Hi,
The author of *backtrader* here.
0. Some Linkes
1. Open source is preferred.
2. A long development history. I don't want to learn a system that's abandoned soon.
- Well, it was developed to have as many features as possible and be easy to use ... over a long time of period.
3. Widely used with a big developer base.
- (See above for the references) Without trying to be cocky, I believe it is by far the most used Python backtester and it could well be that it is the mostly used. It has been used in companies, banks, quant firms, energy trading, universities, for cryptocurrency bots, in university courses, thesis and PHDs, listed by many as a skill in the LinkedIn profile and also in hire-for-work-sites.
4. A community such as a forum is a big plus.
- See above for the community link
5. Robust, flexible, etc.
- You probably want to read through the docs (see above for the link)
- 100% flexible. Everything can be plugged-in. It is a kit with many off-the-shelf blocks but you can plug your own.
- Robust, well ... that depends on your definition. If your code is clean it will be robust. But one of the design decisions was to let Exceptions propagate to the end user: the reason ... if your code produces an exception during backtesting it may well do the same during live trading ... so you'd better catch the errors it before you put some money in the market.
6. Back-test is more important than live trading for me at the stage.
- It is also for me, by far. The goal was to have a tool which allows rapid prototyping of ideas in the form of indicators, strategies, performance analysis, visualization ... but the platform took over and grew up to be what it is.
******** Some additional notes *********
7. For example `pyqstrat` was mentioned here. You can see in the PyPi package that *backtrader* is one source of inspiration ( have not checked if it is the source of some source code too

)
It's not the only package which draws on *backtrader.*
8. It is pure Python. A single `pip install backtrader` suffices (visualization requires *matplotlib*, but these days most people use Conda or WinPython or something with lots of packages precompiled and installed)
9. One of the most important things ... it has to suit you. You may not like the API, the object orientation, the need to plug in things or ... or ... Even if I think *backtrader* is the best by far (and have seen people often in reddit which say so) it doesn't have to be your cup of tea. One size cannot fit all needs and tastes.
10. I may of course be completely wrong about *backtrader* being used widely and much more than other platforms ...
Best regards