Simple Implementation of HashTable in Python

hashtable.py

class HashTable(object):

    def __init__(self, size=100):
        self._size=size
        self._table=[[] for i in range(self._size)]
        
    def get(self, key):
        hash = self._hash(key)
        bucket = self._table[hash]
        for x in bucket:
            if x[0] == key:
                return x[1]
        return None

    def put(self, key, value):
        if key is None or value is None:
            raise ValueError('Bad Key or Value')
        hash = self._hash(key)
        bucket = self._table[hash]
        isExist = False
        for x in bucket:
            if x[0] = key
                x[1] = value
                isExist = True
        if not isExist:
            bucket.append((key, value))

    def _hash(self, key):
        string = str(key)
        sum = 0
        for c in string:
            sum += ord(c)

        return sum % self._size