step1: add calculator backend and tests

api/app.py

@@ -0,0 +1,35 @@
+from flask import (
+    Flask,
+    request,
+)
+
+from calculator.calculator import Calculator
+
+app = Flask(__name__)
+
+@app.route('/api/add', methods=['POST'])
+def add():
+    return operation('add', 2)
+
+@app.route('/api/subtract', methods=['POST'])
+def subtract():
+    return operation('subtract', 2)
+
+@app.route('/api/multiply', methods=['POST'])
+def multiply():
+    return operation('multiply', 2)
+
+@app.route('/api/divide', methods=['POST'])
+def divide():
+    return operation('divide', 2)
+
+def operation(method, num_factors):
+    factors = []
+    if num_factors == 2:
+        factors.append(float(request.json.get('x')))
+        factors.append(float(request.json.get('y')))
+
+    return str(getattr(Calculator, method)(*factors))
+
+
+app.run(host='0.0.0.0', port=8080)

api/calculator/calculator.py

@@ -0,0 +1,14 @@
+class Calculator:
+    def add(x, y):
+        return x + y
+
+    def subtract(x, y):
+        return x - y
+
+    def multiply(x, y):
+        return x * y
+
+    def divide(x, y):
+        if y == 0:
+            return 'Cannot divide by 0'
+        return x * 1.0 / y

api/calculator/test_calculator.py

@@ -0,0 +1,30 @@
+from .calculator import Calculator
+
+
+def test_add():
+    assert Calculator.add(1, 2) == 3.0
+    assert Calculator.add(1.0, 2.0) == 3.0
+    assert Calculator.add(0, 2.0) == 2.0
+    assert Calculator.add(2.0, 0) == 2.0
+    assert Calculator.add(-4, 2.0) == -2.0
+
+def test_subtract():
+    assert Calculator.subtract(1, 2) == -1.0
+    assert Calculator.subtract(2, 1) == 1.0
+    assert Calculator.subtract(1.0, 2.0) == -1.0
+    assert Calculator.subtract(0, 2.0) == -2.0
+    assert Calculator.subtract(2.0, 0.0) == 2.0
+    assert Calculator.subtract(-4, 2.0) == -6.0
+
+def test_multiply():
+    assert Calculator.multiply(1, 2) == 2.0
+    assert Calculator.multiply(1.0, 2.0) == 2.0
+    assert Calculator.multiply(0, 2.0) == 0.0
+    assert Calculator.multiply(2.0, 0.0) == 0.0
+    assert Calculator.multiply(-4, 2.0) == -8.0
+
+def test_divide():
+    assert Calculator.divide(1, 2) == 0.5
+    assert Calculator.divide(1.0, 2.0) == 0.5
+    assert Calculator.divide(0, 2.0) == 0
+    assert Calculator.divide(-4, 2.0) == -2.0