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I am trying to readline from stdout of subprocess. Sometimes device "123" doesn't respond and won't provide and data in stdout. In that case line out = proc.stdout.readline() is getting stuck forever. How to come out of loop if there is no response from device.

I am trying to read stdout of a subprocess. below is the code.

cmd = ["node", "transformerMonitor.js", "-h", "sample.com", "-s", "123"] proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, text=True) time_out = 120 start_time = time.time() while time.time() - start_time < time_out: out = proc.stdout.readline() print(out)

if device doesn't respond. out = proc.stdout.readline() is stuck forever. How to break the loop if there is no response.

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    just to know proc.wait and proc.communicate doc have a timeout parameter and processes may receive signals Commented Apr 8, 2023 at 16:56
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    Does this answer your question? Using module 'subprocess' with timeout Commented Apr 9, 2023 at 6:55

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Once the subprocess finishes/fails and there are no more responses from the device, out will be an empty string. We can break out of the loop using this fact.

Code:

import subprocess import time cmd = ["node", "transformerMonitor.js", "-h", "sample.com", "-s", "123"] proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, text=True) time_out = 120 start_time = time.time() while time.time() - start_time < time_out: out = proc.stdout.readline() # If empty string output (No Response from Device), break out of loop. if not out: break print('Code Finished')

Output:

node:internal/modules/cjs/loader:998 throw err; ^ Error: Cannot find module 'C:\Users\sapta\Documents\testing\transformerMonitor.js' at Module._resolveFilename (node:internal/modules/cjs/loader:995:15) at Module._load (node:internal/modules/cjs/loader:841:27) at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12) at node:internal/main/run_main_module:23:47 { code: 'MODULE_NOT_FOUND', requireStack: [] } Node.js v18.12.1 Code Finished
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Since there are no new lines/data , line 'out = proc.stdout.readline()' will stuck forever code wont go to next line.

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