Why does IDLE use a subprocess?

[Chris Angelico]

On Wed, 31 May 2023 at 12:03, James Schaffler via Python-list
<python-list at python.org> wrote:
>
> On Tuesday, May 30th, 2023 at 9:14 PM, Greg Ewing wrote:
> > Globals you create by executing code in the REPL have their own
> > namespace. But everything else is shared -- builtins, imported
> > Python modules, imported C extension modules, etc. etc.
>
> Thanks for the explanation. Could you elaborate on precisely how "everything else is shared"? As far as I understand, if you run the following code:
>
> from code import InteractiveInterpreter
> interp = InteractiveInterpreter()
> import numpy as np
> interp.runcode("np.__name__")
>
> this will result in the error
> Traceback (most recent call last):
>   File "<string>", line 1, in <module>
> NameError: name 'np' is not defined
>
> which seems to imply that imports in the parent shell are not shared with interpreters and vice versa (if you swap the places of the import and the __name__ call).
>

Yep, what you're seeing there is the namespace and nothing else. But
if you mess with an actual builtin object, it'll be changed for the
other interpreter too.

>>> import ctypes
>>> ctypes.cast(id(42), ctypes.POINTER(ctypes.c_int))[6] = 43
>>> 41+1
43
>>> from code import InteractiveInterpreter
>>> interp = InteractiveInterpreter()
>>> interp.runcode("print(41+1)")
43

(Note that this only works in CPython and only with integers small
enough to be in the cache, meaning that there is only one such object
representing that integer.)

The same is true of C extensions, which often have their own internal
state, and that state isn't isolated to a single interpreter.

Better isolation is coming with PEP 554
https://peps.python.org/pep-0554/ which also has some great
information about what currently is NOT isolated. (Also, even then,
some things won't be fully isolated; I think that the ctypes trick
above might still affect a subinterpreter even in a post-PEP554
world.)

ChrisA