[kristjansson]

> if you later choose to want to release that tool it really aught to follow the terms of the GPL and be under a licence compatible with the GPL

This may be in the spirit of Free-as-in-freedom software, but I don't think it's grounded in the reality of copyright and the GPL. With the usual disclaimer that IANAL: copyright attaches to the work (the code), and is retained by the author. The author grants you certain rights to use, modify, and redistribute the code subject to conditions (the GPL). Those constraints only encumber your works that are derivative of the GPL-licensed code i.e. either direct modifications of original source or linking original/derived object code.

A product that directly and necessarily imported perfplot would probably itself need to be GPL licensed to be distributed. A product developed with merely the assistance of something like perfplot would not need to be licensed in any particular way, any more than software written in Emacs needs to be GPL licensed.

[cozzyd]

As far as I understand, I don't think even importing is an issue, unless you are distributing the module too as a combined artifact (e.g. docker image). See e.g. https://github.com/pyFFTW/pyFFTW/issues/229

There's a lot of GPL FUD though from commercial interests though.

[kristjansson]

Yeah, there's probably some untested boundaries at the intersection of the GPL, interpreted languages, and their libraries.

Those boundaries are nowhere close to the situation in the OP though. I guess the FUD is working...