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python 全局变量的import机制介绍

作者：loleina

在之前学习python设计模式（工厂模式实践篇），希望使用全局变量代替c++的宏完成服务自动注册功能时，遇到过一个问题，全局变量的定义和使用放在同一个可执行脚本中的问题

先把有问题的代码晒一下：

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。

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