Problem Statement

def validate(A, B):
  if not A or not B or not A[0] or not B[0]:
    raise ValueError("Matrices must be non-empty")
  n, k = len(A), len(A[0])
  k2, m = len(B), len(B[0])
  if any (len(r) != k for r in A): raise ValueError("Matrix A is jagged")
  if any (len(r) != m for r in B): raise ValueError("Matrix B is jagged")

  if k != k2 : 
    raise ValueError("Matrices dimensions mismatch")
  return n, k, m

# Compute a single row 
def compute_row(i, A, B, m, k):
  a_row = A[i]
  return [sum(a_row[p] * B[p][j] for p in range(k)) for j in range(m)]


def multiply_serial(A,B):
  n,k,m = validate(A,B)
  return [compute_row(i,A,B,m,k) for i in range(n)]
print(multiply_serial(A,B))
  

from concurrent.futures import ThreadPoolExecutor

def multiple_threaded(A,B, max_workers=None):
  n, k, m =  validate(A,B)
  C = [[0.0]*m for _ in range(n)]

  def work(i):
    C[i] = compute_row(i,A,B,m,k)
  
  with ThreadPoolExecutor(max_workers=max_workers) as pool:
    list(pool.map(work, range(n)))
  return C
  
print(multiple_threaded(A,B))