Заполнение массива диагоналями

def gen2d(ar,c=2):
    max_len = max([len(str(e)) for r in ar for e in r])+c
    s=""
    for row in ar:
        for el in row:
            s+="{:>{length}}".format(el,length=max_len)
        s+="\n"
    return s

def genTable(ar):
    max_len=0

    for i in range(len(ar)):
        for j in range(len(ar[i])):
            if len(str(ar[i][j]))>max_len:
                max_len=len(str(ar[i][j]))
    max_len+=2
    s="┏"+("━"*max_len+"┳")*len(ar[i])
    s=s[:len(s)-2]+"┓\n"
    for i in range(len(ar)):
        for j in range(len(ar[i])):
            t="┃"+str(ar[i][j]).center(max_len)
            s+=t
        s=s[:len(s)-1]+"┃\n┣"+("━"*max_len+"╋")*len(ar[i])
        s=s[:len(s)-2]+"┫\n"
    t=len(s)-len("┃\n┣"+("━"*max_len+"╋")*len(ar[len(ar)-1]))
    #print(t)
    s=s[:t+1]+"\n"+"┗"+("━"*max_len+"┻")*len(ar[i])
    s=s[:len(s)-2]+"┛"
    return s

def print2d(ar,type_=1):
    if type_==0:
        print(genTable(ar))
    else:
        print(gen2d(ar))


n=int(input("Please, input a number of rows in matrix, n= "))
m=int(input("Please, input a number of columns in matrix, m= "))

matrix = [[0 for _ in range(m)] for _ in range(n)]
count = 0
start = 0
for k in range(n):
    for i in range(start, m):
        line = k
        for j in range(i,-1,-1):
            if line < n:
                matrix[line][j] = count
                count += 1
                line += 1
        start = i
print2d(matrix)