Handling errors within loops through exceptions - error-handling

Question

Tried my first python program to read temp sensor and output to influxdb
Occasionally temp sensor gives error "IndexError: list index out of range" and loop ends
I want loop to wait 15 seconds on this error and then continue the loop (sensor usually corrects itself by then on the next read)
My code:
import os
import glob
import time
import urllib
import urllib2
import httplib
import json
from influxdb import InfluxDBClient
client = InfluxDBClient(host='192.168.1.7', port=8086)
#client.get_list_database()
client.switch_database('influxdb1')
os.system('modprobe w1-gpio')
os.system('modprobe w1-therm')
base_dir = '/sys/devices/w1_bus_master1/'
device_folder = glob.glob(base_dir + '28*')[0]
while True:
device_file = device_folder + '/w1_slave'
def read_temp_raw():
f = open(device_file, 'r')
lines = f.readlines()
f.close()
return lines
def read_temp():
lines = read_temp_raw()
while lines[0].strip()[-3:] != 'YES':
time.sleep(0.2)
lines = read_temp_raw()
equals_pos = lines[1].find('t=')
if equals_pos != -1:
temp_string = lines[1][equals_pos+2:]
temp_c = float(temp_string) / 1000.0
return temp_c
temp = float(read_temp())
json_body = [
{
"measurement": "YOUR_MEASUREMENT",
"tags": {
"Device": "YOUR_DEVICE",
"ID": "YOUR_ID"
},
"fields": {
"outside_temp": temp,
}
}
]
client.write_points(json_body)
time.sleep(60)
******************************************************
which works ok :)
When I edit the code to catch the exception.....
******************************************************
while True:
except IndexError:
time.sleep(15)
continue
device_file = device_folder + '/w1_slave' # store the details
def read_temp_raw():
f = open(device_file, 'r')
lines = f.readlines() # read the device details
f.close()
return lines
def read_temp():
lines = read_temp_raw()
while lines[0].strip()[-3:] != 'YES':
time.sleep(0.2)
lines = read_temp_raw()
equals_pos = lines[1].find('t=')
if equals_pos != -1:
temp_string = lines[1][equals_pos+2:]
temp_c = float(temp_string) / 1000.0
return temp_c
temp = float(read_temp())
json_body = [
{
"measurement": "YOUR_MEASUREMENT",
"tags": {
"Device": "YOUR_DEVICE",
"ID": "YOUR_ID"
},
"fields": {
"outside_temp": temp,
}
}
]
client.write_points(json_body)
time.sleep(60)
************************************************************
I get following error...
File "temptoinfluxdb2.py", line 22
except IndexError:
^
SyntaxError: invalid syntax
Where am i going wrong please?

Answer 1

You will always need to use the except block in combination with a try block.
So the code in the try block is executed until an exception (in that case IndexError) occurs.
try:
# Execution block
except IndexError:
# Error handling
You could also use a more general approach with except Exception as e, which catches not just the IndexError but any exception.
Check the official documentation for further information.