While I was looking the other way, Python got advanced static types! Here’s matrix multiplication, describing the input shapes and its output shape: def mat_mul[ N, K, M ]( m1: Mat[N, M], m2: Mat[M, K], ) -> Mat[N, K]: return m1 @ m2 There’s a lot going on here! In traditional Python, we’d write: def mat_mul(m1, m2): return m1 @ m2 Then if we used the wrong shapes, we’d get a runtime error, like this: >>> m1 @ m2 Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: ...