python 全局變量的import機制介紹

更新時間：2017年09月07日 17:01:22 作者：loleina

在之前學習python設計模式（工廠模式實踐篇），希望使用全局變量代替c++的宏完成服務自動注冊功能時，遇到過一個問題，全局變量的定義和使用放在同一個可執(zhí)行腳本中的問題

先把有問題的代碼曬一下：

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
  # 接收到報文后，根據(jù)報文的內容，從db中獲取到serverType，假設獲取到的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())

代碼內已經加了調試的部分信息，運行CreatFactory.py。調用DoWithA失敗，提示AttributeError: 'str' object has no attribute 'DoWithA'。運行結果如下：

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

從運行的結果，可以看到：GLOBAL_class_dic 被定義了2次。有兩個不同的id，第一次定義分配了一塊內存，第二次不明原因的又重新分配了一塊內存，然后服務的自動注冊全部注冊在這塊內存中，等到main函數(shù)使用的使用，又使用的是第一次申請的內存，所以導致程序運行失敗。那問題就來了，為什么會被重新又分配了一次？

之所以會被重新定義一次全局變量，是因為在執(zhí)行CreatFactory.py時，最開始定義了全局變量，此時該命名空間可使用的函數(shù)和變量打印：['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打印出可使用的函數(shù)和變量時，['IServer', '__builtins__', '__doc__', '__file__', '__name__', '__package__', 'serverType']，就沒有GLOBAL_class_dic，程序發(fā)現(xiàn)沒有，就又重新聲明了一遍。似乎問題原因已經找到了。

python在導入的時候，有2種場景，一種就是在文件前普通的import語句，還有一種就是特殊的場景：__main__模塊是相對于Python的導入系統(tǒng)。在最開始運行CreatFactory.py文件時，__name__打印的值是__main__，而再子類再次導入時，會在當前命名空間查找是否已經導入__name__=CreatFactory，發(fā)現(xiàn)這個模塊不存在，故此又導入了一遍，全局變量由此又被重新定義分配了內存，后期全局變量在子類業(yè)務的使用就都使用該值，而在main函數(shù)里，使用的又是當前的作用域內的第一次定義的全局變量。

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