Python Interpreter within Interpreter

Question

So I was learning about the Python subprocess module. Withing the interpreter, I typed subprocess.call("python") and another interpreter opened within the existing interpreter, and things got pretty weird. While typing, I noticed some characters weren't being echoed properly and I had to type them repeatedly. Plus, plain Enters (or returns) were adding lot more than just chevrons on the screen. Here's part of the output:

Can someone explain what was happening? I know the question may be too broad or vague, but I'm unable to be specific here.

@HFBrowning I was trying quit() and just pressing return repeatedly. — Sam Chats
– Sam Chats, Commented May 29, 2017 at 21:18
Hm, just tried it myself with no problem. Maybe someone with a similar set up to you (I'm on Windows) has an idea. — HFBrowning
– HFBrowning, Commented May 29, 2017 at 21:20
Well, you have opened another interpreter in the current one, but the current one already fetches the input, therefore to "pass" the input further you'll need to do it each second time. For example after Popen the object is created, then <enter>, and then the new interpreter prints to the current one. If you then type right after the interpreter is opened e.g. help(), it'll be interpreted in the first one, while now the second one waits for your input. If now you give it <enter>, the second interpreter just "passes" and the help() output continues... — Peter Badida
– Peter Badida, Commented May 29, 2017 at 21:25
... Windows, 3.5.2, can't explain it better. Basically it fights for input and output because you're doing something stupid. If you need to use it, just interpret a file and definitely not in an already opened interpreter, but also in a separate file - python main.py, where in main.py you call that subprocess and the string is also something like python second_main.py. Maybe it'd be better to explain it with two input() functions waiting for each other? — Peter Badida
– Peter Badida, Commented May 29, 2017 at 21:25

Peter Badida · Accepted Answer · 2017-05-29 22:20:31Z

I'll try to make it straightforward to reproduce, so that you can see the collision.

python
import os; os.getpid(); # we'll need this, get it to clipboard or something

296236 # for me
import subprocess
subprocess.Popen('python')

<subprocess.Popen object at somewhere_over_the_rainbow> # object is created, python probably waits for a chance to push it to the console as a command "python"

>>>

<import&get pid>

296236 # the command is pushed, the new process spawned

You spawned another Python process into a current open console that already waits for an input. The "child" process now exists but still can't demand to get an input, because the main one is still "on the turn". Imagine it like a queue. The output for import os;os.getpid() now will still be the main process' PID and now the child one gets its turn.

<import&get pid>

296236

>>> Python 3.5.2 (v3.5.2:4def2a2901a5, Jun 25 2016, 22:01:18) [MSC v.1900 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information. # from second process, it did its stuff in its turn, now main process wants to talk

<import&get pid> # in main process

296236

<import&get pid> # in second process

296800

<import&get pid> # in main process

296236

<import&get pid> #in second process

296800

...and alternating again and again. The fun begins when you want to exit(), because the fight for input is still ongoing:

import os;os.getpid();exit() #in second process

296800

and a clean empty line

import os;os.getpid();exit() #in main process

296236

C:\some folder>

Basically it fights for an IO operation in a queue because you're doing something stupid. If you need to use it, just interpret a file and definitely not in an already opened interpreter, but also in a separate file. For example:

main.py

import subprocess
subprocess.Popen('python other.py')

other.py

print('hello')
exit()

console

python main.py

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