索引超出范围，仅在函数 python 内部

Question

我真的不明白这里发生了什么。 我有一个功能：

def get_appliance_status(appliance):
appliance_data = None
status_color = SENSOR_STATUS_COLOR_UNKNOWN
status_message = 'No Appliance Data Available'
try:
appliance_data = ApplianceData.objects.filter(appliance=appliance).latest('timestamp')
except Exception as ex:
print ex
else:
if appliance_data:
status_color = SENSOR_STATUS_COLOR_OK
status_message = 'Appliance Temperature and Pressure values are within designed ranges.'
temperature_status = ''
pressure_status = ''

# status of inside box temp and food temps 1-4
if appliance_data.temperature_4_f > appliance.max_temperature or 
appliance_data.temperature_5_f > appliance.max_temperature or 
appliance_data.temperature_6_f > appliance.max_temperature or 
appliance_data.temperature_7_f > appliance.max_temperature or 
appliance_data.temperature_8_f > appliance.max_temperature:
status_color = SENSOR_STATUS_COLOR_WARN
temperature_status = 'Appliance Temperature is outside designed ranges. '
status_message = temperature_status
if est_obj > timestamp + timedelta(minutes=15) or 
est_obj > timestamp + timedelta(minutes=15) and 
appliance_data.temperature_4_f > appliance.max_temperature:
status_color = SENSOR_STATUS_COLOR_CRITICAL
status_message = "Polar Board Has Lost Connection/Power Failure, Critical"
#textclient.text(textclient.Brian, "Polar Board has lost connection" + " " + appliance.name)
if appliance_data.power_1_amps > 0  and 
appliance_data.power_2_amps == 0:
status_color = SENSOR_STATUS_COLOR_EMERGENCY
status_message = "Condenser Motor Fail-Emergency"
if appliance_data.power_1_amps == 0 and 
appliance_data.power_2_amps > 0:
status_color = SENSOR_STATUS_COLOR_EMERGENCY
status_message = "Compressor Motor Fail-Emergency"  
if appliance_data.power_1_amps == 0 and 
appliance_data.power_2_amps == 0 and 
appliance_data.power_3_amps == 0 and 
appliance_data.power_4_amps == 0:
status_color = SENSOR_STATUS_COLOR_EMERGENCY
status_message = "Power failure- Emergency"
if appliance_data.pressure_1_psi < 8 and 
appliance_data.pressure_2_psi < 7 and 
appliance_data.power_1_amps == 0 and 
appliance_data.power_2_amps == 0 and 
appliance_data.power_3_amps > 0 and 
appliance_data.power_4_amps > 0:
status_color = SENSOR_STATUS_COLOR_EMERGENCY
status_message = "No Pressure, Condenser/Compressor failure, Emergency"
if appliance_data.pressure_1_psi > appliance.max_pressure_1 or 
appliance_data.pressure_1_psi < appliance.min_pressure_1 or 
appliance_data.pressure_2_psi > appliance.max_pressure_2 or 
appliance_data.pressure_2_psi < appliance.min_pressure_2:
status_color = SENSOR_STATUS_COLOR_WARN
status_message = ''
pressure_status = 'Appliance Pressure is outside designed ranges.'
status_message = ' '.join((temperature_status, pressure_status))
if appliance_data.power_3_amps == 0 and 
appliance_data.power_4_amps > 0:
status_color = SENSOR_STATUS_COLOR_CRITICAL
status_message = "Evaporator Fan 1 Failure, Critical"
if appliance_data.power_3_amps > 0 and 
appliance_data.power_4_amps == 0:
status_color = SENSOR_STATUS_COLOR_CRITICAL
status_message = "Evaporator Fan 2 Failure, Critical "

for item in my_appliance:
if item.temperature_4_f > appliance.max_temperature:
above_temp.append(item.timestamp.astimezone(local_tz).strftime('%H:%M'))
else:
break
print(above_temp)
return '<span class="status" style="background-color:' + status_color +';" title="' + status_message + '"></span>'

问题就在这里：

above_temp=[]
for item in my_appliance:
if item.temperature_4_f > appliance.max_temperature:
above_temp.append(item.timestamp.astimezone(local_tz)
.strftime('%H:%M'))
else:
break
print(above_temp)

如果我尝试打印above_temp[0]，则会出现超出范围的错误。 我可以使用above_temp[0:1]访问第一个元素。 但是，这将返回此元素的列表，这不是我想要的。 当我print(above_temp)时，我得到了一个[]，然后我得到了我想要的列表。 我真的很想要这个列表的第一个和最后一个元素。 索引中最后一个元素，基于列表的 len。如果我在函数外部运行相同的代码，它会按预期工作。 不过，我需要它在此函数中运行。 为什么它最初返回一个空列表，为什么我不能只使用索引来访问此函数中的元素？ 这是我运行这个时得到的：

[]
['11:17', '11:14', '11:12', '11:10', '11:08', '11:06', '11:04', '11:02',
'11:00', '10:58', '10:56', '10:53', '10:51', '10:49', '10:47', '10:45', 
'10:43', '10:41', '10:39', '10:37', '10:35', '10:32', '10:30', '10:28', 
'10:26', '10:24', '10:22', '10:20', '10:18', '10:16', '10:14', '10:11', 
'10:09', '10:07', '10:05', '10:03', '10:01', '09:59', '09:57', '09:55', 
'09:53', '09:50', '09:48', '09:46', '09:44', '09:42', '09:40', '09:38', 
'09:36', '09:34', '09:32', '09:29']

Answer 1

列表切片更宽松 - 如果列表不需要您从中请求的元素，它甚至可以工作，在这种情况下，它将返回一个空列表[]。 在不存在的位置索引到列表将返回错误。

可通过以下方式重现：

tata = [] 
print  tata[0:200] 
try:
print tata[0] 
except IndexError as idxerr:
print idxerr 

输出：

[]
list index out of range

---

请参阅有关字符串部分中的简介：

word = 'Python'

尝试使用太大的索引将导致错误：

word[42]  # the word only has 6 characters Traceback (most recent call last):   
File "<stdin>", line 1, in <module> IndexError: string
index out of range 

但是，处理超出范围的切片索引 用于切片时优雅：

word[4:42] 'on'
word[42:] ''

---

如果您需要访问列表的第一个和最后一个元素，您可以考虑这样做：

firstVal, lastVal = above_temp[0:1],above_temp[-1:] # both wont throw error, but 
# might be [] afterwards

演示：

tata = [2,3]
firstVal, lastVal = tata[0:1],tata[-1:] 
print firstVal , lastVal
tata = []
firstVal, lastVal = tata[0:1],tata[-1:] 
print firstVal , lastVal

输出：

[2] [3]
[] []