只有當數據可用時才從USB條形碼掃描儀讀取

我已經成功地使用Python讀取USB條形碼掃描儀。我希望能夠逃脫fp.read()如果是空的，並檢查用戶按下左LCD按鈕

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

由於條碼掃描器不發送EOF使用fp.read()不帶參數不會返回，如果它是空的。任何幫助表示讚賞。我不願意使用中斷或超時，因爲我不想在掃描條形碼的過程中結束該功能。

在此先感謝。

來源

2014-10-11 Tom

我只是想有一個合法的答案更新。我認爲這個問題最好用evdev來解決。這是我拼湊在一起的功能。

基本上，它被稱爲它會從條形碼掃描器event = dev.read_one()讀取一個事件。如果沒有，它會一直移動。

當它遇到一個字符時，它會一直檢查它的內容，直到它得到一個返回鍵事件。此時，條碼已完成並返回。要退出循環，用戶可以按下後退按鈕lcd.LEFT被按下

一些註釋，在進入函數或開始之前讀取每個事件可能是個好主意。我不認爲這個函數清除了所有的事件，並且如果事件發生在函數調用之前（即掃描條形碼），它可能會捕獲最後幾個字符。

#!/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

來源

2014-10-18 17:39:30 Tom