Serhiy's analysis is correct.  If anything more than a comment is going
to make its way to the code, I'd suggest to move dictionary building to
its own function, so that it can be called either on first use --like
now-- or before threading if the user is concerned.

I agree there is nothing wrong with multiple builds.  My point is just a
minor, bearable inefficiency.  It can be neglected.  Its most annoying
case is probably with test suites, which are more likely to shoot up a
bunch of new threads all at once.

Greetings
Ale