HashSets and HashTables in Python

Question

Is there any HashSet implementation in Python? I know HashTable can be represented using dictionaries, but how do we represent HashSet implementation.

I am NOT looking for a data structure with the same methods as HashSets but rather someone with a CONSTANT lookup time, or the order of O(1);

Also, I want to know if the lookup time in a Python Dictionary is constant aka O(1)

Answer 1

I think the HashSet implementation you are looking is set(). This answer may help you: What's the difference between HashSet and Set?

And yes, the average time complexity for python dictionary O(1). You may read on why we use the term: "Average time complexity": Time complexity of accessing a Python dict

Answer 2

I guess this is what you want. You may define a hash function yourself, like what I did in HashSet or just use the built-in hash() function in Python.

class HashSet:
    CONST = 2 ** 61 - 1

    def __init__(self, size = 10_000):
        self.size = size * 2
        self.contents = [None] * self.size

    def hash(self, x):
        return x % CONST

    def put(self, key):
        idx = self.hash(key) % self.size
        arr = self.contents[idx]
        if arr is None:
            self.contents[idx] = [key]
        elif key not in arr:
            arr.append(key)
        return None

    def get(self, key):
        idx = self.hash(key) % self.size
        arr = self.contents[idx]
        if arr is None or key not in arr:
            return False
        return True


myset = HashSet()
myset.put(123)
myset.put(145)
myset.put(138)
res = myset.get(145)
print(res)
res = myset.get(10)
print(res)


class HashMap:
    def __init__(self, size = 10_000):
        self.size = size * 2
        self.contents = [None] * self.size

    class __Pair:
        def __init__(self, key, value):
            self.key = key
            self.value = value

    def find(self, arr, key):
        for pair in arr:
            if pair.key == key:
                return pair
        return None

    def put(self, key, value):
        idx = hash(key) % self.size
        pair = self.__Pair(key, value)
        arr = self.contents[idx]
        if arr is None:
            self.contents[idx] = [pair,]
            return None

        t = self.find(arr, key)
        if t != None:
            t.value = value
        else:
            arr.append(pair)

    def get(self, key):
        idx = hash(key) % self.size
        arr = self.contents[idx]
        if arr == None:
            raise KeyError(f'{key} is not a valid key')
        t = self.find(arr, key)
        if t == None:
            raise KeyError(f'{key} is not a valid key')
        return t.value


mymap = HashMap()
mymap.put('abc', [123,456])
mymap.put('def', [456,789])
res = mymap.get('abc')
print(res)
res = mymap.get('def')
print(res)
res = mymap.get('defx')
print(res)