find if the sum of any two numbers in a list equates to a given number

TwoSum.py

class TwoSum(object):
    """
    find if the sum of any two numbers in a list equates
    to a given number
    """
    def __init__(self):
        self.nums = []
        self.sum = 0

    def add_num(self, num):
        self.nums.append(num)
        self.sum += num

    def find_sum(self, sum):
        if self.sum - sum in self.nums:
            return True
        else:
            return False

ts = TwoSum()

ts.add_num(1)
ts.add_num(-1)
ts.add_num(2)
ts.add_num(-2)
ts.add_num(3)

print ts.find_sum(-1)
print ts.find_sum(1)
print ts.find_sum(6)
print ts.find_sum(0)

# False
# True
# False
# True