Hashmap in Python without dictionaries

hashmap.py

# Implement a hashmap in Python without using a dictionary     
# A simple toy example, look to improve when the hashes start clashing with each other either by adapting a dynamic size
# Or using a better hash function

class HashMap(object):

    def __init__(self, size, hash_function):

        self.array = [None] * size
        self.size = size
        self.hash_function = hash_function

    def __getitem__(self, item):

        index = self._get_index(item)
        value = self.array[index]
        if value is None:
            raise KeyError("Value does not exist")
        return value

    def _get_index(self, val):

        return self.hash_function(val) % self.size

    def __setitem__(self, item, value):

        if item is None:
            return ValueError("NoneTypes are not hashable")
        if value is None:
            return ValueError("NoneTypes cannot be stored")
        index = self._get_index(item)
        self.array[index] = value


def simple_hash_function(value):

    str_hash = str(value)
    hash_strs = 'abcdefghijklmnopqrstuvwxyz'
    hash_strs = hash_strs + hash_strs.upper()
    hash_strs = hash_strs + '0123456789'
    hash_dict = {v:i for i,v in enumerate(hash_strs)}

    value = [hash_dict[v] for v in value]
    return sum(value)



example_hashmap = HashMap(30, simple_hash_function )
example_hashmap['toy'] = 123
example_hashmap['duck'] = 'quack'
assert example_hashmap['toy'] == 123
assert example_hashmap['duck'] == 'quack'
print(example_hashmap['toy'])
print(example_hashmap['duck'])