Complex Number Multiplication LeetCode solution in Python

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class Solution:
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    def complexNumberMultiply(self, num1: str, num2: str) -> str:
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        i1 = num1.index("+")
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        i2 = num2.index("+")
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        a = int(num1[0:i1])
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        x = int(num2[0:i2])
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        b = int(num1[i1 + 1 : len(num1) - 1])
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        y = int(num2[i2 + 1 : len(num2) - 1])
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        ans1 = a * x + (-1) * b * y
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        ans2 = a * y + b * x
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        return str(ans1) + "+" + (str(ans2) + "i")