Python3集合，相交和字符串列表的并集

Question

Question: How come the following basic math formula is not working with the python3 intersect and union calculation?

len(q1) + len(q2) - intersection = union

Input

q1 = ['How', 'does', 'the', 'Surface', 'Pro', 'himself', '4', 'compare', 'with', 'iPad', 'Pro', '?']
q2 = ['Why', 'did', 'Microsoft', 'choose', 'core', 'm3', 'and', 'not', 'core', 'i3', 'home', 'Surface', 'Pro', '4', '?']

intersect = set(q1).intersection(q2)
union_length = list(set(q1).union(q2))

print('q1_len',len(q1))
print('q2_len',len(q2))
print('union',len(union_length))
print('intersect',len(intersect))

output

q1_len 12
q2_len 15
union 21
intersect 4

12 + 15 - 4 should be 23 not 21.

Answer 1

The rule applies to set not list, so if you print:

print('q1_len',len(set(q1)))
print('q2_len',len(set(q2)))
print('union',len(union_length))
print('intersect',len(intersect))

Output:

('q1_len', 11)
('q2_len', 14)
('union', 21)
('intersect', 4)

The formula (11 + 14 - 4 = 21) holds true.