Is there a way to use pre-trained R ML model in python web app?
Not sure what calling R code from Python has to do with ML models.
If you have a trained model, you can try converting it into ONNX format (emerging standard), and try using the result from Python.
More of a theoretical question: 
Use case: Create an API that takes json input, triggers ML algorithm inside of it and returns result to the user. 
I know that in case of python ML model, I could just pack whole thing into pickle and use it easily inside of my web app. The problem is that all our algorithms are currently written in R and I would rather avoid re-writing them to python. I have checked a few libraries that allow to run R code within python but I cannot find a way to pack it "in a pickle way" and then just utilize. 
It may be stupid but I have not had much to do with R so far.
Thank you in advance for any suggestions!