Stdin redirection from Python

Question

Say I have a program called some_binary that can read data as:

some_binary < input

where input is usually a file in disk. I would like to send input to some_binary from Python without writing to disk.

For example input is typically a file with the following contents:

To simulate something like that in Python I have:

import numpy as np

# Random binary numbers
first_column = np.random.random_integers(0,1, (6,))

# Random numbers between 0 and 1
second_column = np.random.random((6,))

How can I feed the concatenation of first_column and second_column to some_binary as if I was calling some_binary < input from the command line, and collect stdout in a string?

I have the following:

def run_shell_command(cmd,cwd=None,my_input):
      retVal = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stdin=my_input, cwd=cwd);
      retVal = retVal.stdout.read().strip('\n');
      return(retVal);

But I am not sure I am heading in the right direction.

Johannes Overmann · Accepted Answer · 2013-03-26 22:49:19Z

1

Yes, you are heading in the right direction.

You can use pythons subprocess.check_output() functions which is a convenience wrapper around subprocess.Popen(). The Popen needs more infrastructure. For example you need to call comminucate() on the return value of Popen in order for things to happen.

Something like

output = subprocess.check_output([cmd], stdin = my_input)

should work in your case.

edited Mar 26, 2013 at 22:49

answered Mar 26, 2013 at 22:43

Johannes Overmann

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1 Comment

Amelio Vazquez-Reina Over a year ago

Thanks. How do I build my_input from first_column and second_column to achieve the desired effect?

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