Convert string to signed integer

str_to_int.py

"""
Convert string to integer
"""
import unittest

class TestStringMethods(unittest.TestCase):

    def test1(self):
        self.assertEqual(myInt("123"), 123)

    def test2(self):
        self.assertEqual(myInt("-1"), -1)

    def test3(self):
        self.assertEqual(myInt("0"), 0)

    def test4(self):
        self.assertEqual(myInt("-0"), 0)

    def test5(self):
        self.assertRaises(TypeError, myInt, "")

    def test6(self):
        self.assertRaises(TypeError, myInt, "12q3")



def myInt(s):
    num = 0
    if len(s) == 0:
        raise TypeError("String must be non-empty")

    sign = 1

    if s[0] == "-":
        sign = -1
        s = s[1:]

    power = 0
    for i in range(len(s)-1, -1, -1):
        if ord(s[i]) in range(ord("0"), ord("9")+1):
            num += (ord(s[i]) - ord("0")) * 10**power
            power += 1
        else:
            raise TypeError("String must contain only digits and minus sign")

    return sign * num

unittest.main()

print myInt("-112490") # -112490