[Tutor] Multiprocessing

[Cameron Simpson]

On 22Sep2020 06:57, Stephen M Smith <stephen.m.smith at comcast.net> wrote:
>Thanks - I don't get all of the subtleties of python yet. I am picking it up
>after a long career in much older technology. I don't think in python yet -
>may never. In any event I understand what has happened and the change to
>make, don't really understand the why.

It is only partly Python; the other part is how the multiprocessing 
module is implemented.

When you "import" a module - any of those "import" statements at the top 
of the script, the code which implements the module is actually run at 
that time. It only happens once - importing the same module again 
elsewhere just uses the already imported module.

The main programme you've written is itself a small module, whose name 
is "__main__", which is what the if-statement down the bottom checks.  

That magic peice of boilerplate code lets you write modules which can 
run as a standalone programme. If you import it, it just defines the 
various functions and whatnot. But if you run it as a main programme, it 
is imported with the special name "__main__", and the if-statement at 
the bottom actually runs the main programme, _after_ defining functions 
and whatnot.

So, what has this to do with multiprocessing?

When you kick off a subprocess with the multiprocessing module, it 
starts a shiny new Python programme for the subprocess, and has that 
import your code. As a module. But _not_ named "__main__"!

So when you start your programme, it runs as "__main__" and the stuff 
inside the if-statement at the bottom runs in addition to all the stuff 
before.

Each subprocess _also_ runs your script, but not as "__main__". So it 
runs the stuff before the if-statement (to define the functions and 
whatnot), but skips the stuff inside the if-statement.

In this way the subprocess loads your functions, so that it can be told 
to run one of them.

Because your first print() call is outside the if-statement, it is part 
of the stuff which is always executed. So it runs once for you in the 
main programme, and also by every subprocess.

Cheers,
Cameron Simpson <cs at cskk.id.au>