仅在数据可用时从USB条形码扫描仪读取

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更新时间：

我已经成功地用Python读取了一个USB条码扫描器。如果是空的，我希望能够转义fp.read()，并检查用户是否按下了左LCD按钮

def read Barcode
lcd.backlight(lcd.GREEN)
hid = { 4: 'a', 5: 'b', 6: 'c', 7: 'd', 8: 'e', 9: 'f', 10: 'g', 11: 'h', 12: 'i', 13: 'j', 14: 'k', 15: 'l', 16: 'm', 17: 'n', 18: 'o', 19: 'p', 20: 'q', 21: 'r', 22: 's', 23: 't', 24: 'u', 25: 'v', 26: 'w', 27: 'x', 28: 'y', 29: 'z', 30: '1', 31: '2', 32: '3', 33: '4', 34: '5', 35: '6', 36: '7', 37: '8', 38: '9', 39: '0', 44: ' ', 45: '-', 46: '=', 47: '[', 48: ']', 49: '\\', 51: ';' , 52: '\'', 53: '~', 54: ',', 55: '.', 56: '/'  }
hid2 = { 4: 'A', 5: 'B', 6: 'C', 7: 'D', 8: 'E', 9: 'F', 10: 'G', 11: 'H', 12: 'I', 13: 'J', 14: 'K', 15: 'L', 16: 'M', 17: 'N', 18: 'O', 19: 'P', 20: 'Q', 21: 'R', 22: 'S', 23: 'T', 24: 'U', 25: 'V', 26: 'W', 27: 'X', 28: 'Y', 29: 'Z', 30: '!', 31: '@', 32: '#', 33: '$', 34: '%', 35: '^', 36: '&', 37: '*', 38: '(', 39: ')', 44: ' ', 45: '_', 46: '+', 47: '{', 48: '}', 49: '|', 51: ':' , 52: '"', 53: '~', 54: '<', 55: '>', 56: '?'  }
backPressed = False
lcd.clear()
lcd.message("accn: \nLocation: ")
lcd.setCursor(7, 1)
print("Scan Next Accn:")
while not backPressed:
  fp = open('/dev/hidraw0', 'r')
  accn = ""
  shift = False
  done = False
  print("should loop")
  r, w, e = select.select([ fp ], [], [], 0)
  print("Should be a line here")
  if fp in r:
    print("Fp is in r")
    while not done:
        print("looping")
        buffer = os.read(fp.fileno(), 8)
        for c in buffer:
          if ord(c) > 0:
             if int(ord(c)) == 40:
                done = True
                fp.flush()
                fp.close()
                print("Done = True")
                break;
             if shift: 
                if int(ord(c)) == 2 :
                   shift = True
                else:
                   accn += hid2[ int(ord(c)) ]
                   shift = False
             else:
                if int(ord(c)) == 2 :
                   shift = True
                else:
                   accn += hid[ int(ord(c)) ]
    print("accn: " + accn)
    fileAccn(accn)
    fp.close()
    backPressed = lcd.buttonPressed(lcd.LEFT)
    if(backPressed):
      lcd.backlight(lcd.WHITE)
      return
  backPressed = lcd.buttonPressed(lcd.LEFT)
  if(backPressed):
    return
return

因为条码扫描器不使用fp.read()发送没有参数的EOF，所以如果它是空的，就不会返回。如有任何帮助，我们将不胜感激。我不想使用中断或超时，因为我不想在扫描条形码的过程中结束此功能。

提前谢谢。

Answer 1

或许这个问题的答案能有所帮助。它建议对指定的字节数使用os.read()，而不是常规的read，因为os.read()将在读取任何内容后返回。

Answer 2

我只想更新一个合法的答案。我认为这个问题最好用evdev来解决。 这是我拼凑的函数。

基本上，当它被调用时，它将从条形码扫描器event = dev.read_one()读取一个事件。 如果连它都没有移动。

当它命中一个字符时，它将继续检查even的内容，直到它得到一个返回键事件。此时，条形码完成并返回。要退出循环，用户可以按下后退按钮lcd.LEFT

注意，在进入函数之前或在开始时阅读每个事件可能是一个好主意。我认为这个函数不会清除所有事件，如果事件发生在调用函数之前（即扫描条形码），它可能会捕获最后几个字符。

#!/usr/bin/env python

from evdev import *
import signal, sys, time

keys = {
    # Scancode: ASCIICode
    0: None, 1: u'ESC', 2: u'1', 3: u'2', 4: u'3', 5: u'4', 6: u'5', 7: u'6', 8: u'7', 9: u'8',
    10: u'9', 11: u'0', 12: u'-', 13: u'=', 14: u'BKSP', 15: u'TAB', 16: u'Q', 17: u'W', 18: u'E', 19: u'R',
    20: u'T', 21: u'Y', 22: u'U', 23: u'I', 24: u'O', 25: u'P', 26: u'[', 27: u']', 28: u'CRLF', 29: u'LCTRL',
    30: u'A', 31: u'S', 32: u'D', 33: u'F', 34: u'G', 35: u'H', 36: u'J', 37: u'K', 38: u'L', 39: u';',
    40: u'"', 41: u'`', 42: u'LSHFT', 43: u'\\', 44: u'Z', 45: u'X', 46: u'C', 47: u'V', 48: u'B', 49: u'N',
    50: u'M', 51: u',', 52: u'.', 53: u'/', 54: u'RSHFT', 56: u'LALT', 100: u'RALT'
}
dev = InputDevice('/dev/input/event0')
def scanBarcode():
  barcode = ''
  while True:
      event = dev.read_one()
      if event is None and barcode == '':
        #There are blank events in between characters, 
        #so we don't want to break if we've started
        #reading them
        break #nothing of importance, start a new read. 
      try:
        if event is not None:
          if event.type == ecodes.EV_KEY:
            data = categorize(event)
            if data.keystate == 0 and data.scancode != 42: # Catch only keyup, and not Enter
              if data.scancode == 28: #looking return key to be pressed
                return barcode
              else:
                  barcode += keys[data.scancode] # add the new char to the barcode
      except AttributeError:
          print "error parsing stream"
          return 'SOMETHING WENT WRONG'
      if lcd.buttonPressed(lcd.LEFT):
        return

Answer 3

查看选择模块。

下面是它的使用示例： http://stromberg.dnsalias.org/~strombrg/reblock.html