If I bought an 18 foot python and it got my hand in its mouth...

Do you think it could swallow my arm to the shoulder before it had to stop?

It could probably swallow you all the way.
First a constrictor snake kills its prey by crushing/suffocating it. Then it swallows it's prey, usually head first. Since a snake dislocates its jaw, your head won't be a problem. Maybe your shoulders, but even if they were a problem, you would already be dead.

That snake could probably swallow you whole...

If you manage to avoid letting it wrap itself around you to suffocate you, you can save yourself by prying open its mouth and push your hand further inwards before pulling it out, because their teeth grow inwards (if you just pull your hand out, you will rip your flesh open pretty bad).

it can swallow you hole
thats big python, not intented to be kept as a pet unless you know exactly what it needs , how to handle it, and a very big inviroment/tank, ect to keep it in
its mouth can loosen to a lot wider than it is now
it wont stop at your shoulder
it will either
swallow you whole
or it could swallow you whole and regurgitate you later on,
youd still be dead though so it wouldnt matter
if you wanta snake, get a much much smaller one