python 全局变量的import机制介绍_Python

先把有问题的代码晒一下：

python 全局变量的import机制介绍

IServer.py

									from abc import ABCMeta, abstractmethod

									print __name__

									class IServer:

									  def __init__(self):

									    pass

									  @abstractmethod

									  def DoWithA(self):

									    pass

									  @abstractmethod

									  def DoWithB(self):

									    pass

IServer_A.py

									import IServer

									serverType ='1001'

									print __name__

									dir()

									from CreatFactory import GLOBAL_class_dic

									dir()

									class IServer_A(IServer.IServer):

									  def __init__(self):

									    pass

									  def DoWithA(self):

									    print 'Server_A do with interface A'

									  def DoWithB(self):

									    print 'Server_A do with interface B'

									global GLOBAL_class_dic

									print 'the id of GLOBAL_class_dic in A is:',id(GLOBAL_class_dic)

									GLOBAL_class_dic[serverType] = IServer_A

									print 'GLOBAL_class_dic in a is:', GLOBAL_class_dic

IServer_B.py

									import IServer

									serverType ='1002'from CreatFactory import GLOBAL_class_dic

									print __name__

									class IServer_B(IServer.IServer):

									  def __init__(self):

									    pass

									  def DoWithA(self):

									    print 'Server_B do with interface A'

									  def DoWithB(self):

									    print 'Server_B do with interface B'

									print 'the id of GLOBAL_class_dic in B is:',id(GLOBAL_class_dic)

									GLOBAL_class_dic[serverType] = IServer_B

									print 'GLOBAL_class_dic in b is:', GLOBAL_class_dic

CreatFactory.py

									#coding:UTF-8

									import os;

									import sys;

									import threading

									from misc import *

									global GLOBAL_class_dic

									GLOBAL_class_dic ={1:1}

									print 'GLOBAL_class_dic in define is:', GLOBAL_class_dic

									print 'the id of GLOBAL_class_dic in define is:', id(GLOBAL_class_dic)

									dir()

									import IServer_A

									import IServer_B

									def CreateServer(serverType):

									  global GLOBAL_class_dic

									  print 'GLOBAL_class_dic in use is:', GLOBAL_class_dic

									  print 'the id of GLOBAL_class_dic in USE is:', id(GLOBAL_class_dic)

									  if GLOBAL_class_dic.has_key(serverType):

									    return GLOBAL_class_dic[serverType]

									  else:

									    return 'no'

									if __name__ == '__main__':

									  pass

									  # 接收到报文后，根据报文的内容，从db中获取到serverType，假设获取到的serverType=1001

									  print 'main'

									  print 'GLOBAL_class_dic in main A is:', GLOBAL_class_dic

									  serverType = '1002'

									  server = CreateServer(serverType)

									  print 'GLOBAL_class_dic in main B is:', GLOBAL_class_dic

									  print 'server :',server

									  server.DoWithA(server())

代码内已经加了调试的部分信息，运行CreatFactory.py。调用DoWithA失败，提示AttributeError: 'str' object has no attribute 'DoWithA'。运行结果如下：

									D:\Python27\python.exe "D:/DesignMode/Server --00/CreatFactory.py"

									GLOBAL_class_dic in define is: {1: 1}

									the id of GLOBAL_class_dic in define is: 36230176

									['GLOBAL_class_dic', 'Misc', '__builtins__', '__doc__', '__file__', '__name__', '__package__', 'binascii', 'inspect', 'minidom', 'os', 'struct', 'sys', 'threading']

									IServer

									IServer_A

									['IServer', '__builtins__', '__doc__', '__file__', '__name__', '__package__', 'serverType']

									GLOBAL_class_dic in define is: {1: 1}

									the id of GLOBAL_class_dic in define is: 36230032

									['GLOBAL_class_dic', 'Misc', '__builtins__', '__doc__', '__file__', '__name__', '__package__', 'binascii', 'inspect', 'minidom', 'os', 'struct', 'sys', 'threading']

									['IServer', '__builtins__', '__doc__', '__file__', '__name__', '__package__', 'serverType']

									['GLOBAL_class_dic', 'IServer', '__builtins__', '__doc__', '__file__', '__name__', '__package__', 'serverType']

									IServer_B

									the id of GLOBAL_class_dic in B is: 36230032

									GLOBAL_class_dic in b is: {1: 1, '1002': <class IServer_B.IServer_B at 0x022C2ED8>}

									['GLOBAL_class_dic', 'IServer', '__builtins__', '__doc__', '__file__', '__name__', '__package__', 'serverType']

									the id of GLOBAL_class_dic in A is: 36230032

									GLOBAL_class_dic in a is: {1: 1, '1002': <class IServer_B.IServer_B at 0x022C2ED8>, '1001': <class IServer_A.IServer_A at 0x02273420>}

									main

									GLOBAL_class_dic in main A is: {1: 1}

									GLOBAL_class_dic in use is: {1: 1}

									the id of GLOBAL_class_dic in USE is: 36230176

									GLOBAL_class_dic in main B is: {1: 1}

									server : no

									Traceback (most recent call last):

									 File "D:/DesignMode/Server --00/CreatFactory.py", line 38, in <module>

									  server.DoWithA(server())

									AttributeError: 'str' object has no attribute 'DoWithA'

									Process finished with exit code 1

从运行的结果，可以看到：GLOBAL_class_dic 被定义了2次。有两个不同的id，第一次定义分配了一块内存，第二次不明原因的又重新分配了一块内存，然后服务的自动注册全部注册在这块内存中，等到main函数使用的使用，又使用的是第一次申请的内存，所以导致程序运行失败。那问题就来了，为什么会被重新又分配了一次？

之所以会被重新定义一次全局变量，是因为在执行CreatFactory.py时，最开始定义了全局变量，此时该命名空间可使用的函数和变量打印：['GLOBAL_class_dic', 'Misc', '__builtins__', '__doc__', '__file__', '__name__', '__package__', 'binascii', 'inspect', 'minidom', 'os', 'struct', 'sys', 'threading'，然后在import IServer_A，在IServer_A.py中，import IServer后，在from CreatFactory import GLOBAL_class_dic打印出可使用的函数和变量时，['IServer', '__builtins__', '__doc__', '__file__', '__name__', '__package__', 'serverType']，就没有GLOBAL_class_dic，程序发现没有，就又重新声明了一遍。似乎问题原因已经找到了。

python在导入的时候，有2种场景，一种就是在文件前普通的import语句，还有一种就是特殊的场景：__main__模块是相对于Python的导入系统。在最开始运行CreatFactory.py文件时，__name__打印的值是__main__，而再子类再次导入时，会在当前命名空间查找是否已经导入__name__=CreatFactory，发现这个模块不存在，故此又导入了一遍，全局变量由此又被重新定义分配了内存，后期全局变量在子类业务的使用就都使用该值，而在main函数里，使用的又是当前的作用域内的第一次定义的全局变量。

原文链接：http://www.cnblogs.com/loleina/p/7469218.html