RingMorphism defined by a python function

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Is there a way to define a RingHomomorphism object that calls a user defined function? I know this is possible for SetMorphism, but I can't find a way to do the same for rings.

My use case: I have a python (or cython) function defining an embedding of a (finite) field into another. Of course, I could evaluate it on the generators of the domain and create a RingHomomorphism_im_gens object, but my function is much faster to evaluate.