Python Raspberry pi - If path doesn't exist, skip the loop

问题

I have a function to collect temperature (values from text files) which uses a partly predefined path. However, sometimes the path does not exist if the temperature sensor wasn't loaded (is disconnected). How can I set a condition or exception to skip a loop if a path is not available?

I wanted to use continue, but i have no idea what condition to set with it.

def read_all(): 

    base_dir = '/sys/bus/w1/devices/'
    sensors=['28-000006dcc43f', '28-000006de2bd7', '28-000006dc7ea9', '28-000006dd9d1f','28-000006de2bd8']

    for sensor in sensors:

        device_folder = glob.glob(base_dir + sensor)[0]
        device_file = device_folder + '/w1_slave'

        def read_temp_raw():
            catdata = subprocess.Popen(['cat',device_file], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
            out,err = catdata.communicate()
            out_decode = out.decode('utf-8')
            lines = out_decode.split('\n')
            return lines

回答1:

Use os.path.isfile and os.path.isdir() to check.

for sensor in sensors:
    device_folders = glob.glob(base_dir + sensor)
    if len(device_folders) == 0:
        continue
    device_folder = device_folders[0]
    if not os.path.isdir(base_dir):
        continue
    device_file = device_folder + '/w1_slave'
    if not os.path.isfile(device_file)
        continue
    ....

回答2:

I'm not sure why you are using subprocess.Popen to read the file. Why not just open() it and read()?.

A python way to handle missing dir or file is something like this:

for sensor in sensors:
    try:
        device_folder = glob.glob(base_dir + sensor)[0]
        device_file = device_folder + '/w1_slave'
        with open(device_file) as fd: # auto does fd.close()
            out = fd.read()
    except (IOError,IndexError):
        continue
    out_decode = out.decode('utf-8')
    ...

If you want to avoid hanging in open() or read() you can add a signal handler and give yourself an alarm signal after, say, 5 seconds. This will interrupt the function, and move you into the except clause.

Setup the signal handler at the start:

import signal
def signal_handler(signal, frame):
    raise IOError
signal.signal(signal.SIGALRM, signal_handler)

and modify your loop to call alarm(5) before the part that might hang. Call alarm(0) at the end to cancel the alarm.

for sensor in sensors:
    signal.alarm(5)
    try:
        device_file = ...
        with open(device_file) as fd:
            out = fd.read()
    except (IOError,IndexError):
        continue
    finally:
        signal.alarm(0)
    print "ok"
    ...

来源：https://stackoverflow.com/questions/32269775/python-raspberry-pi-if-path-doesnt-exist-skip-the-loop

标签

python

raspberry-pi2

temperature