Question about list and while function

[adt]

the problem is that list must always have 10 elements and "a" and "b" will be input by the user

Will this meet your needs?

a = 1
b = 5
mlist= [0]*(a - 1) + [a * (b - x) for x in range(a, b+1)] + [0]*(10 - b)
print(mlist)

Output:
[4, 3, 2, 1, 0, 0, 0, 0, 0, 0]

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If you need to have the calculated values in ascending order, you could try the following:

a = 1
b = 5
mlist= [0]*(a - 1) + [a * (b - x) for x in range(b, a-1, -1)] + [0]*(10 - b)
print(mlist)

Output:
[0, 1, 2, 3, 4, 0, 0, 0, 0, 0]