mot-clé global utilisé dans un contexte global python

Question

Que signifie un clavier global utilisé dans un contexte global en python? Je sais qu'il vous permet de modifier les variables globales si elles sont utilisées à l'intérieur d'une fonction, mais je ne sais pas comment cela fonctionne lorsque vous déclarez une variable globale en utilisant global car je pense qu'elles sont déjà globales lorsque vous déclarez en dehors des fonctions. Comme dans ce code ci-dessous:

from ctypes import * 

import ctypes 

import os 
import cv2 
import time 
import Image 
import argparse 
import numpy as np 
import thread as thread 

from select import select 
from evdev import InputDevice 

import ArducamSDK 

COLOR_BYTE2RGB = 49 
CAMERA_MT9F002 = 0x4D091031 
SensorShipAddr = 32 
I2C_MODE_16_16 = 3 
usbVid = 0x52cb 
Width = 4384 
Height = 3288 
cfg ={"u32CameraType":CAMERA_MT9F002, 
     "u32Width":Width,"u32Height":Height, 
     "u32UsbVersion":1, 
     "u8PixelBytes":1, 
     "u16Vid":0x52cb, 
     "u8PixelBits":8, 
     "u32SensorShipAddr":SensorShipAddr, 
     "emI2cMode":I2C_MODE_16_16 } 
# *********** Here is where I'm talking about**************** 
global saveNum,saveFlag,downFlag,flag,handle,openFlag 
openFlag = False 
saveNum = 0 
handle = {} 
downFlag = False 
flag = True 
saveFlag = False 


regArr= [[0x301A, 0x0018], 

    [0x0103, 0x1 ], 

    [0x31AE, 0x301 ], 
    [0x0112, 0x0C0C], 
    [0x301A, 0x0010], 
    [0x3064, 0x0805], 
    [0x301E, 0x00A8], 
    [0x301A, 0x10DC], 
    [0x0104, 0x1 ], 
    [0x0100, 0x0 ], 

    [0x0300, 0x4 ], 
    [0x0302, 0x01 ], 
    [0x0304, 0x03 ], 
    [0x0306, 0x6E ], 
    [0x0308, 0x0C ], 
    [0x030A, 0x01 ], 

    [0x3016, 0x111 ], 
    [0x0344, 0x090 ], 
    [0x0348, 0x11AF], 
    [0x0346, 0x020 ], 
    [0x034A, 0xCF7 ], 
    [0x3040, 0x0041], 
    [0x0400, 0x0 ], 
    [0x0404, 0x10 ], 
    [0x034C, 0x1120], 
    [0x034E, 0xCD8 ], 
    [0x0342, 0x33C1], 
    [0x0340, 0x0D67], 
    [0x3010, 0x00CF], 
    [0x3012, 0x0010], 
    [0x3014, 0x1F50], 
    [0x3018, 0x0000], 

    [0x0104, 0x0], 
    [0x0100, 0x1], 
    [0x0304, 8], 

    [0x301a, 0x5ccc],   
    [0x3012, 500], 
    [0x0206, 33], 
    [0x0208, 50], 
    [0x020a, 50], 
    [0x020c, 33], 

    [0xffff, 0xffff], 
    [0xffff, 0xffff] 
] 


ap = argparse.ArgumentParser() 
ap.add_argument("--type",default = "jpg",required = False, help = "type to the image file") 
ap.add_argument("--name",required = False, help = "name to the image file") 
args = vars(ap.parse_args()) 

def readThread(threadName,read_Flag): 
    global flag,handle 
    data = {} 
    cv2.namedWindow("MT9F002",1) 
    while flag: 
     if ArducamSDK.Py_ArduCam_availiable(handle) > 0: 

      res,data = ArducamSDK.Py_ArduCam_read(handle,Width * Height) 
      if res == 0: 
       ArducamSDK.Py_ArduCam_del(handle) 
      else: 
       print "read data fail!" 

     else: 
      print "is not availiable" 
     if len(data) >= Width * Height: 
      show(data) 
      flag = False 
     else: 
      print "data length is not enough!" 
     if flag == False:  
      break 

thread.start_new_thread(readThread,("Thread-2", flag,)) 

pass 

def show(bufferData): 
    global downFlag,saveFlag,saveNum 
    image = Image.frombuffer("L",(Width,Height),bufferData) 
    img = np.array(image) 
    height,width = img.shape[:2] 
    img2 = cv2.cvtColor(img,COLOR_BYTE2RGB) 
     saveNum += 1 

    if args["name"] != "": 
       name = args["name"] 
     else: 
       name = str(saveNum) 
    if "bmp" == args["type"]:   
       name += ".bmp" 
     if "png" == args["type"]:   
       name += ".png" 
     if "jpg" == args["type"]:   
       name += ".jpg" 
    cv2.imwrite(name,img2) 


def video(): 
    global flag,regArr,handle 
    regNum = 0 
    res,handle = ArducamSDK.Py_ArduCam_autoopen(cfg) 
    capFlag = True 
    if res == 0: 
     openFlag = True 
     print "device open success!" 
     while (regArr[regNum][0] != 0xFFFF): 
      ArducamSDK.Py_ArduCam_writeSensorReg(handle,regArr[regNum][0],regArr[regNum][1]) 
      regNum = regNum + 1 
     res = ArducamSDK.Py_ArduCam_beginCapture(handle) 

     if res == 0: 
      print "transfer task create success!" 
      while flag : 
           if capFlag: 
             res = ArducamSDK.Py_ArduCam_capture(handle) 
             if res != 0: 
               print "capture fail!" 
               break 
             else: 
               capFlag = False 
           time.sleep(0.4) 
       if flag == False:  
        break 
     else: 
      print "transfer task create fail!" 
     res = ArducamSDK.Py_ArduCam_close(handle) 
     if res == 0: 
      openFlag = False 
      print "device close success!" 
     else: 
      print "device close fail!" 
    else: 
     print "device open fail!" 

if __name__ == "__main__":  
    video() 

Answer 1

Absobleepinglutely rien. global est un indice pour le compilateur qu'il doit vérifier les étendues externes pour le nom de code dans la portée actuelle. Comme la portée globale est la portée la plus externe, il n'y a aucun effet.

>>> import dis 
>>> def foo(): 
... global bar 
... bar = 1 
... baz = 2 
... 
>>> dis.dis(foo) 
    3   0 LOAD_CONST    1 (1) 
       3 STORE_GLOBAL    0 (bar) 

    4   6 LOAD_CONST    2 (2) 
       9 STORE_FAST    0 (baz) 
      12 LOAD_CONST    0 (None) 
      15 RETURN_VALUE   
>>> def quux(): 
... global bar 
... print bar 
... baz = 1 
... print baz 
... 
>>> dis.dis(quux) 
    3   0 LOAD_GLOBAL    0 (bar) 
       3 PRINT_ITEM   
       4 PRINT_NEWLINE  

    4   5 LOAD_CONST    1 (1) 
       8 STORE_FAST    0 (baz) 

    5   11 LOAD_FAST    0 (baz) 
      14 PRINT_ITEM   
      15 PRINT_NEWLINE  
      16 LOAD_CONST    0 (None) 
      19 RETURN_VALUE