Generate numbers with strictly decreasing digits

I am trying to generate numbers of maximum length n whose all digits are strictly decreasing.

If ‍‍n = 5‍, then numbers would be :

Any hints on how to proceed?

Thank you in advance

Edit - My current code looks like this and I am not getting all the results.

def func(arr):
for x in arr:
    token=x[-1]
    k=int(token)-1
    while k>1:
        for i in range(1,k):
            f.write(x)
            for j in range(k,i,-1):
                f.write(str(j))
        k-=1

f = open ("demo.txt", "w")

arr = ["987"]
for i in range(986,321,-1):
    tens = i
    units = i%10
    tens = int(tens)/10
    hundreds = int(tens)/10 
    tens%=10
    hundreds%=10
    if tens >= hundreds or units>= tens:
        continue
    arr.append(str(i))

func(arr)
f.close()

Solution

Here is a recursive generator function:

def nums(n):
    if n:
        yield from (int(f"{n}{num}") for num in nums(n-1))
        yield n
        yield from nums(n-1)
       

[*nums(1)]
# [1]
[*nums(2)]
# [21, 2, 1]
[*nums(3)]
# [321, 32, 31, 3, 21, 2, 1]
[*nums(4)]
# [4321, 432, 431, 43, 421, 42, 41, 4, 321, 32, 31, 3, 21, 2, 1]

Or using the itertools.combinations approach:

from itertools import combinations

def nums(n):
    for l in range(n, 0, -1):
        for c in combinations(range(n, 0, -1), l):
            yield int("".join(map(str, c)))

[*nums(2)]
# [21, 2, 1]
[*nums(3)]
# [321, 32, 31, 21, 3, 2, 1]
[*nums(4)]
# [4321, 432, 431, 421, 321, 43, 42, 41, 32, 31, 21, 4, 3, 2, 1]