Complexity of converting a set to a frozenset in Python

Question

What is the computational complexity of "freezing" a set in Python?

For example, does the second line in

a = {1,2,3}
b = frozenset(a)

require O(n) time? Or is it simply a "view" created in constant time?

Answer 1

b is not a view of a . You can check this via id :

a = {1, 2, 3}
b = a

id(a) == id(b)  # True

b = frozenset({1, 2, 3})

id(a) == id(b)  # False

Hence a change in b will not be reflected in a . You can, of course, test this yourself. Since frozenset takes an iterable as an argument, it must iterate over each argument. This is an O( n ) process.

As an aside, there's nothing special about frozenset , even creating a set from a set has O( n ) time complexity:

for i in [10**5, 10**6, 10**7]:
    a = set(range(i))
    %timeit set(a)

100 loops, best of 3: 3.33 ms per loop
10 loops, best of 3: 30.2 ms per loop
1 loop, best of 3: 421 ms per loop