How to implement dependency injection via constructor - Python

Question

I have rewritten a C# dependency inversion example in Python. I have created an interface called CalculatorOperation and trying to implement dependency injection via the Calculator constructor. The problem is when I run the code, it returns None. If I change self.calc_op = CalculatorOperation() to self.calc_op = Addition(), it returns the correct value of 15.

class CalculatorOperation():
    def calculate(self, num_1, num_2):
        pass


class Calculator:
    def __init__(self, calc_op):
        self.calc_op = CalculatorOperation()

    def solve(self, num_1, num_2):
        return self.calc_op.calculate(num_1, num_2)


class Addition(CalculatorOperation):
    def calculate(self, num_1, num_2):
        return num_1 + num_2


class Subtraction(CalculatorOperation):
    def calculate(self, num_1, num_2):
        return num_1 - num_2


def main():
    addition = Calculator(Addition())
    print(addition.solve(10, 5))


if __name__ == '__main__':
    main() # None

Slobodan Ilic · Accepted Answer · 2020-01-04 12:39:09Z

2

Just pass in the object that you created in main (don't re-do the initialization from within the __init__ method), like so:

class Calculator:
    def __init__(self, calc_op):
        self.calc_op = calc_op

answered Jan 4, 2020 at 12:39

Slobodan Ilic

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