Hi,
Just to make sure I understand the question -- you're poking registers via stm.memX to configure the DMA device, but would like to implement one of the ISR vectors (e.g.) DMAx_StreamN from Python?
If so, sounds like you'd be much better off doing this in C, and then you'd have the benefit of the ST HAL and the code that already supports DMA for SPI/SDCard/etc.