python运行出错

File "./DHT11.py", line 20
    for i in range(0,500):
    ^
IndentationError: unexpected indent

如何解决

以下是全部代码：

#!/usr/bin/python

import RPi.GPIO as GPIO
import time

def bin2dec(string_num):
    return str(int(string_num, 2))

    data = []

    GPIO.setmode(GPIO.BCM)

    GPIO.setup(4,GPIO.OUT)
    GPIO.output(4,GPIO.HIGH)
    time.sleep(0.025)
    GPIO.output(4,GPIO.LOW)
time.sleep(0.02)

GPIO.setup(4, GPIO.IN, pull_up_down=GPIO.PUD_UP)
    for i in range(0,500):
        data.append(GPIO.input(4))

        bit_count = 0
        tmp = 0
        count = 0
        HumidityBit = ""
        TemperatureBit = ""
        crc = ""

        try:
        while data[count] == 1:
        tmp = 1
        count = count + 1

        for i in range(0, 32):
            bit_count = 0

            while data[count] == 0:
            tmp = 1
            count = count + 1

            while data[count] == 1:
            bit_count = bit_count + 1
            count = count + 1

            if bit_count > 3:
            if i>=0 and i<8:
            HumidityBit = HumidityBit + "1"
            if i>=16 and i<24:
            TemperatureBit = TemperatureBit + "1"
            else:
            if i>=0 and i<8:
            HumidityBit = HumidityBit + "0"
            if i>=16 and i<24:
            TemperatureBit = TemperatureBit + "0"

            except:
    print "ERR_RANGE"
exit(0)

    try:
    for i in range(0, 8):
        bit_count = 0

        while data[count] == 0:
        tmp = 1
        count = count + 1

        while data[count] == 1:
        bit_count = bit_count + 1
        count = count + 1

        if bit_count > 3:
        crc = crc + "1"
        else:
        crc = crc + "0"
        except:
    print "ERR_RANGE"
exit(0)

    Humidity = bin2dec(HumidityBit)
Temperature = bin2dec(TemperatureBit)

    if int(Humidity) + int(Temperature) - int(bin2dec(crc)) == 0:
    print Humidity
    print Temperature
    else:
    print "ERR_CRC"

来自这个网址：
http://www.raspberrypi.org/forums/viewtopic.php?t=69427&p=505235