using subpocess for both reading and writing simultaneously

[killerrex]

Command string as parameter may only be provided when it is coupled with shell=True; with default value False list is required (as you have shown)

I try not to use the shell=True mode... some bad experiences with the shell expansions.
I have done some additional tests and you are right that when you want to pass parameters with shell=False you need to pass them in a list. Is logical as Popen will try to blindly search for a file with the full name:

>>>with subprocess.Popen(['ls'], stdout=subprocess.PIPE) as p:
>>>    print(''.join(b.decode() for b in p.stdout.readlines()))
cpt.py
subp.py
>>> with subprocess.Popen('ls', stdout=subprocess.PIPE) as p:
>>>    print(''.join(b.decode() for b in p.stdout.readlines()))
cpt.py
subp.py
>>> with subprocess.Popen(['ls', '-r'], stdout=subprocess.PIPE) as p:
>>>    print(''.join(b.decode() for b in p.stdout.readlines()))
subp.py
cpt.py
>>> with subprocess.Popen('ls -r', stdout=subprocess.PIPE) as p:
>>>    print(''.join(b.decode() for b in p.stdout.readlines()))
Traceback (most recent call last):
  File "subp.py", line 27, in <module>
    with subprocess.Popen('ls -r', stdout=subprocess.PIPE) as p:
  File "/usr/lib64/python3.6/subprocess.py", line 709, in __init__
    restore_signals, start_new_session)
  File "/usr/lib64/python3.6/subprocess.py", line 1344, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'ls -r': 'ls -r'

But for a python script that is in the same folder works in both ways as long as you set it executable.
If the script is not executable it produces an error like:

Error:
Traceback (most recent call last):
  File "subp.py", line 7, in <module>
    with subprocess.Popen(['./cpt.py'], stdout=subprocess.PIPE) as p:
  File "/usr/lib64/python3.6/subprocess.py", line 709, in __init__
    restore_signals, start_new_session)
  File "/usr/lib64/python3.6/subprocess.py", line 1344, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
PermissionError: [Errno 13] Permission denied: './cpt.py'