How do I make a unitary matrix in python 3 using lists and sublists?

Question

I was trying to make a unitary matrix, (a square grid the size defined by the user and to have the diagonal from left to right become ones and I couldn’t make it only change the diagonal with:
for i in len(matrix):
Matrix[i][i]=1
Please help
I can’t use imports and it should be simple ( the most advanced things would be functions and sublists for this, sorry for the limitations.

Answer 1

Sorry I, didn’t specify but cant use imports only self made programs

Answer 2

If I have to use it in a quiz I wont be able to though

Answer 3

Oh nice thanks a bunch