My Python backend (Django) has to request to a C++ library to get a result (with help of ctypes module).
Is it normal to call a C++ method directly? Or may be I need an intermediate thread manager that starts a new thread when python script wants a result?
Basically you have to decide what kind of operation flow you want. If you prefer synchronous processing you can call you method directly, if you favor asynchronous processing you will need an intermediate solution.
However, you have to be aware, that when you call the C++ routine directly form your Django app the call will end in the execution path that is triggered via the web application. If the processing takes more time than you want to wait, a job management system will be the better choice.
In any case I would recommend such a solution if the execution of your C++ routine takes too much time. You could then use polling to wait until the result is ready using e.g. Web Sockets.
High Perfomance: Do call C++ methods directly from a python backend?