python模块查找机制探究

先上段上代码：

[root@localhost ~]# vi SocketServer.py

importSocketServer

classMyTCPHandler(SocketServer.BaseRequestHandler):

    def handle(self):
        self.data =self.request.recv(1024).strip()
        print"{} wrote:".format(self.client_address[0])
        printself.data
        self.request.sendall(self.data.upper())

if __name__ =="__main__":
    HOST, PORT ="localhost",9999
    server =SocketServer.TCPServer((HOST, PORT),MyTCPHandler)
    server.serve_forever()

这段程序语法上是正确的，但就是运行时报如下的错：

[root@localhost ~]# python SocketServer.py
Traceback(most recent call last):
  File"SocketServer.py", line 1,in
    ?importSocketServer
  File"/root/SocketServer.py", line 3,in
    classMyTCPHandler(SocketServer.BaseRequestHandler):
AttributeError:'module'object has no attribute 'BaseRequestHandler'

这个问题是一个刚学python不久的新手遇到的，拿着这个报错信息在网上搜也找不到答案，原因是什么呢？我想大家看到上面我贴出代码就知道是啥原因了，因为这个脚本的名字为SocketServer.py，而在这个脚本中import了SocketServer这个模块，python把当前目录下的SocketServer.py当成了SocketServer模块，解决方法很简单，把脚本的文件名改一下就可以了，这个问题解决，接下来看一下python加载模块的机制，用strace来跟一下这个程序

stat("/root",{st_mode=S_IFDIR|0550, st_size=4096,...})=0
stat("/root",{st_mode=S_IFDIR|0550, st_size=4096,...})=0
stat("/root/SocketServer",0x7fffc5280bf0)=-1 ENOENT (No such file or directory)
open("/root/SocketServer.so", O_RDONLY)=-1 ENOENT (No such file or directory)
open("/root/SocketServermodule.so", O_RDONLY)=-1 ENOENT (No such file or directory)
open("/root/SocketServer.py", O_RDONLY)=3
fstat(3,{st_mode=S_IFREG|0644, st_size=449,...})=0
open("/root/SocketServer.pyc", O_RDONLY)=4
fstat(4,{st_mode=S_IFREG|0644, st_size=856,...})=0
mmap(NULL,4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS,-1,0)=0x7f9ec67dc000
read(4,"\3\363\r\n\353\t\36Pc\0\0\0\0\0\0\0\0\3\0\0\0@\0\0\0sf\0\0\0d\0"...,4096)=856
fstat(4,{st_mode=S_IFREG|0644, st_size=856,...})=0
read(4,"",4096)                       =0
close(4)                                =0
munmap(0x7f9ec67dc000,4096)            =0
close(3)                                =0

这段是加载模块的地方，我们可以看到python加载模块时，首先是在当前目录下搜索，先stat SocketServer，看SocketServer这个目录是否存在，然后再搜索SocketServer.so，接着搜SocketServermodule.so，最后搜索SocketServer.py和SocketServer.pyc，直到找到，我们发现另外一个现象就是python已经找到了.py文件了，并且打开了，但还是会继续找.pyc文件，这个地方有点奇怪。接着我们跟一下把名字改掉后的脚本：

stat("/tmp",{st_mode=S_IFDIR|S_ISVTX|0777, st_size=12288,...})=0
stat("/tmp",{st_mode=S_IFDIR|S_ISVTX|0777, st_size=12288,...})=0
stat("/tmp/SocketServer",0x7fff13219a70)=-1 ENOENT (No such file or directory)
open("/tmp/SocketServer.so", O_RDONLY)  =-1 ENOENT (No such file or directory)
open("/tmp/SocketServermodule.so", O_RDONLY)=-1 ENOENT (No such file or directory)
open("/tmp/SocketServer.py", O_RDONLY)  =-1 ENOENT (No such file or directory)
open("/tmp/SocketServer.pyc", O_RDONLY)=-1 ENOENT (No such file or directory)
stat("/usr/lib64/python2.6/SocketServer",0x7fff13219a70)=-1 ENOENT (No such file or directory)
open("/usr/lib64/python2.6/SocketServer.so", O_RDONLY)=-1 ENOENT (No such file or directory)
open("/usr/lib64/python2.6/SocketServermodule.so", O_RDONLY)=-1 ENOENT (No such file or directory)
open("/usr/lib64/python2.6/SocketServer.py", O_RDONLY)=3
fstat(3,{st_mode=S_IFREG|0644, st_size=21914,...})=0
open("/usr/lib64/python2.6/SocketServer.pyc", O_RDONLY)=4
fstat(4,{st_mode=S_IFREG|0644, st_size=21222,...})=0
mmap(NULL,4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS,-1,0)=0x2b268d545000
read(4,"m\362\r\nz\17\305Lc\0\0\0\0\0\0\0\0\v\0\0\0@\0\0\0s.\2\0\0d\0"...,4096)=4096
close(4)                                =0
munmap(0x2b268d545000,4096)            =0

这个看的更加明白了，python会在当前目录下找，然后接着到系统的路径的路径下去找，也就是sys.path

>>>import sys
>>> sys.path
['','/usr/lib64/python26.zip','/usr/lib64/python2.6','/usr/lib64/python2.6/plat-linux2','/usr/lib64/python2.6/lib-tk','/usr/lib64/python2.6/lib-old','/usr/lib64/python2.6/lib-dynload','/usr/lib64/python2.6/site-packages']

我们只要把python模块放在这个sys.path里所在路径下，python就可以找到我们的模块，如果我们python模块不在当前路径下也不在sys.path里怎么办呢？我们有两种方式：

第一种，设置PYTHONPATH：

假如我将模块放在/home/admin/下，在终端下执行如下命令：
$ export PYTHONPATH=$PYTHONPATH:/home/admin/
$ echo $PYTHONPATH
:/home/admin/
>>> sys.path
['','/tmp','/home/admin','/usr/lib64/python26.zip','/usr/lib64/python2.6','/usr/lib64/python2.6/plat-linux2','/usr/lib64/python2.6/lib-tk','/usr/lib64/python2.6/lib-old','/usr/lib64/python2.6/lib-dynload','/usr/lib64/python2.6/site-packages']

第二种，在程序运行时修改sys.path就可以

>>>import sys
>>> sys.path
['','/usr/lib64/python26.zip','/usr/lib64/python2.6','/usr/lib64/python2.6/plat-linux2','/usr/lib64/python2.6/lib-tk','/usr/lib64/python2.6/lib-old','/usr/lib64/python2.6/lib-dynload','/usr/lib64/python2.6/site-packages']
>>> sys.path.append("/home/admin")
>>> sys.path
['','/usr/lib64/python26.zip','/usr/lib64/python2.6','/usr/lib64/python2.6/plat-linux2','/usr/lib64/python2.6/lib-tk','/usr/lib64/python2.6/lib-old','/usr/lib64/python2.6/lib-dynload','/usr/lib64/python2.6/site-packages','/home/admin']

看完了python的模块查找机制后，总感觉这个查找机制不是很高效，如果模块在sys.path这个list最后一个，那么python要把list中所有路径都遍历一遍，假如sys.path的路径很多，遍历一遍还是需要花一些时间。