number between 0000-9999 but not the same.py

# 0001 -  9999

num = []
for i in range(100,10000):
    n = str(i)
    if len(n) != 4 :
        n = '0' + n
    if n[0] != n[1] and n[0] != n[2] and n[0] != n[3] and n[1] != n[2] and n[1] != n[3] and n[2] != n[3] :
        num.append(n)

import csv
with open('num.csv','w', newline='') as f:
    f.write(f'number between 0000-9999 but not the same\n')
    for n in num :
        f.write(f'{n}\n')

    f.write(f'total number is {len(num)}')