Python subprocess.Popen does not work with stdout

Question

I need to implement an external application to calculate CRC values for Modbus communication. The executable requires an input string and gives back output like this:

CRC16 = 0x67ED / 26605 CRC16 (Modbus) = 0x7CED / 31981

I call the programm and afterwards type in the input manually.

p = Popen(["some_file.exe", "-x"], stdin=PIPE) p.communicate("some_string")

This is working fine so far.

However, I want to save the output to a variable or something (no extra file) for further uses.

I know there are the stdout and stderr arguments, but when typing

p = Popen([file, "-x"], stdin=PIPE, stdout=PIPE, stderr=PIPE)

nothing happens at all.

Does anyone has an idea what to do?

Thanks in advance.

PS: Using Python 2.7 on Windows 7.

Community · Accepted Answer · 2017-05-23 12:30:33Z

To get the output of ls, use stdout=subprocess.PIPE.

proc = subprocess.Popen('ls', stdout=subprocess.PIPE) output = proc.stdout.read() print output

Obtained from: Pipe subprocess standard output to a variable

NOTE:

If you use stdin as PIPE you must assign a value, like in this example:

grep = Popen('grep ntp'.split(), stdin=PIPE, stdout=PIPE) ls = Popen('ls /etc'.split(), stdout=grep.stdin) output = grep.communicate()[0]

if value is given by console using a PIPE, you must assign the stdin value reading sys.stdin

Thanks for your response. I dont know, but this does not seem to work. I need to give an input to the programm, so there needs to be a stdin=PIPE. Using just this argument is fine, but adding the stdout=PIPE is not. I dont even get to the point where I can write to a variable output=...
@mulm if you use stdin=PIPE you must assign a value to stdin, else it will be null.

Community · Accepted Answer · 2017-05-23 12:19:46Z

Ok, I figured it out.

It says in an older post: How do I write to a Python subprocess' stdin?

The p.communicate() just waits for the input in the following form:

p = Popen(["some_file.exe", "-x"], stdout=PIPE, stdin=PIPE, stderr=PIPE) output = p.communicate(input="some_string")[0]

Then output has all the information that is received.

Collectives™ on Stack Overflow

Python subprocess.Popen does not work with stdout

2 Answers 2

2 Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Linked

Related