def ascending( na, nb ):
        assert nb>=na
        # split each number into a list of digits
        a = list( int(x) for x in str(na))
        b = list( int(x) for x in str(nb))
 
        d = len(b) - len(a)
 
        # if both numbers have different length add leading zeros 
        if d>0:
            a = [0]*d + a  # add leading zeros
        assert len(a) == len(b)
        n = len(a)
 
 
        # check if the initial value has increasing digits as required,
        # and fix if necessary
        for x in range(d+1, n):
            if a[x] <= a[x-1]:
                for y in range(x, n):
                    a[y] = a[y-1] + 1
                break
 
        res = [] # result set
 
        while a<=b:
            # if we found a value and add it to the result list
            # turn the list of digits back into an integer
            if max(a) < 10:
                res.append( int( ''.join( str(k) for k in a ) ) )
 
            # in order to increase the number we look for the
            # least significant digit that can be increased
            for x in range( n-1, -1, -1): # count down from n-1 to 0
                if a[x] < 10+x-n:
                    break
            # digit x is to be increased
            a[x] += 1
            # all subsequent digits must be increased accordingly
            for y in range( x+1, n ):
                a[y] = a[y-1] + 1
 
        return res
 
print( ascending( 5000, 9000 ) )
 

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