Contains of HashSet<Integer> in Python

In Java we have HashSet<Integer>, I need similar structure in Python to use contains like below:

A = [1, 2, 3]
S = set()
S.add(2)
for x in A:
    if S.contains(x):
        print "Example"

Could you please help?

Solution 1:

Just use a set:

>>> l = set()
>>> l.add(1)
>>> l.add(2)
>>> 1 in l
True
>>> 34 in l
False

The same works for lists:

>>> ll = [1,2,3]
>>> 2 in ll
True
>>> 23 in ll
False

Edit: Note @bholagabbar's comment below that the time complexity for in checks in lists and tuples is O(n) on average (see the python docs here), whereas for sets it is on average O(1) (worst case also O(n), but is very uncommon and might only happen if __hash__ is implemented poorly).

Solution 2:

In Python, there is a built-in type, set. The major difference from the hashmap in Java is that the Python set is not typed, i.e., it is legal to have a set {'2', 2} in Python.

Out of the box, the class set does not have a contains() method implemented. We typically use the Python keyword in to do what you want, i.e.,

A = [1, 2, 3]
S = set()
S.add(2)
for x in A:
  if x in S:
    print("Example")

If that does not work for you, you can invoke the special method __contains__(), which is NOT encouraged.

A = [1, 2, 3]
S = set()
S.add(2)
for x in A:
  if S.__contains__(x):
    print("Example")