Set input length for embedded python block in gnu radio

I am having a "Stream to Vector " block which gives me 10 complex elements for another custom embedded block. How can I set the input type and length for my ambedded block to merge the size? I have already tried the official suggested method: in_sig = [(np.complex64,10)] but it doesn’t work.
Can someone post the entire code for the desired block, please?
Thank you!

I think, you can use indirect way to do that. I mean two block streams to vector and vector to streams, try to have de-interleaver (stream operator)