmac : RFC 2104 Keyed-Hashing Message Authentication.
ar,pw,res = (sys.argv,lambda u:hmac.new(sys.argv[1],u).hexdigest(),re.search)
# A multiple assignment.
# ar < - sys.argv : the argument list.
# pw <- lambda u:hmac.new(sys.argv[1],u).hexdigest() :
# a function makes an HMAC digest from a URL.
# INPUT: a string, u, which is a URL here.
# OUTPUT: a hexdecimal HMAC digest.
# DESCRIPTION:
# 1\. Creates a HMAC object from the URL using network's password,
# sys.argv[1], as the key.
# 2\. Returns a hexdecimal digest of the HMAC object.
# res <- re.search : alias for the regular expression search function.
pxy,xs = (xmlrpclib.ServerProxy,SimpleXMLRPCServer.SimpleXMLRPCServer)
# A multiple assignment.
# pxy <- xmlrpclib.ServerProxy : alias for the ServerProxy class.
# xs <- SimpleXMLRPCServer.SimpleXMLRPCServer : alias for the SimpleXMLRPCServer class.
def ls(p=""):return filter(lambda n:(p=="")or res(p,n),os.listdir(os.getcwd()))
# a function lists directory entries.
# INPUT: a string, p, which is a regular expression pattern.
# OUTPUT: a list of directory entries matched the pattern.
# DESCRIPTION:
# 1\. Creates a function using lambda expression that takes a pathname as its
# parameter. The function returns true if the pattern is empty or the
# pathname matches the pattern.
# 2\. Finds out what is the current working directory.
# 3\. Retrieves a list of directory entries of current working directory.
# 4\. Filters the list using the lambda function defined.
if ar[2]!="client":
# Running in server mode...
myU,prs,srv = ("http://"+ar[3]+":"+ar[4], ar[5:],lambda x:x.serve_forever())
# A multiple assignment.
# myU <- "http://"+ar[3]+":"+ar[4] : server's own URL.
# prs <- ar[5:] : URL's of other servers in the network.
# srv <- lambda x:x.serve_forever() :
# a function to start a SimpleXMLRPCServer.
# INPUT: a SimpleXMLRPCServer object, x.
# OUTPUT: (none)
# DESCRIPTION:
# Calls the server's serve_forever() method to start handling request.
def pr(x=[]): return ([(y in prs) or prs.append(y) for y in x] or 1) and prs
# a function returns the server list.
# INPUT: a list, x, of servers' URLs to be added to the server list.
# OUTPUT: the updated server list.
# DESCRIPTION:
# 1\. For each URL in x, checks whether it's already in the server list.
# If it's not in the list, appends in onto the list.
# 2\. Returns the updated server list.
def c(n): return ((lambda f: (f.read(), f.close()))(file(n)))[0]
# a function returns content of the specified file.
# INPUT: a string, n, which is a filename.
# OUTPUT: the content of the file in a string.
# DESCRIPTION:
# 1\. Creates a function using lambda expression that takes a file object, f,
# as its parameter. The function reads the content of the file, then
# closes it. The results of the read and close are put into a tuple, and
# the tuple is returned.
# 2\. Creates a file object with the filename. Passes it to the lambda
# function.
# 3\. Retrieves and returns the first item returned from the lambda function.
f=lambda p,n,a:(p==pw(myU))and(((n==0)and pr(a))or((n==1)and [ls(a)])or c(a))
# a request handling function, depending on the mode, returns server list,
# directory entries, or content of a file.
# INPUT: a string, p, which is a hexdecimal HMAC digest.
# a mode number, n.
# if n is 0, a is a list of servers to be added to server list.
# if n is 1, a is a pattern string.
# if n is anything else, a is a filename.
# OUTPUT: if n is 0, returns the server list.
# if n is 1, returns directory entries match the pattern.
# if n is anything else, returns content of the file.
# DESCRIPTION:
# 1\. Verifies the password by comparing the HMAC digest received and the
# one created itself. Continues only when they match.
# 2\. If n is 0, calls pr() to add list, a, and returns the result.
# If n is 1, calls ls() to list entries match pattern a, and returns
# the result enclosed in a list.
# If n is any other value, retreives and return content of the file
# with filename specified in a.
def aug(u): return ((u==myU) and pr()) or pr(pxy(u).f(pw(u),0,pr([myU])))
# a function augments the network.
# INPUT: a string, u, which is a URL.
# OUTPUT: a list of URL's of servers in the network.
# DESCRIPTION:
# 1\. If the URL, u, equals to my own URL, just returns the server list.
# 2\. Otherwise, creates a ServerProxy object for server u. Then calls its
# request handling function f with a HMAC digest, mode 0, and server
# list with myself added.
# 3\. Calls pr() with the result returned from server u to add them to my
# own list.
# 4\. Returns the new list.
pr() and [aug(s) for s in aug(pr()[0])]
# 1\. Checks the server list is not empty.
# 2\. Takes the first server on the list. Asks that server to augment its
# server list with my URL.
# 3\. For each server on the returned list, asks it to add this server to its
# list.
(lambda sv:sv.register_function(f,"f") or srv(sv))(xs((ar[3],int(ar[4]))))
# Starts request processing.
# 1\. Defines a function with lambda expression that takes a SimpleXMLRPCServer
# object, registers request handling function, f, and starts the server.
# 2\. Creates a SimpleXMLRPCServer object using hostname (ar[3]) and portnum
# (ar[4]). Then feeds the object to the lambda function.
# Running in client mode...
for url in pxy(ar[3]).f(pw(ar[3]),0,[]):
# 1\. Create a ServerProxy object using the serverurl (ar[3]).
# 2\. Calls the remote server and retrieves a server list.
# 3\. For each URL on the list, do the following:
for fn in filter(lambda n:not n in ls(), (pxy(url).f(pw(url),1,ar[4]))[0]):
# 1\. Create a ServerProxy object using the URL.
# 2\. Calls the remote server to return a list of filenames matching the
# pattern (ar[4]).
# 3\. For each filename doesn't exist locally, do the following:
(lambda fi:fi.write(pxy(url).f(pw(url),2,fn)) or fi.close())(file(fn,"wc"))
# 1\. Define a lambda function that takes a file object, calls remote server
# for the file content, then closes the file.
# 2\. Create a file object in write and binary mode with the filename. (I
# think the mode "wc" should be "wb".)
# 3\. Passes the file object to the lambda function.
如果直接运行上面的代码会有问题（不知道作者那里为什么会正常运行），会有类似这样的错误提示：
Traceback (most recent call last):
File "tinyp2p.py", line 14, in ?
for url in pxy(ar[3]).f(pw(ar[3]),0,[]):
File "/usr/lib/python2.4/xmlrpclib.py", line 1096, in __call__
return self.__send(self.__name, args)
File "/usr/lib/python2.4/xmlrpclib.py", line 1383, in __request
verbose=self.__verbose
File "/usr/lib/python2.4/xmlrpclib.py", line 1147, in request
return self._parse_response(h.getfile(), sock)
File "/usr/lib/python2.4/xmlrpclib.py", line 1286, in _parse_response
return u.close()
File "/usr/lib/python2.4/xmlrpclib.py", line 744, in close
raise Fault(**self._stack[0])
xmlrpclib.Fault: <Fault 1: 'exceptions.TypeError:coercing to Unicode:
need string or buffer, list found'>
经google的帮助在这里找到的解决方案：
pr() and [aug(s) for s in aug(pr()[0])]
to
pr() and [aug(s) for s in aug(pr()[0])] or pr([myU])
代码中大量应用了lambda，还有and,or，虽然这种代码风格不值得我们学习，我们可以写出更清昕的代码，但作者是为了证明P2P应用程序是如何容易实现，作者更希望可以把代码写的更简洁，可以用在他的签名里。
还有perl写的6行代码，我看了一下grep,cat这些都出来了，是shell版吧。
Comments
luguo: 那6行Perl在哪里？？ Perl自己有内置的grep，也很nb。虽然没cat，但可以定义个子函数叫cat。
luguo: 还有，你最后那段Python不对啊！人家是说把上面那行换成下面那行，那个“to”明显不是Python源代码啊！
cocobear: http://ansuz.sooke.bc.ca/software/molester/molester-min 这里有一个，貌似我上次看到的不是这个。 最后那一段只是个说明，并不是python源代码。
luguo: lambda，map，reduce啊，都是很nb的！ 弱弱地问一下：p2p和xmlrpc有啥关系？？
cocobear: XMLRPC可以实现分布式处理，这样P2P可以使用多个Server，俺这样觉得 。
