diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 73f0c17..b241c82 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -16,9 +16,9 @@ jobs:
         with:
           submodules: "true"
       - name: Install dependencies
-        run: pip install -r src/scripts/app/requirements.txt
+        run: pip install -r app/requirements.txt
       - name: Run tests and collect coverage
-        run: pytest src/scripts/app/ --cov
+        run: pytest app/ --cov
 
       - name: Upload coverage to Codecov (script)
         uses: ./
@@ -50,7 +50,7 @@ jobs:
           token: ${{ secrets.CODECOV_TOKEN }}
 
   run-macos-latest-xlarge:
-    if: github.head.repo.full_name == 'codecov/codecov-action'
+    if: github.event.pull_request.head.repo.full_name == 'codecov/codecov-action'
     runs-on: macos-latest-xlarge
     steps:
       - name: Checkout
@@ -58,9 +58,9 @@ jobs:
         with:
           submodules: "true"
       - name: Install dependencies
-        run: pip install -r src/scripts/app/requirements.txt
+        run: pip install -r app/requirements.txt
       - name: Run tests and collect coverage
-        run: pytest src/scripts/app/ --cov
+        run: pytest app/ --cov
       - name: Upload coverage to Codecov (script)
         uses: ./
         with:
diff --git a/.gitignore b/.gitignore
index a310cc1..91945ba 100644
--- a/.gitignore
+++ b/.gitignore
@@ -93,3 +93,6 @@ public/
 
 # macOS Finder metadata
 .DS_Store
+
+# pycache dirs
+__pycache__/
diff --git a/app/__init__.py b/app/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/app/calculator.py b/app/calculator.py
new file mode 100644
index 0000000..8a976b2
--- /dev/null
+++ b/app/calculator.py
@@ -0,0 +1,15 @@
+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
diff --git a/app/requirements.txt b/app/requirements.txt
new file mode 100644
index 0000000..c75c448
--- /dev/null
+++ b/app/requirements.txt
@@ -0,0 +1 @@
+pytest-cov
diff --git a/app/test_calculator.py b/app/test_calculator.py
new file mode 100644
index 0000000..f564193
--- /dev/null
+++ b/app/test_calculator.py
@@ -0,0 +1,31 @@
+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
+    # assert Calculator.divide(2.0, 0.0) == 'Cannot divide by 0'