How to Run Function in a Subprocess Without Threading or Writing a Separate File/Script

Is it possible to run function in a subprocess without threading or writing a separate file/script.

I think you're looking for something more like the multiprocessing module:

http://docs.python.org/library/multiprocessing.html#the-process-class

The subprocess module is for spawning processes and doing things with their input/output - not for running functions.

Here is a multiprocessing version of your code:

from multiprocessing import Process, Queue

# must be a global function    
def my_function(q, x):
    q.put(x + 100)

if __name__ == '__main__':
    queue = Queue()
    p = Process(target=my_function, args=(queue, 1))
    p.start()
    p.join() # this blocks until the process terminates
    result = queue.get()
    print result

How to run second Python script from first one?

Try this:

process = subprocess.Popen([r'app_virtual_helper.bat'])

So that your script doesn't wait for the call on your second script to end

Call a function defined in a shell script from a subprocess

You should read the file with source or just . then invoke function. Source will load bash script defined functions.
The right way will be source my_scripts.sh && func1.

EDIT:
I came with this solution.

bash
#!/bin/bash
function func1() {
    echo "func1 completed"
}
function invoke_python(){
    python a.py
}
if [ -z "$@" ]; then
    invoke_python 
else
    "$@"
fi

and for python I got that.

import subprocess
p = subprocess.Popen("./a.sh func1",
        stdout = subprocess.PIPE,
        shell = True)
p.wait()

Launch a python script from another script, with parameters in subprocess argument

The subprocess library is interpreting all of your arguments, including demo_oled_v01.py as a single argument to python. That's why python is complaining that it cannot locate a file with that name. Try running it as:

p = subprocess.Popen(['python', 'demo_oled_v01.py', '--display',
'ssd1351', '--width', '128', '--height', '128', '--interface', 'spi',
'--gpio-data-command', '20'])

See more information on Popen here.

Constantly print Subprocess output while process is running

You can use iter to process lines as soon as the command outputs them: lines = iter(fd.readline, ""). Here's a full example showing a typical use case (thanks to @jfs for helping out):

from __future__ import print_function # Only Python 2.x
import subprocess

def execute(cmd):
    popen = subprocess.Popen(cmd, stdout=subprocess.PIPE, universal_newlines=True)
    for stdout_line in iter(popen.stdout.readline, ""):
        yield stdout_line 
    popen.stdout.close()
    return_code = popen.wait()
    if return_code:
        raise subprocess.CalledProcessError(return_code, cmd)

# Example
for path in execute(["locate", "a"]):
    print(path, end="")

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