How to make this code using loop(for i in range)

Question

hello all i want to ask about loop that output like
1
12
123
1234
12345
1234
123
12
1
how to make that code using for i in range

Answer 1

https://code.sololearn.com/c6ddrpISD5Ak/?ref=app

Answer 2

@Emma
a.append(str(i+1))
print("".join(a))
what is that function of that code?