Python Subprocess Empty Output

Question

I have this code:

pprocess = subprocess.Popen('PROGRAM', stdout=subprocess.PIPE)

while True:
    output = pprocess.stdout.readline()
    print 'output = ', output

But the output is not being printed using the python code, but instead it is being printed on the console and it appears to be printed directly from the process.

Does anyone came across this issue?

what is PROGRAM returning when you call it IN TERMINAL?

user1767754
– user1767754

2018-01-10 09:17:33 +00:00
Commented Jan 10, 2018 at 9:17 — user1767754
– user1767754, Commented Jan 10, 2018 at 9:17
It prints the output correctly on the terminal

Jay R. W
– Jay R. W

2018-01-10 09:19:12 +00:00
Commented Jan 10, 2018 at 9:19 — Jay R. W
– Jay R. W, Commented Jan 10, 2018 at 9:19
You need to move the popen as well into the while loop.

user1767754
– user1767754

2018-01-10 09:25:25 +00:00
Commented Jan 10, 2018 at 9:25 — user1767754
– user1767754, Commented Jan 10, 2018 at 9:25
Why? It will cause the popen to be opened every time

Jay R. W
– Jay R. W

2018-01-10 11:51:46 +00:00
Commented Jan 10, 2018 at 11:51 — Jay R. W
– Jay R. W, Commented Jan 10, 2018 at 11:51

heemayl · Accepted Answer · 2018-01-10 10:18:54Z

2

But the output is not being printed using the python code, but instead it is being printed on the console

It seems the output is being printed on the STDERR stream, rather than the STDOUT stream; you're only capturing the STDOUT.

So, consume the stderr stream (too):

pprocess = subprocess.Popen('PROGRAM', stdout=subprocess.PIPE, stderr=subprocess.PIPE)
# Checking just the STDERR here
for line in pprocess.stderr:
    print(line)

As a side note, rather that using readline() in a while loop, just iterate over the file-like object provided by Popen.stdout/Popen.stderr, which are iterators.

answered Jan 10, 2018 at 10:18

heemayl

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3 Comments

Jay R. W Over a year ago

The " for line in pprocess.stderr: " line appears to be blocking, the flow stops there

heemayl Over a year ago

@JayR.W From terminal, run program 2>/dev/null . Do you get any output? (This is to confirm that the output actually is being written to STDERR)

Jay R. W Over a year ago

The output that is being written is different from the output I am receiving from this line of code: " print = self.sip_process.stderr.readline() "

user1767754 · Accepted Answer · 2018-01-10 09:27:22Z

-1

Once you read from the stdout stream by doing stdout.readline it becomes empty. You either have to move your Popen call into the while loop or change your logic:

while True:
    pprocess = subprocess.Popen('PROGRAM', stdout=subprocess.PIPE)
    output = pprocess.stdout.readline()
    print 'output = ', output

answered Jan 10, 2018 at 9:27

user1767754

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