UnboundLocalError: local variable 'signaloff' referenced before assignment

Question

我正在尝试在 raspberry pi 上按下按钮时读取超声波范围。

我收到一个随机错误，大约每三次发生 1 次。还尝试了运行 'print reading(0)' 三次，每次尝试之间等待 2 秒，有时会成功，有时会在第一次尝试时失败。

错误是：

Traceback (most recent call last):
  File "test.py", line 37, in <module>
    print reading(0)
  File "test.py", line 30, in reading
    timepassed = signalon - signaloff
UnboundLocalError: local variable 'signaloff' referenced before assignment

密码是：

import RPi.GPIO as GPIO
import time
GPIO.setwarnings(False)
GPIO.setmode(GPIO.BCM)

# btn on pin 18
GPIO.setup(18, GPIO.IN, pull_up_down=GPIO.PUD_UP)
# LED on pin 24
GPIO.setup(24, GPIO.OUT)
# GPIO output = the pin that's connected to "Trig" on the sensor
# GPIO input = the pin that's connected to "Echo" on the sensor
GPIO.setup(17,GPIO.OUT)
GPIO.setup(27,GPIO.IN)

def reading(sensor):

    if sensor == 0:
        GPIO.output(17, GPIO.LOW)
        time.sleep(0.3)

        GPIO.output(17, True)
        time.sleep(0.00001)
        GPIO.output(17, False)
        while GPIO.input(27) == 0:
            signaloff = time.time()

        while GPIO.input(27) == 1:
            signalon = time.time()

        timepassed = signalon - signaloff
        distance = timepassed * 17000

        return distance
    else:
        print "Incorrect usonic() function varible."

while True:
    input_state = GPIO.input(18)
    if input_state == False:
        print('Button Pressed')
        GPIO.output(24, True)
        print reading(0)
        time.sleep(2)
        GPIO.output(24, False)

Answer 1

如果GPIO.input(27) returns 0第一次调用，永远不会进入while循环，永远不会设置signaloff。对于设置 signalon 的循环实际上也是如此，尽管这个问题可能比较少见。

UnboundLocalError: local variable 'signaloff' referenced before assignment

UnboundLocalError: local variable 'signaloff' referenced before assignment

python

raspberry-pi