Solution 1

``````class OperationInvalidException(Exception):
pass

class Operation(object):
def __init__(self, *args):
self.args = args

def operate(self):
raise NotImplementedError()

def operate(self):
return sum(self.args)

class SubtractOperation(Operation):

def operate(self):
if not self.args:
return 0
return self.args[0] - sum(self.args[1:])

class Calculator(object):

def __init__(self, operations):
self.operations = operations

def calculate(self, *args):
numbers, operation = args[:-1], args[-1]
if operation not in self.operations:
raise OperationInvalidException
operation = self.operations[operation](*numbers)
return operation.operate()
``````

You'll need to build a better version of your calculator using OOP and inheritance:
A calculator can be built with different operations. An Operation is an abstract class for which you'll define
subclasses.

Example:

``````calc1 = Calculator(operations={
'subtract': SubtractOperation})

calc2 = Calculator(operations={
``````

The calculator has 1 generic method `calculate` that will receive the arguments
and the operation to perform. For example:

``````calc1.calculate(2, 1, 5, 'add')  # Should return 2 + 1 + 5 = 8
calc2.calculate(1, 5, 'add')  # Should return 1 + 5 = 6
``````

IMPORTANT: The number of arguments should be variable

The Calculator will check if it has that computation present and
invoke the operation. Operations are initialized with the arguments to compute:

``````op = AddOperation(2, 1, 5)
``````

Once you have an operation object created you should be able to invoke the `operate`
method PRESENT IN EVERY OPERATION.

``````op.operate()  # Should return 8
``````

Important notes:
The only method that you must implement for every operation (descendant from Operation) is the `operate` method.
If the operation is not supported by the calculator (for example invoking `multiply` on calc1) the calculator should raise a custom exception (built by you) named `OperationInvalidException`.

### Test Cases

test calculator with one operation -

``````def test_calculator_with_one_operation():
calc = Calculator(
operations={
}
)

assert calc.calculate(1, 5, 13, 2, 'add') == 21
``````

test calculator with multiple operations -

``````def test_calculator_with_multiple_operations():
calc = Calculator(
operations={
'subtract': SubtractOperation
}
)
assert calc.calculate(1, 5, 13, 2, 'add') == 21
assert calc.calculate(13, 3, 7, 'subtract') == 3
``````

test subtract operation -

``````def test_subtract_operation():
op = SubtractOperation(10, 1, 3, 2, 1)
assert op.operate() == 3
``````

test calculator invoked with an invalid operation -

``````import pytest

def test_calculator_invoked_with_an_invalid_operation():
calc = Calculator(
operations={
``````def test_add_operation():