side_list = ['opposite', 'adjacent', 'hypotenuse']
side_values = []

for item in side_list:
    side_known = input(f"Do you have the length of the {item}? ")
    if side_known == 'yes':
        side_values.append(int(input("What is its length? ")))
    else:
        side_values.append(None)

print(side_values)