Python TypeError: 'type' object does not support item assignment

Question

I have to design and implement a TwoSum class. It should support the following operations:

• add - Add the number to an internal data structure.
• find - Find if there exists any pair of numbers which sum is equal to the value.

Here is my code:

class TwoSum(object):

    dict = {}

    def add(self,n):
        dict[n] = n #TypeError: 'type' object does not support item assignment

    def find(self,n):
        for i in range(0,len(dict)+1):
            if dict[i] == None:
                continue

            val = n - dict[i]
            if dict[val] != None and val != i+1:
                return True
        return False

test = TwoSum()
test.add(1)
test.add(3)
test.add(5)
print(test.find(4))  # True
print(test.find(7))  # False

I got error message

TypeError: 'type' object does not support item assignment for "dict[n] = n"

Any help or suggestion? Thank you so much!

Answer 1

Lot of issues here, I'll try to go through them one by one

The data structure

dict = {}

Not only is this overwriting python's dict, (see mgilson's comment) but this is the wrong data structure for the project. You should use a list instead (or a set if you have unique unordered values)

Using the data structure

The data structure is an instance variable, it needs to be defined with self and inside the __init__ function. You should be using something like this:

class TwoSum(object):
    def __init__(self):
        self.numbers = []

def add

def add(self,n):
        dict[n] = n

Assigning items to a dictionairy is not the way to do it. You should instead append to your list. Additionally you need to append to the list for that instance using self.variableName = value

def find

That range is wrong, and you would need a nested range, or itertools.combinations since you have to check for any two numbers that sum to a certain value, pythons sum() is handy here.

To loop through the numbers you can use two ranges or itertools.combinations

The code

import itertools

class TwoSum(object):
    def __init__(self):
        self.numbers = []

    def add(self, num):
        self.numbers.append(num)

    def find(self, desiredSum):
        for nums in itertools.combinations(self.numbers, 2):
            if sum(nums) == desiredSum:
                return True
        return False


test = TwoSum()
test.add(1)
test.add(3)
test.add(5)
print(test.find(4))
print(test.find(7))
#True
#False

Def find without itertools

def find(self, desiredSum):
        for num1 in self.numbers:
            for num2 in self.numbers:
                if num1 + num2 == desiredSum and num1 != num2:
                    return True
        return False